Enterohemorrhagic Escherichia coli, particularly the O157:H7 serogroup, has become a worldwide public health concern because it is the cause of ‘hamburger disease’. This serotype was first recognized as a pathogen in 1982 after two human illness outbreaks in Oregon and Michigan. Since then, E. coli O157:H7 has caused major human illness outbreaks worldwide, including one affecting 600 people (two deaths) in the western USA and another 10 000 people (11 deaths) in Japan.
Cattle feces are a major source of E. coli O157:H7 that cause human diseases, most of which have been traced to consumption of undercooked beef that had been contaminated with bovine feces. In contrast to humans, cattle infected with E. coli O157:H7 remain free of disease and are tolerant of E. coli O157:H7 for most of their lives. E. coli O157:H7 strains are not pathogenic in calves over 3 weeks of age.
As a consequence, considerable resources have been devoted to defining the epidemiology and ecology of E. coli O157:H7 in the cattle environment so that control might begin at the farm level. Sheep also harbor E. coli O157:H7 and non-E. coli O157:H7 verotoxin-producing E. coli at rates similar to or higher than those reported in cattle.
E. coli O157:H7 is the major serotype that was first recognized as a cause of human illness. E. coli O157:H7 is one of more than 60 serotypes of verotoxin-producing E. coli that cause a variety of human illnesses such as mild diarrhea, hemorrhagic colitis and hemolytic–uremic syndrome. These illnesses result from verotoxins (VT1 and VT2) similar to those produced by Shigella dysenteriae and are known for their toxic effects on African–Green Monkey kidney (Vero cells in cultures). These toxins are different proteins, are encoded by different genes and have similar toxic effects.
Several non-O157:H7 verotoxin-producing E. coli (O6:H31, O26:H- (nonmotile), O26:H11, O48:H21, O9:H-, O104:H21, O113:H21, and O26:H2) have been associated with human disease outbreaks in the USA and in other countries.1 Because most of these outbreaks have been traced to consumption of undercooked beef that had been contaminated with bovine feces, cattle have been considered to be reservoirs of verotoxin-producing E. coli. Sheep may harbor E. coli O157:H7 and non-O157:H7 verotoxin-producing E. coli at rates similar to or higher than in cattle.
Naturally occurring cases of attaching and effacing lesions of the intestines in calves with diarrhea and dysentery and infected with E. coli O126:H11, the predominant verotoxin-producing E. coli in humans, have been described in the UK.2 Verotoxin-producing E. coli and eae-positive non-verotoxin-producing E. coli have been isolated from diarrheic dairy calves 1–30 days of age.3
The literature of the epidemiological surveys on the prevalence of contamination of healthy cattle with Escherichia coli O157:H7 has been reviewed,4 as has the literature on the epidemiology and ecology of E. coli O157:H7 in bovine production environments.5
Cattle are nonclinical natural reservoirs of E. coli O157:H7. Estimates of the prevalence of verotoxin-producing E. coli fecal carriage among populations of cattle vary considerably.
The herd prevalence of infection with E. coli O157:H7 in North American and European cattle herds ranges from 3–8% and 0.5–1.0% of animals, with a higher prevalence up to 5% in weaned calves and heifers.4 Some surveys reported the organism in dairy and beef cattle at 0.28% and 0.71%, respectively.6 Other studies found much higher levels of prevalence, such as 15.7% of cattle over a 1-year period. Monthly prevalences ranged from 4.8–36.8% and were highest in the spring and late summer.
The prevalence of E. coli O157:H7 is influenced by numerous variables, including the season, the scope, frequency and timing of sampling, and the conditions of sampling and storage. The organism can be found widely distributed in samples from several types of cattle including beef calves, stocker cattle, feedlot cattle, adult beef cows, dairy calves, water sources and wildlife.7 The prevalence is commonly higher in feeder-age cattle.
The prevalence of E. coli O157:H7 infection in range beef calves at weaning prior to arrival at the feedlot varies from 1.7–20.0% with an average of 7.4%.8 All herds had high prevalence of anti-O157 antibodies, ranging from 63–100% of individuals within herds. Most calves (83%) and all herds (100%) have been exposed to E. coli O157:H7. In a naturally infected cow/calf herd followed over a period of 2 years, postpartum shedding of E. coli O157:H7 and cow–calf or calf–calf transmission under confined conditions in the postpartum period appeared to be potentially important factors in initial infection of beef calves.9 Parturition, calving pens and weaning appear to be important factors in maintaining E. coli O157:H7 infections in some herds.
Different serotypes of verotoxin-producing E. coli known to cause human illness have been found in the feces of culled beef cows at the time of shipping to slaughter.10
The diversity, frequency, and persistence of E. coli O157:H7 strains in cattle, wildlife and water sources within range cattle production environments indicates that the molecular epidemiology of the organism is very complex.11
The prevalence of E. coli O157:H7 in dairy cattle is similar worldwide and ranges from 1–5% in heifers and less than 1% in adult cattle. The herd-level prevalence varies from 0–100%.4 The herd-prevalence of E. coli O157:H7 in dairy cattle in the subtropical southeastern USA in the summer months was 38.5%, and the cow-level prevalence was 6.5%.12 Among positive herds, prevalence ranged from 3–35%. The prevalence was higher during the spring and summer months compared to the winter season. Site-specific prevalences of E. coli O157:H7 from the oral cavity, skin, and fecal samples were 0%, 0.7%, and 25.2% respectively. Interestingly, in these same areas, reported cases of E. coli O157:H7 infections in humans are relatively uncommon and do not support the hypothesis that E. coli O157:H7 infections in humans are rare in southern states because of the low prevalence in cattle.
Viable E. coli O157:H7 can be isolated from the oral cavity, multiple hide surfaces and feces of a high percentage of feedlot beef cattle, and bacterial culture of feces alone generally underestimates the prevalence of E. coli O157:H7 in feedlot cattle.13
A survey of 60 Danish dairy cattle farms found a herd-prevalence of E. coli O157:H7 of 17% and an individual animal prevalence of 3.6%.14 The high-risk age group for E. coli O157:H7 was calves between 2 and 6 months of age (8.6% positive), in contrast to calves under 2 months of age (0.7%) and cows (2.4%).14
E. coli O157:H7 is widespread in cattle in England and Wales. During a survey between June and December, the herd-prevalence on positive farms ranged from 1.1–51.4%, and the overall individual animal prevalence was 4.2%, and 10.3% among animals in positive herds.15 The prevalence of excretion was least in calves under 2 months of age, peaked in calves between 2 and 6 months of age and declined thereafter. Multiple phage types of E. coli O157:H7 were isolated from almost a third of the herds, and the phages types were similar to those responsible for human infections in England and Wales during the same period. Similar prevalences have been found in dairy cattle in Brazil.16
Attaching and effacing E. coli (AEEC) of various strains have been associated with naturally occurring and experimental dysentery in calves, and outbreaks of hemorrhagic enteritis with attaching and effacing lesions of the colon associated with E. coli O126 infection have occurred in heifers 8–12 months of age.17 E. coli O126 is recognized as an enterohemorrhagic pathogen in humans.
Based on fecal sampling, the prevalence rate of infection of cattle with E. coli O157:H varies considerably. Based on fecal samples from the rectums of cattle at slaughter in the UK, the animal-level prevalence was 7.5% and the group prevalence was 40.4%.18 Of the infected animals, 9% were high shedders whose feces contained E. coli O157:H7 at concentrations of more than 104 colony-forming units (cfu)/g. This 9% also represented more than 96% of the total E. coli O157:H7 produced by all animals tested. This indicates that the presence of high-shedding animals at the abattoir increases the potential risk of beef contamination during the slaughtering process and stresses the need for correct hazard analysis and critical control points procedures.
There is a correlation between the prevalence of E. coli O157:H7 in the feces, hides and carcasses of beef cattle during slaughter.19 Overall, the prevalence of E. coli O157:H7 in feces and on hides was 28% and 11%, respectively. Carcass samples were taken at three points during processing: pre-evisceration, post-evisceration before antimicrobial intervention, and postprocessing after carcasses entered the cooler. The prevalence of E. coli O157:H7 in the three post processing samples was 43%, 18%, and 2%, respectively. Antimicrobial intervention included steam pasteurization, hot water washes, organic acid washes, or combinations of these treatments. The reduction in carcass prevalence from pre-evisceration to postprocessing suggests that sanitary procedures can be effective within processing plants. Fecal and hide prevalence were significantly correlated with carcass contamination, indicating a role for control of E. coli O157:H7 in live cattle.
The prevalence of fecal carriage of E. coli O157:H7 in cattle and sheep sampled throughout the year at the abattoir before slaughter was 4.7% for cattle and 1.7% for sheep.20 The most frequently recovered E. coli O157:H7 isolates were phage types 2, 8, and 21/28 in cattle and 4 and 32 in sheep, the five most frequently isolated phage types associated with illness in people in Great Britain during the same survey period. In a commercial beef abattoir in Ireland, carcass contamination with E. coli O157:H7 can occur during removal of the hide and tying the bung, and this contamination can remain on the carcass during subsequent processing.21
The prevalence of attaching and effacing E. coli isolated from cattle, sheep, and pigs at slaughter in England and Wales was 8.1%, with a frequency of 18% in sheep, 6% in cattle, and less than 1% in pigs.22 Several different eae-positive serogroups were isolated; some possessed stx1 but none had stx2.
An overall prevalence of E. coli O157:H7 fecal shedding by New York cull dairy cattle of 1.3% was found in specimens just before processing the packing plant.23
In a survey of downer cattle submitted to two slaughter facilities in Wisconsin, the prevalence of E. coli O157:H7 in the feces and/or tissues of downer dairy cattle was 4.9% compared to 1.5% in healthy cattle.24
PCR has been used to detect virulence genes and molecular epidemiology of E. coli O157:H7 isolates from abattoirs.25 Samples included swabs of tools, knives and saws, fecal samples, carcass samples and ears removed after slaughter. From 1432 samples, 143 E. coli O157:H7 strains were isolated. These results indicate the increase in contamination frequencies during transportation to the abattoir and the lairage period before slaughter, as a result of cross-infection caused by mixing of animals from different sources.25 Surveys at the abattoir could be useful in detecting infected cattle herds and allow focusing on potentially infected herds, in concert with on-farm surveys.
Sheep and goats can be naturally infected with E. coli O157:H7 and sheep have been used as a model of ruminant infection.26 Sheep may harbor E. coli O157:H7 and non-O157:H7 verotoxin-producing E. coli at rates similar to or higher than in cattle. Prevalence rates of 67% and 45% have been reported in Germany and Australia, respectively.1 Worldwide, sheep have been shown to shed several non-O157 verotoxin-producing E. coli in their feces.1 Several of these verotoxin-producing E. coli serotypes have been associated with sporadic cases or major outbreaks of human illnesses. Thus lamb, mutton and their products share a food safety risk factor similar to that of beef. Non-O157:H7 verotoxin-producing E. coli have been found in sheep grazing irrigated pasture or arid rangeland forage in Nevada.1 In Norway, sheep have carried Shiga-toxin-producing E. coli serogroups O5, O91, and O128, and their virulence factors have been characterized.27 In Brazil, Shiga-toxin-producing E. coli occurred in the feces of 51% of healthy sheep grazing on pasture.28 The serotypes could cause severe disease in humans.
Based on fecal samples of deer submitted by hunters, E. coli O157:H7 have been found in the feces of free-ranging white-tailed deer in Nebraska at a rate of 0.25%.29 The prevalence of infection of E. coli O157:H7 in white-tailed deer sharing rangeland with cattle was 2.4%.29 The low overall prevalence of E. coli O157:H7 and the identification of only one site with positive deer suggest that wild deer are not a major reservoir of E. coli O157:H7 in the southeastern USA.30
E. coli O157:H7 has been found in fecal samples of finished pigs at the time of slaughter but the prevalence was very low at 0.08%. In experimentally infected pigs, E. coli O157:H7 can persist for more than 2 months.31 Pigs may have the potential to be reservoirs hosts for E. coli O157:H7 but the magnitude of the risk needs to be determined.
Healthy normal cattle are a major reservoir for enterohemorrhagic E. coli O157:H7 strains, which cause disease in humans. Cattle that are infected with E. coli O157:H7 remain free of disease and are tolerant of E. coli O157:H7 for their entire lives. Neonatal calves under 36 hours of age are susceptible to experimental inoculation with E. coli O157:H7 and develop attachment and effacing lesions in both the large and small intestines.32 Neither experimentally infected calves nor older cattle with E. coli O157:H7 develop extraintestinal vascular lesions. The lack of vascular receptors for Stx in cattle renders them resistant to the Stx.33 Neither viable E. coli O157:H7 nor Stx-containing extracts caused fluid accumulation in ligated ileal loops of newborn calves, whereas doses of Stx did cause fluid accumulation experimentally in rabbits.33
The factors associated between management, climate and the prevalence of E. coli O157:H7 in feedlot-water tanks and in feedlot-cattle feed have been examined in selected feedlots in the USA.34 E. coli O157:H7 was isolated from 13% of the water tanks and at least one water tank was positive on 60% of the feedlots. The factors associated with E. coli O157:H7 in water were a greater percentage of cattle shedding E. coli O157:H7 in the feces within the same pen, higher concentrations of total E. coli in the water, lack of clarity of the water, the use of fly traps, the reported frequency of rodent sightings in the pen or alley area, and the weather at the time of sampling. E. coli O157:H7 was isolated from 14.9% of the feed samples obtained from the feedbunks. Factors positively associated with E. coli O157:H7 in the feed were higher heat index at the time of sampling, the presence of cottonseed meal in the ration, and the feedlot location.
The association between management, climate and E. coli O157:H7 in the feces of feedlot cattle in the midwestern USA has been examined.34 The prevalence of E. coli O157:H7 was 10.2% at the sample level, 52.0% at the pen-level and 95.9% at the feedlot level. The factors associated with the presence of E. coli O157:H7 in cattle feces were the frequency of observing cats in the pens or alleys, the presence of E. coli O157:H7 in the water tanks (positive association), the historical use of injectable mass medication (positive association), the use of antibiotics in the ration or water (negative association), the wetness of the pen, number of cattle in the pen (negative association), wind velocity (positive association), and height of the feed bunk (positive association).
Environmental dissemination of an inoculated strain of E. coli O157:H7 given to dairy calves spreads more quickly when calves are housed in groups compared to calves housed in individual pens from 7–110 days of age.35 This indicates that control may depend on reduction of horizontal transmission within cattle groups, thus decreasing prevalence. The use of segregated penning systems rather than group housing of weaning calves may reduce the prevalence of these potential pathogens within the calf unit. If this results in a reduction in the general herd or farm Shiga-toxin-producing E. coli prevalence, then such changes in calf-rearing practice may offer a control point for preharvest Shiga-toxin-producing E. coli risk on dairy farms.
The housing of beef suckler cows in Scotland during the winter months was associated with increased level of shedding of E. coli O157:H7.36
The primary feature of Shiga-toxin-producing E. coli isolates is their ability to produce potent cytotoxins encoded by stx1 and stx2. They also have the ability to adhere to the intestinal mucosa in an intimate manner through the attachment and effacement protein intimin, encoded by the eaeA gene, and most produce a plasmid-encoded enterohemolysin, encoded by the elixA gene. Shiga-toxin-producing E. coli isolates that cause disease in humans usually have one or both of these virulence-associated factors and have been referred to as complex Shiga-toxin-producing E. coli (cSTEC). The most often reported Shiga-toxin-producing E. coli serotype causing diseases in humans worldwide is E. coli O157:H7, but non-O157 serotypes such as O111:H- and O113:H21 are commonly found to cause diseases such as hemolytic–uremic crisis. There are over 160 Shiga-toxin-producing E. coli serotypes that have been isolated from human patients around the world. A broad range of cSTEC serotypes have been isolated in Australian cattle37 and in Argentina.38
Molecular typing of E. coli O157:H7 strains is done using pulse-field gel electrophoresis (PFGE) and is accepted as the standard technique in epidemiological investigations. The technique can be used to demonstrate horizontal transmission of a single strain type among animals within a farm and comparing sporadic isolates between animals and humans. The technique has been used to study E. coli O157:H7 isolates from cattle and human cases isolated in France and in northern Italy and from French and Spanish cattle imported to Sicily.39
E. coli O157:H7 is extremely acid-resistant, which contributes to the low infectious dose for humans, which has been estimated to be fewer than 700 cells and possibly even as low as 10.26 Certain strains of E. coli O157:H7 have been considered to be more acid-tolerant than some commensal E. coli. In addition, E. coli O157:H7 strains may become acid-habituated by exposure to weak acids in the rumen. Consequently, E. coli O157:H7 may survive passage through the acid barrier in the stomach, colonizing and replicating in the ruminant colon. The numbers of E. coli O157:H7 are much higher in the colon than the rumen.40
The acid-resistance characteristics of E. coli O157:H7 led to the hypothesis that feeding grain to cattle created an ideal environment in the gastrointestinal tract to promote the growth and persistence of the organism. The research data on the effects of grain versus forage feeding to cattle and its effects on fecal E. coli O157:H7 are limited and conflicting. Some early research indicated that grain feeding increased the dissemination of acid-resistant E. coli by cattle, and that feeding hay for a brief period immediately before slaughter would decrease the shedding of E. coli O157:H7. The numbers, persistence and acid resistance of generic coliforms and E. coli O157:H7 from various gastrointestinal tract sites of cattle fed grain or hay were compared. Grain-feeding or hay-feeding did not affect survival of E. coli O157:H7 in the rumen, nor its passage through the abomasum (pH 2.0) to the duodenum.41 Generic coliforms from the rumen and rectum of hay-fed animals were more sensitive to an acid shock than coliforms from gut locations in grain-fed animals. Thus E. coli O157:H7 in bovine ingesta are acid-resistant regardless of animal diet.41 Abruptly switching cattle from a high-grain diet to a high-quality hay-based diet has been shown to reduce generic E. coli and E. coli O157:H7 populations but the magnitude of the reduction has varied among studies.42,43
Other work has shown that, in the context of acid tolerance, E. coli O157:H7 does not appear to be greatly different from commensal E. coli.44 However, E. coli O157:H7 may have the ability to generate a higher ‘tail’ population and this, together with the low infective dose and changes in our lifestyle, has allowed the organism to become a major pathogen. Recent studies on the effect of forage or grain diets have shown that cattle fed forage diets had ruminal persistence of fecal E. coli O157:H7 at quantifiable concentrations for twice as long as cattle fed grain diets.45 Diets high in grain generate high volatile fatty acid concentrations and low pH, creating a less conducive environment for E. coli O157:H7, whereas lower volatile fatty acid concentrations and higher pH in forage-fed cattle may be more conducive to the growth and survival of the organism. Monensin supplementation decreased the duration of shedding with forage diet, and the cecum and colon were culture-positive for E. coli O157:H7 more often than the rumen of cattle.
The prevalence of antimicrobial resistance among isolates of E. coli O157:H7 recovered from clinical cases in humans, pigs, cattle, and food over a 15-year period (1985–2000) in the USA has been described.46 There was a high prevalence of resistance to tetracycline, sulfamethoxazole, cephalothin, and ampicillin. The highest prevalence occurred among isolates from pigs, where more than 50% of all isolates were resistant to sulfamethoxazole, cephalothin, or tetracycline and more than 20% were resistant to ampicillin or gentamicin.
E. coli O157:H7 is a transient inhabitant of the gastrointestinal tract of normal healthy ruminants. Cattle and sheep feces serve as sources for contamination of feed and water sources. Fecal shedding is transient in cattle, often lasting 1–3 months or less, but the organism can persist on individual farms for up to 2 years.47 Longitudinal surveys have shown that maintenance of E. coli O157:H7 in cattle herds relies on continual reinoculation of individual cattle.6 Repeated isolations of E. coli O157:H7 from healthy beef and dairy cattle demonstrate that cattle are asymptomatic carriers of the organism. Short periods of relatively high prevalence of excretion are separated by longer periods of reduced or undetectable shedding. This has contributed to the variance in prevalence data reported in the literature.
Fecal shedding is more prevalent in the USA and Canada during the summer months and is more prevalent in the UK in the spring and fall. Fecal shedding also varies among different classes of animal. Weaned heifers between 3 months of age and breeding age are more likely to shed E. coli O157:H7 in feces than adult cattle or younger calves. Increased shedding is associated with weaning and with the first month of lactation in dairy herds, and culled dairy cattle have a higher prevalence than previously reported. Contaminated water troughs, particularly those that are allowed to develop sediments, provide an environment for survival, proliferation and horizontal spread of E. coli O157:H7. The organism can also proliferate to very high levels in moist silage.
In Alberta, Canada, the prevalence of E. coli O157:H7 was 12.4% of fecal samples from yearling cattle and 2.0% of the fecal samples from cull cows.48 The prevalence of E. coli O157:H7 in yearling cattle increased from 1.4% in the winter months to 40% in the summer. In feedlots, E. coli O157:H7 was isolated in preslaughter pens of cattle from the feces (0.8%), feedbunks (1.7%), water troughs (12%) and incoming water supplies (4.5%) but not from fresh total mixed rations. Fresh total mixed rations did not support the growth of E. coli O157:H7 from feces following experimental inoculation. Many different subtypes of E. coli O157:H7 were isolated from the feces, water and feed in pens of feedlot cattle. This suggests that methods to control E. coli O157:H7 in feedlot cattle will have to center not only on reducing fecal shedding of the organism in cattle but on the potential of reinfection from environmental sources, such as water and feed, both at the feedlot and before the cattle arrive on the premises.
The pattern of fecal carriage of E. coli O157:H7 in cattle finished under modern intensive feedlot management conditions has been examined.49 E. coli O157:H7 was isolated from 13% of fecal samples, with highest prevalence values of the organism in pens supplied with chlorinated drinking water compared with nonchlorinated water pens. Over a period of 7 months from April to September, certain specific clonal types of E. coli O157:H7 persisted and predominated despite massive cattle population turnover. This suggests that the farm environment, and not necessarily the incoming cattle, is an important potential source of E. coli O157:H7 on farms. In a longitudinal study of E. coli O157:H7 in a cattle-finishing unit in Finland most farm isolates belonged to one PFGE genotype (79.6%) and the remainder to closely related PFGE genotypes. Thus the finishing unit rather than the introduction of new cattle was the source of E. coli O157:H7 at the farm and E. coli O157:H7 seemed to persist well on barn surfaces.50
One dairy calf infected with and shedding E. coli O157:H7 can infect other negative calves in a confined environment within 8 days, and the duration of shedding may range from 17–31 days.51 Experimentally, some calves may begin prolonged, high-level shedding of E. coli O157:H7 after only very low exposure doses, and other calves exposed to calves excreting E. coli O157:H7 in their feces are at high risk of infection.52 Experimentally infected calves may begin shedding within 6 days after oral inoculation and continue shedding for up to 70 days.
E. coli O157:H7 subtypes indistinguishable from those detected in cattle have been found in pigeons, geese, horses, dogs, opossums, and flies. E. coli O157:H7 also has been isolated from insects in cattle environments but their role in dissemination is uncertain.
There are many possible sources of E. coli O157:H7 in the farm environment, including manure piles, ponds, dams and wells, barns, calf hutches, straw and other bedding, feed and feed troughs, water and water troughs, farm equipment, ground surface and pasture, and watercourses. Once in the environment, the organism can be transferred to other sites by rainwater, wind, and removal and spreading of manure, including animals and humans.6
Drinking water offered to cattle is often of poor microbiological quality and the daily exposure of animals to E. coli O157:H7 from this source can be substantial.54 The degree of E. coli exposure is positively associated with proximity of water troughs to the feedbunk, protection of the trough from sunlight, and warmer weather. Cattle water troughs can serve as environmental reservoirs for E. coli O157:H7 and as a long-term source infection for cattle.55
The experimental inoculation of E. coli O157:H7 with 1 L of water into dairy calves in a confined environment resulted in shedding of the organism by the calves within 24 hours after administration.51 The duration of shedding varied from 18 to more than 43 days and the number of doses necessary to initiate shedding varied among calves.
E. coli O157:H7 is present in as many as 10% of water troughs. E. coli O157:H7 was present in the water or water-tank sediment in 13.1% of water tanks in a feedlot in the USA and 60% of feedlots had at least one positive tank.56 Cattle were more likely to be shedding E. coli O157:H7 in pens with positive water tanks, and water was more likely to be positive when E. coli O157:H7 was detected in the sediment.
Chlorination of input water in feedlots was unable to reduce the prevalence of E. coli O157:H7-contaminated water troughs.49
Water trough sediments with feces from cattle excreting E. coli O157:H7 may serve as a long-term reservoir of the organism on farms and a source of infection for cattle. The accumulation of large amounts of organic matter would be expected to rapidly inactivate the biocidal activity of chlorine and provide an ideal niche for the survival of the organism.55 E. coli O157:H7 can survive in farm water under field and shed conditions at temperatures less than 15°C for up to 24 days.57 The addition of feces to water outdoors resulted in survival for 24 days.
E. coli O157:H7 has been isolated from surface waters collected from a Canadian watershed.58 Systematic sampling of surface water within the Oldman River basin in southern Alberta reveals that it often contaminated with E. coli O157:H7 and Salmonella spp. The prevalence of E. coli O157:H7 and Salmonella spp. in water samples was 0.9% and 6.2%, respectively. The region surveyed is noted for high cattle density as well as for one of the highest incidences of gastroenteritis in Canada, resulting from infection by Salmonella spp. and E. coli O157:H7. While the data indicated a relationship between high livestock density and high pathogen levels in southern Alberta, analysis of the point source data indicates that the predicted manure output from cattle, pig, and poultry feeding operations was not directly associated with the prevalence of either Salmonella spp. or E. coli O157:H7. Variations in time, amount and frequency of manure applications on to agricultural lands may have influenced levels of surface-water contamination with these bacterial pathogens.
The prevalence of E. coli O157:H7 in cattle feeds in feedlots was 14.9%, higher than previously reported, which may be due to more sensitive detection methods.59 Feed may be a vehicle for dissemination and colonization; however, the source of the E. coli O157:H7 contamination in cattle feed is uncertain. Possible sources include saliva and fecal contamination by cattle or other species, or by wildlife, including birds, rodents, and insects. Another possible source is contaminated feed components mixed into the feed. PFGE profiles of E. coli O157:H7 isolated from a component feed sample closely resembled that isolated later from the same farm, suggesting that cattle feed may be an important vector for the transmission of E. coli O157:H7.60
Survival of E. coli O157:H7 in manure and manure slurry has been observed under various experimental and environmental conditions. The use of manure as fertilizer could explain foodborne outbreaks of E. coli O157:H7 associated with unpasteurized apple cider, potatoes, and other vegetables. Because E. coli O157:H7 can survive for extended periods of time, proper manure management is of major importance in preventing the spread of this organism to the environment. Composting is an effective method for eliminating pathogens such as E. coli O157:H7 from manure.
E. coli O157:H7 inoculated into loam and clay soils can survive for 25 weeks and in sandy soil for 8 weeks. The organism was detectable for up to 7 days after inoculation into the uppermost 2.5 cm of the soil, and for up to 7 days on grass plots inoculated with a fecal slurry from dairy cattle at an application rate of E. coli O157:H7 of 660 cfu/m2.
The organism can be cultured from rope devices in a feedlot pen that cattle rub or chew and there is a correlation with the prevalence of cattle shedding the organism in the feces from within the same pen.61 This pen-test strategy may be useful to identify pens of cattle posing a higher risk to food safety.
The Esp and Tir proteins secreted by E. coli O157:H7 play critical roles in the development of the attaching and effacing lesions and are recognized serologically in human patients with the hemolytic crisis syndrome. Antibodies to intimin, Esp and Tir proteins have been detected in HUS patients following infections with E. coli O157:H7.26
In contrast, little is known about the immune responses of cattle to infection with E. coli O157:H7.26 E. coli O157:H7 and other enterohemorrhagic E. coli are shed sporadically by cattle, and it appears that natural exposure to E. coli O157:H7 does not confer protection on the host. Calves 13–30 days of age developed anti-O157 IgG responses following experimental oral inoculation with E. coli O157:H7.62 Mature cows did not develop a significant increase in their serum anti-O157 IgG levels following oral inoculation. These observations suggest that local immunity to E. coli O157:H7 may not develop to any degree in the intestine and that immunization to reduce fecal shedding of E. coli O157:H7 may not be effective.
Vaccination of cattle with antigenic bacterial proteins involved in colonization can significantly reduce fecal shedding and prevalence of E. coli O157:H7 in cattle. Vaccination of cattle with E. coli O157:H7 type III secreted proteins can reduce the numbers of E. coli O157:H7 shed in the feces, and the duration of shedding in experimentally challenged cattle, and in feedlot cattle under field conditions.63 Vaccination of pregnant gilts with intimin from E. coli O157:H7 induced high intimin-specific immune responses in the serum and colostrum, and suckling neonatal piglets had reduced bacterial colonization and intestinal lesions following experimental challenge.64 These results suggest that vaccination may be a useful preharvest strategy for reducing the prevalence of E. coli O157:H7 infection in cattle.
However secure and well-regulated civilized life may become, bacteria, protozoa, viruses, and infected fleas, lice, ticks, mosquitoes, and bedbugs will always lurk in the shadows ready to pounce when neglect, poverty, famine, or war lets down the defenses. And even in normal times they prey on the weak, the very young and the very old, living along with us in mysterious obscurity awaiting their opportunities.
The above wisdom relates to E. coli O157:H7.
Enterohemorrhagic strains of E. coli, especially serotype E. coli O157:H7, have been linked in humans with hemorrhagic colitis, hemolytic–uremic syndrome and thrombocytopenic purpura from eating contaminated foods, such as beef and dairy products, vegetables, and apple cider, from contaminated drinking water or from contact with infected animals or contaminated environments. As few as 10 E. coli O157:H7 bacteria can cause illness in humans.
In 1999, the Centers for Disease Control and Prevention estimated that 73 480 people per year in the USA were infected with E. coli O157:H7 and that 61 of these cases were fatal.26 Most cases of E. coli O157:H7 illness are attributable to food-borne infection; however, acquisition of disease by direct contact with animals and manure at petting zoos and dairy farms are of increasing concern. Among E. coli O157:H7 foodborne outbreaks in 1999, one-third were due to beef; historically, undercooked ground beef is the most common vehicle. Consumption of pink hamburgers at home or in restaurants is a risk factor for E. coli O157:H7 infection. Microbiological testing of ground beef patties from a large outbreak that occurred in the Pacific northwest between November 1992 and February 1993 suggested that the infectious dose for E. coli O157:H7 is fewer than 700 organisms. This represents a strong argument for enforcing zero tolerance for this organism in processed food and for markedly decreasing contamination of raw ground beef. A major source of the bacteria in ground beef is bovine feces, which contaminates carcasses before evisceration; the organism is thought to be spread from contaminated hides to the surfaces of carcasses at slaughter. In addition to feces and hides, E. coli O157:H7 has been isolated from the oral cavities of cattle.
In May 2000, E. coli O157:H7 and Campylobacter jejuni contaminated the drinking water supply in Walkerton, Ontario, Canada.65,66 As a result, seven people died and over 2000 became ill. The pathogens causing the outbreak were attributed to contamination of the town’s water well arising from cattle manure from a nearby cattle farm following a period of heavy spring rainfall. Failure to adequately chlorinate the water supply resulted in the contaminated water being consumed by the people in the town.
Argentina has one of the highest recorded incidence of hemolytic–uremic crisis in the world at 300–400 cases per year.38 It also has the highest per capita consumption of beef of any country in the world.
Visits to farms for recreational or educational purposes have become an important part of the tourism and leisure industries in some countries.67 The emergence of E. coli O157:H7, with its very low infectious dose and associated risks of serious human illness, has greatly increased the potential for zoonotic disease acquired from livestock, including those on open farms. The livestock of these farms may include sheep, goats, mature cattle and calves, pigs, donkeys, ponies, rabbits, guinea pigs, chipmunks, laying hens, bantams, ducks, geese, and a variety of waterfowl. Outbreaks of E. coli O157:H7 infection have occurred in people visiting these farms and the E. coli O157:H7 has been isolated primarily from the calves and goats.
In a large outbreak of E. coli O157:H7 infections among visitors to a dairy farm, predominantly children, high rates of carriage of E. coli O157:H7 among calves and young cattle most probably resulted in contamination of both the hides of the animals and the environment.68,69 Contact with calves and their environment was associated with an increased risk of infection, whereas hand-washing was protective. Thirteen percent of the cattle were colonized with E. coli O157:H7, which had the same distinct pattern on PFGE found in isolates from the patients. The organism was also recovered from surfaces that were accessible to the public.
Phenotyping and genotyping of E. coli O157:H7 has been used to verify transmission of the organism from dairy cattle to humans with E. coli O157:H7 infections.70 The isolates from cattle and humans were indistinguishable, which indicates that the infection originated from the farms. However, it may not be possible to determine whether the source was unpasteurized milk or direct or indirect contact with cattle.
Transmission of E. coli O157:H7 occurs by three major routes: food items such as undercooked meat or unpasteurized milk, person-to-person spread, and direct or indirect contact with animals. Infections have been associated with visits to cattle farms and farms open to the public, with consumption of farm products, and with camping on a cattle-grazing site. Infections have also been described in farm family members and other farm dwellers.
Undercooked beef products, unpasteurized milk and dairy products, and contaminated water are all potential vehicles for human infection with E. coli O157:H7.
The economic consequences of beef contaminated with E. coli O157:H7 are enormous. Since 1994 in the USA, millions of kilograms of ground beef have been recalled from retail outlets because of contamination with E. coli O157:H7. Such beef products must be destroyed and not used for animal or human food. Human illness associated with the most common food-borne pathogens alone cost the US economy more than $7 billion each year.71 Some of these human outbreaks have been linked to the consumption of meat-based products or to contact with animals and their wastes.
Enterohemorrhagic E. coli are characterized by the presence of Shiga toxin (Stx) genes, locus for enterocyte effacement (LEE) and a high-molecular-weight plasmid that encodes for a hemolysin.47 These three virulence factors are present in most E. coli associated with bloody diarrhea and hemolytic–uremic crisis in humans.
The LEE is a large cluster of genes that are collectively responsible for the intimate attachment of the bacterium to the apical membrane of the enterocyte and subsequent destruction or effacement of the microvilli. The intimate attachment of the bacterial cell to the epithelium is attributed to the adhesin intimin and Tir, a bacterial protein, which is inserted into the host membrane and serves as the response for intimin. Both factors are part of the LEE in enteropathogenic E. coli and enterohemorrhagic E. coli. Intimin appears to be an essential component in initiating attachment, colonization and the subsequent pathological changes that follow infection with enteropathogenic E. coli and enterohemorrhagic E. coli.
E. coli O157:H7 also possesses a high-molecular-weight plasmid that contains several putative virulence genes, including a pore-forming hemolysin. Virulence plasmids are common features of pathogenic E. coli, encoding toxins, adhesins and other factors necessary for colonization, survival, and ability to cause disease in its animal host.
In ruminants, E. coli O157:H7 persists and proliferates in the lower gastrointestinal tract and does not remain for long periods in the ruminant stomachs or duodenum.34 E. coli O157:H7 exhibits a novel tropism for the terminal rectum in cattle. In calves experimentally infected with E. coli O157:H7, in almost all persistently colonized animals, the majority of tissue-associated bacteria identified are in a region within 3–5 cm proximal to the rectoanal junction.72 This region contains a high density of lymphoid follicles, and microcolonies of the bacterium are readily detectable on the epithelium of this region by immunofluorescence microscopy. As a consequence of this specific distribution, E. coli O157:H7 are present predominantly on the surface of the fecal mass. Sampling the feces and terminal rectum of cattle immediately after slaughter found higher numbers of E. coli O157:H7 at the site closer to the rectoanal junction, and low- and high-level carriers were identified.73 High-level carriage was detected in 3.7% of the animals, and carriage on the mucosal surface of the terminal rectum was associated with high-level fecal excretion. This supports the finding than the mucosal epithelium of the terminal rectum of cattle is an important carriage site for E. coli O157:H7, and that high-level fecal shedding of E. coli O157:H7 results from colonization of this site.
Experimentally, E. coli O157:H7 causes fatal ileocolitis in newborn calves under 36 hours of age.27 Affected calves developed diarrhea and enterocolitis with attaching and effacing lesions in both the large and small intestines by 18 hours after inoculation.
Natural and experimental infection of calves from 13–30 days of age and mature cows with E. coli O157:H7 does not result in any clinical signs of disease and no lesions were present at necropsy.62 A serological response occurred in the calves but not in the cows.
Attaching and effacing intestinal lesions can be produced by experimental inoculation of 6-day-old conventionally reared lambs with E. coli O157:H7.74 All animals remain normal clinically but attaching and effacing lesions occur in the cecum at 12 and 36 hours post-inoculation and in the terminal colon and rectum at 84 hours. This indicates that the well-characterized mechanisms for intimate attachment encoded by the locus for enterocyte effacement of E. coli O157:H7 may contribute to the initial events of colonization. Similar lesions can be produced in ligated intestine loops of 6-month-old sheep using E. coli O157:H7.75
A review is available of the details of the culture techniques for the detection of E. coli O157:H7 in the feces of cattle.76
A number of culture methods for the screening of fecal specimens for E. coli O157:H7 are available but no standard protocol is recommended.77 Because none of the techniques provides 100% sensitivity, the isolation rates of E. coli O157:H7 from bovine feces using only one test will result in an underestimation of the incidence of the organism in bovine feces. Performing more than one test must be considered.
Feces may be directly plated on to selective media and/or differential agars or feces may be selectively enriched in a variety of broth enrichment protocols followed by plating on to selective agars. This enrichment step may be followed by immunomagnetic separation with beads coated with O157-specific antibody before plating on to agar.76 Because E. coli O157:H7 often occurs in small numbers in bovine feces, immunomagnetic separation is now in common use. Enrichment broths are comparable to each other but they are superior to direct plating.23 In addition, regardless of the culture protocol used, recovery of E. coli O157:H7 is more likely from fresh fecal samples than from frozen samples.23
PFGE has been used extensively to investigate the epidemiology of E. coli O157:H7, although it has not been evaluated as a tool for establishing genetic relationships.78 PCR has been used to detect virulence genes and molecular epidemiology of E. coli O157:H7 isolates from abattoirs.25
A real-time PCR kit for the detection of E. coli O157:H7 in bovine fecal samples is commercially available.79 Both the sensitivity and specificity of the assay are 99% for isolates in pure culture and the assay detects 1 cfu/g of E. coli O157:H7 in artificially inoculated bovine feces following enrichment.
The culture of swabs of the rectoanal junction mucosa is as sensitive and may be more sensitive than culture of feces for the detection of E. coli O157:H7 in cattle.80 This is because the sample site is the location of E. coli O157:H7 colonization, which contains high numbers of the organism. For both experimentally and naturally infected cattle, the rectoanal mucosa predicted the duration of infection. Cattle transiently shedding E. coli O157:H7 for less than 1 week were positive by fecal culture only and not by rectoanal mucosa culture, whereas colonized animals were positive early on by rectoanal mucosa junction culture.
A rapid, specific and quantitative method to detect E. coli O157:H7 in ground beef in a combined immunomagnetic separation for cell capture and concentration with real-time PCR has been developed.81
In spite of the large amount of information generated about various aspects of E. coli O157:H7 since 1982, reliable management practices to control the infection effectively at all stages of beef production from the farm to the slaughter plant, the retail handling and processing, and finally the consumer, have not been examined scientifically so that recommendations could be made at each stage of production that would result in control of the organism at a very low level.82 Studying E. coli O157:H7 during the entire cattle-production process is problematic because of the complexity of the system and the complexity of the ecology of the organism. The development of economically feasible intervention strategies that are effective in reducing foodborne pathogens is a priority for both the beef and dairy industries.
The effective control of E. coli O157:H7 will require the implementation of several different infectious disease control strategies and management procedures extending from the farm environment to the meat processing plant, the retail handling and processing of meat products and the handling and cooking of beef products in the home.
The features of the ecology of E. coli O157:H7 that are important to consider in a control program include:83
• Lack of a host specificity such that indistinguishable isolates can be found in a variety of species
• Near ubiquitous distribution on cattle farms
• Transient residence in the gastrointestinal tract flora of individual animals that is not associated with disease
• Temporal clustering at the population level such that most fecal shedding is confined to sharp bursts in a high percentage of animals separated by much longer periods of very low prevalence
• A higher prevalence in young animals compared to mature ones
• A higher prevalence in animals with gastrointestinal flora disturbances such as those associated with transit, feed changes, or antimicrobial dosing
• A markedly higher prevalence during warm months
• Molecular subtyping of E. coli O157:H7 indicate that certain subtypes can persist on cattle farms for years, supporting the conclusion that cattle farms represent a reservoir
• New subtypes are periodically found on particular farms, and indistinguishable subtypes can be found on farms separated by hundreds of kilometers even in the absence of any obvious animal movements between them
• Commercial feeds are sometimes contaminated with E. coli O157:H7 and it seems likely that feeds represent an important route of dissemination
• Mixed feeds collected from feeding troughs are commonly positive for E. coli O157:H7, as are water troughs, and feed and water probably represent the most common means of infection
• Environmental replication in feeds and in the sediments of water troughs occurs and may account for the higher level of fecal shedding in the summer months
• Since E. coli O157:H7 has been found to persist in and remain infective for at least 6 months in water trough sediments, this may be an important environmental niche where the organism survives during periods when it cannot be detected, especially during cold months
• Traditional means of controlling infectious diseases, such as eradication or test and removal of carrier animals, do not appear to be feasible
• Certain farm management practices, especially those related to maintenance and multiplication of E. coli O157:H7 in feed and water, may provide practical means to substantially reduce the prevalence of these agents in cattle on farms and in those arriving at slaughter plants
• It is virtually impossible to exclude E. coli O157:H7 from beef-processing plants and carcasses82
• Cross-contamination of whole carcasses with fecal-derived bacteria occurs as a result of airborne transmission (during removal of the hide). Contaminated equipment and cross-contamination is inevitable during boning-out and grinding (where portions of carcasses from a large number of animals are commingled or make contact with a common piece of equipment)82
• The very small numbers of E. coli O157:H7 predicted to contaminate carcasses under highly effective control could be spread to a large volume of beef product during processing and multiply if the product experienced temperature abuse. Because the dose of E. coli O157:H7 to cause human illness is very low, this dispersion of the organism throughout a high volume of product may constitute the greatest risk to public health.
The control of E. coli O157:H7 will depend on implementation of management procedures which extend from the farm (preharvest), slaughtering process (postharvest) and retail handling and processing, to ultimately the consumer.
Preharvest beef safety production programs consist of policies, strategies, and procedures that are carried out on food-producing animal farms with the objective of producing safe and wholesome product free of antibiotic or chemical residues and with a minimum of pathogens that could be transferred through meat to humans. Some examples follow here.
The Canadian on-Farm Food Safety (COFFS) Program is a producer-led, industry/government partnership that provides national commodity groups with the opportunity to develop the strategies and the necessary tools to educate producers and to implement national on-farm food safety initiatives consistent with the Codex Alimentarius Hazard Analysis Critical Control Points (HACCP) definitions and principles and with the Canadian Food Inspection Agency’s Food Safety Enhancement Program.
The COFFS Program established in May 1997 is funded by a grant from Agriculture and Agri-food, Canada’s Canadian Adaptation and Rural Development Fund. Technical advice is provided by the Canadian Food Inspection Agency and the program is administered by the Canadian Federation of Agriculture.
The beef cattle industry in Canada has begun excellent programs with Canadian Cattlemen – Quality Starts Here: Good Production Practices for Cow-Calf Producers and Recommended Operating Procedures for Feedlot Animal Health.
The Canadian Cattlemen’s Association has developed a number of quality assurance schemes for the various segments of its industry. The Quality Starts Here program was developed through collaborative discussions with all those along the food chain – cow–calf operators, feedlots, packers, veterinarians, and pharmaceutical companies. The objective was to develop a set of good production practices to deal with sanitation and feeding issues and to minimize problems arising from lesions and bruising at injection sites and from drug residues. It is important to note that this program was developed to improve the beef supply chain as a whole and to augment the processing industry’s in-plant HACCP programs. In part, it attempts to reduce information costs along the supply chain.
Manuals have been produced and distributed to those interested including: Good Practice Guides for cow–calf operators and feedlots and Recommended Operating Procedures for Feedlot Animal Health. The procedures are based on HACCP concepts. At present, the CCA schemes do not include provisions for independent monitoring of cow–calf operations or feedlots. This may be a weakness of the scheme as it may not be accepted by those further along the supply chain (by retailers or the hotels, restaurants, and institutions) and, in particular, export markets. Without independent accreditation, this quality assurance scheme cannot claim to be HACCP-based. If, over time, producers feel that they receive a premium for animals raised according to the specifications of the quality assurance scheme, pressure may increase to have independent verification. The CCA has also endorsed a national animal identification scheme. Technical details still need to be worked out to insure that maximum use can be made of the information. The need for the information to be transferred from the live animal to the carcass remains a technological challenge. However, there is cooperation to find a solution. It is intended that, in the event of an incident that requires traceback, it will be possible with this system to identify the last herd (feedlot, pasture group, etc.) in which the subject animal was located and, from the ear tag, the herd of origin.
The literature on preharvest strategies to reduce the carriage and shedding of E. coli O157:H7 in cattle has been reviewed.71,82 A stochastic simulation model was used to assess the benefit of measures implemented in the preslaughter period that are aimed at reducing the contamination of beef carcasses with Shiga-like-toxin-producing E. coli O157:H7.82 Control measures were based on either reducing the herd prevalence of infection, reducing the opportunity for cross-contamination in the processing plant by reordering of the slaughter procedures, reducing the concentration of E. coli O157:H7 in fresh feces or reducing the amount of feces, mud and bedding (‘tag’) transferred from the hide to the carcass. Simulations suggested that the greatest potential is associated with vaccination and with an agent that reduces shedding of E. coli O157:H7 in feces. An industry-wide reduction in the amount of tag attached to hides and addition of a source of cattle having a prolonged average fasting time were not predicted to have a large impact on mean amount of carcass contamination with E. coli O157:H7.
Interventions at the water trough level offer significant potential to reduce E. coli O157:H7 contamination and cross-contamination. Suggested potential strategies to reduce E. coli O157:H7 survival in the water supply include chlorination, ozonization, frequent cleaning, and screens that reduce organic solids in water troughs. However, field studies found that chlorination of water troughs did not alter the prevalence of E. coli O157:H7 in the troughs, or in the feces of cattle in those pens.49
The survival of E. coli O157:H7 for extended periods of time (weeks to months) in livestock production environments may enable transfer of the organism back to cattle through contaminated feed or water. This creates a cycle of infection allowing E. coli O157:H7 to be maintained in cattle herds. Effective control of E. coli O157:H7 requires suppression at as many points in the cycle of infection as possible in order to reduce its spread. Minimizing contamination of water troughs and feed bunks together with adequate manure management should contribute to a significant reduction in the spread of E. coli O157:H7 in cattle, crops, and water sources.
The fecal prevalence of E. coli O157:H7 among mature dairy cattle is associated with the choice of bedding material used on a farm.84 The use of sawdust for bedding material for lactating dairy cows, as opposed to sand, was associated with a significantly higher fecal prevalence of E. coli O157:H7.84 The overall average herd prevalence was 3.1% and 1.4%, respectively, for cows on sawdust and on sand.84 The total number of days on which herds were positive for E. coli O157:H7 was higher for sawdust-bedded herds than for sand-bedded herds; 22 versus 14, respectively. These results provide evidence that specific farm management practices can influence the prevalence of E. coli O157:H7 on the farm.
The literature on the effects of dietary manipulation of E. coli populations in cattle has been reviewed.43 Feedlot and high-producing dairy cattle are fed rations with a high percentage of grain. When the starches that escape the ruminal microbial degradation move on to the large intestine, enterohemorrhagic E. coli ferment the sugars and the populations of E. coli increase. Cattle fed grain rations shed larger numbers of E. coli, especially E. coli O157:H7 in barley-fed cattle. When cattle are abruptly switched from a high-grain ration to a forage diet, generic E. coli populations decline by 1000-fold within 5 days. Cattle naturally infected with E. coli O157:H7 shed smaller numbers of the organism when the ration is changed to a forage-based diet compared to cattle fed continuously on a high grain diet.43 However, the magnitude of reduction is highly variable between studies and thus is not currently recommended. Fasting for 48 hours and type of diet prior to fasting has no effect on fecal shedding of E. coli O157:H7 in cattle.42 Thus feed withdrawal prior to slaughter should not increase the risk of E. coli O157:H7 entering the food chain. However, re-feeding 100% forage following a 48-hour fast results in a significant increase in the number of animals shedding E. coli O157:H7. This may occur when feeder cattle are moved from one farm to another through a sale barn and may be one of the reasons for the higher incidence of E. coli O157:H7 shedding by cattle when they first enter the feedlot.
Proposals aimed at dietary modifications must be balanced with the practical applications of commercial livestock feeding operations.
Several strategies have been examined that specifically target and directly kill pathogenic bacteria. These include: the use of antibiotics; antimicrobial proteins produced by bacteria; bacteriophages; compounds that specifically target the physiology of pathogenic bacteria; and vaccination.
Some preliminary studies have found that feeding neomycin to cattle for 48 hours reduced the populations of generic E. coli and E. coli O157:H7 in their feces.71
Only limited information is available on their effectiveness.
There is evidence that virulence factors secreted by the type III system can be used as effective vaccine components for the reduction of colonization of cattle by E. coli O157:H7.63 Vaccination of cattle with proteins secreted by E. coli O157:H7, three times at 3-week intervals, significantly reduced the numbers of bacteria shed in feces, the numbers of animals that shed and the duration of shedding in an experimental model. Vaccination of cattle also significantly reduced the prevalence of E. coli O157:H7 in a clinical trial conducted in a typical feedlot. The pretreatment prevalence of animals shedding E. coli O157:H7 averaged 30%. The average proportion of cattle shedding the organism in vaccine-treated pens was 8.8%, and in nonvaccinated pens 21.3%. Since the type III secreted antigens are relatively conserved among non-O157 enterohemorrhagic E. coli serotypes, the vaccine formulation might be broadly cross-protective.
Using the pig as an experimental model, pregnant dams were vaccinated with E. coli O157:H7 adhesin (intiminO157) at 2 and 4 weeks before farrowing.64 E. coli O157:H7 adhesin (intiminO157)-specific antibody titers in colostrum and serum of dams were increased after parenteral vaccination. Neonatal piglets were allowed to suck vaccinated dams for up to 8 hours before being inoculated with a Shiga-toxin-negative strain of E. coli O157:H7. Piglets that had ingested colostrum containing E. coli O157:H7 adhesin (intiminO157)-specific antibodies from vaccinated dams, but not those nursing sham-vaccinated dams, were protected from E. coli O157:H7 colonization and intestinal lesions. This supports the hypothesis that intiminO157 is a potential antigen for an E. coli O157:H7 antitransmission vaccine.
A vaccination field trial evaluated the efficacy of E. coli O157:H7 vaccine in a sample of feedlots in Alberta and Saskatchewan.85 Pens of cattle were vaccinated once on arrival processing and again at reimplanting. The E. coli O157:H7 vaccine included 50 μg of type III secreted proteins. Fecal samples were collected from 30 fresh fecal droppings within each feedlot pen at arrival, at revaccination and within 2 weeks of slaughter. The mean pen prevalence of E. coli O157:H7 in feces was 5.0%, ranging in pens from 0–90%. There was no significant association between vaccination and pen prevalence of fecal E. coli O157:H7 following initial vaccination at reimplanting or prior to slaughter.
The use of native or introduced microflora to reduce pathogenic bacteria in the intestine is termed a ‘probiotic’ or competitive enhancement strategy. The principle is to promote growth of groups of beneficial bacteria that are competitive with, or antagonistic to, pathogens.
Probiotic bacteria are effective in reducing the duration of ruminal carriage of E. coli O157:H7 in cattle.6 Probiotics are live microbial feed supplements that beneficially affect the host animal by improving its intestinal microbial balance. The principle is that these beneficial organisms will combat the effects of stress and prevent undesirable microorganisms from becoming established in the gastrointestinal tract. Dietary supplementation of cattle with Lactobacillus- and Propionibacterium-based direct-fed microbials reduced the prevalence of E. coli O157:H7 in both fecal and hide samples.86
Chlorate supplementation has been investigated as a preharvest strategy to reduce populations of E. coli O157:H7 and Salmonella spp. in food animals.87 Certain bacteria can respire anaerobically by reducing nitrate to nitrite via the intracellular enzyme nitrate reductase. This same enzyme also reduces chlorate to chlorite, a cytotoxic end-product. Chlorate significantly reduced E. coli O157:H7 populations in ruminal fluid incubations, wild-type E. coli, inoculated E. coli O157:H7 and total coliforms in cattle, and inoculated E. coli O157:H7 in sheep. The administration of sodium chlorate in the feed of cattle preharvest for 24 hours reduced the population of E. coli O157:H7 strains approximately by two logs (104 to 102) in the rumen and three logs (106 to 103) in the feces.87
As a result of public concern about E. coli O157:H7, the meat inspection service in many countries has been reorganized to deal with control of the organism in the processing of beef. In the USA, the presence of E. coli O157:H7 in ground beef was declared an adulterant. Surveillance systems have also been established in many countries to obtain more information about the presence of the organism and to report outbreaks, and considerable research has emerged.
Elaborate E. coli O157:H7 detection systems are now in place in abattoirs in many countries as part of the Hazard Analysis of Critical Points System (HACCP) to insure that contamination of beef carcasses with E. coli O157:H7 is below certain legislated levels.
The surfaces of beef cattle carcasses are contaminated with enteric bacteria immediately after removal of the hide during processing following slaughter. Significant correlations between bovine fecal and hide prevalence with beef carcass contamination indicate a role for controlling E. coli O157:H7 in live cattle.19
Major progress has been made in the last decade in the processing of beef carcasses following slaughter to reduce the microbial contamination of beef using the HACCP.
HACCP is a process control system designed to identify and prevent microbial and other hazards in food production.
It includes steps designed to prevent problems before they occur and to correct deviations as soon as they are detected. Such preventive control systems with documentation and verification are widely recognized by scientific authorities and international organizations as the most effective approach available for producing safe food.
In the USA, as of 1996, the US Department of Agriculture (USDA) adopted the Pathogen Reduction HACCP system, which includes four major elements:
• Every plant must adopt and carry out its own HACCP plan, which systematically addresses all significant hazards associated with its products
• Mandatory E. coli testing in slaughter plants. Every plant must regularly test carcasses for E. coli to verify the effectiveness of the plant’s procedures for preventing and reducing fecal contamination
• Pathogen reduction performance standards for Salmonella. All plants and plants producing raw ground products must insure that their Salmonella contamination is below the current national baseline prevalence
• Sanitation standard operating procedures. Every plant must adopt and carry out a written plan for meeting its sanitation responsibilities. Effective sanitation in slaughter and processing plants is essential to prevent adulteration of meat and poultry products.
HACCP is endorsed by such scientific and food safety authorities as the National Academy of Sciences and the National Advisory Committee on Microbiological Criteria for Foods (NACMCF), and by such international organizations as the Codex Alimentarius Commission and the International Commission on Microbiological Specifications for Foods.
HACCP systems must be based on the seven principles articulated by the NACMCF. The seven principles are: (1) hazard analysis, (2) critical control point identification, (3) establishment of critical limits, (4) monitoring procedures, (5) corrective actions, (6) record keeping, and (7) verification procedures.
• Principle 1: Conduct a hazard analysis. Plants determine the food safety hazards and identify the preventive measures the plant can apply to control these hazards
• Principle 2: Identify critical control points. A critical control point (CCP) is a point, step or procedure in a food process at which control can be applied and, as a result, a food safety hazard can be prevented, eliminated, or reduced to an acceptable level. A food safety hazard is any biological, chemical, or physical property that may cause a food to be unsafe for human consumption
• Principle 3: Establish critical limits for each critical control point. A critical limit is the maximum or minimum value to which a physical, biological, or chemical hazard must be controlled at a critical control point to prevent, eliminate, or reduce to an acceptable level.
• Principle 4: Establish critical control point monitoring requirements. Monitoring activities are necessary to insure that the process is under control at each critical control point. The Food Safety and Inspection Service (FSIS) requires that each monitoring procedure and its frequency be listed in the HACCP plan
• Principle 5: Establish corrective actions. These are actions to be taken when monitoring indicates a deviation from an established critical limit. The final rule requires a plant’s HACCP plan to identify the corrective actions to be taken if a critical limit is not met. Corrective actions are intended to insure that no product injurious to health or otherwise adulterated as a result of the deviation enters commerce
• Principle 6: Establish record keeping procedures. The HACCP regulation requires that each plant maintains certain documents, including its hazard analysis and written HACCP plan, and records documenting the monitoring of critical control points, critical limits, verification activities, and the handling of processing deviations
• Principle 7: Establish procedures for verifying the HACCP system is working as intended. Validation insures that the plans do what they were designed to do, i.e. that they are successful in insuring the production of safe product. Plants will be required to validate their own HACCP plans.
HACCP is a system that identifies potential food safety risks, prevents or corrects them, records what was done and verifies that the system works. The objective is to improve food safety for meat and poultry. It is assumed that a reduction in carcass contamination leads to a proportionate reduction in illness and death. Pathogens can contaminate meat and poultry at any step from production through consumption including final food preparation and handling.
The literature on the current methods and technologies used to decontaminate food carcasses in the USA has been reviewed.88 Meat carcasses may become contaminated from fecal material, the stomach contents and the hide. Additional sources of cross-contamination exist in the slaughter process, such as processing tools and equipment, structural components of the facility, human contact, and carcass-to-carcass contact.
Decontamination techniques for carcasses are targeted at reducing or eliminating bacteria that may be human pathogens as well as those that may cause meat spoilage. The pathogenic bacteria of most concern include E. coli O157:H7, Salmonella spp., Listeria monocytogenes, Campylobacter spp., Clostridium botulinum, Clostridium perfringens, Staphylococcus aureus, Aeromonas hydrophilia, and Bacillus cereus.
Meat processors strive to produce raw products that have low levels of bacteria on the surface and no pathogenic bacteria. However, the process is not done in a sterile environment and contamination is unavoidable, and occasionally pathogenic microorganisms may come into contact with the surface of the meat carcass. Routine slaughter practices have evolved over the years to reduce the likelihood of inadvertent microbial contamination. This evolution has led to the adoption of the hurdle technology approach to microbial carcass interventions.
The principles of hurdle technology state that, if the initial microbial load is substantially reduced as a result of carcass decontamination procedures, fewer microorganisms are present, which are then more easily inhibited in subsequent processing steps. The effectiveness of hurdle technology has been demonstrated experimentally for beef decontamination technologies under controlled conditions. The concept of hurdle technology for beef carcass decontamination has also been validated to be effective in field studies in beef-processing facilities.
The following are some of the more widely used and researched intervention strategies:
• Hot water rinse. There is substantial scientific evidence that hot water (> 74°C) will produce a sanitizing effect on beef carcasses, and this is widely practiced in the industry
• Steam pasteurization. The commercialization of the steam pasteurization system has been successful and it is in use in many large beef slaughter facilities in North America. Hot water/steam vacuum systems are designed to remove visible spots of contamination from small areas on the carcass and are used to augment the traditional knife trimming. Steam pasteurization is a process whereby beef carcasses are placed in a slightly pressurized, closed chamber at room temperature and sprayed with steam that blankets and condenses over the entire carcass. This raises the surface temperature to 195°F or 200°F and kills nearly all pathogens. Carcasses then are sprayed with cold water
• Steam vacuum. Steam or hot water is sprayed on a beef carcass followed by vacuuming, which has the combined effect of removing and/or inactivating surface contamination. The hand-held device includes a vacuum wand with a hot water spray nozzle, which delivers water at approximately 82–88°C to the carcass surface, as well as the vacuum unit. Steam vacuuming is approved for use by the USDA-FSIS as a substitute for knife trimming for removing fecal and ingesta contamination when such contamination is less than 2.54 cm at its greatest dimension
• Chemical rinses. Organic acids are typically applied as a rinse to the entire surface of the carcass. The USDA-FSIS approved the use of organic acid solutions such as acetic, lactic, and citric acids at concentrations of 1.5–2.5%. Acetic and lactic acids have been most widely accepted as carcass decontamination rinses. The effectiveness of organic acids is best achieved shortly after hide removal, when the carcass is still warm.
The multiple decontamination processes, as applied in actual plant settings, have resulted in significant improvements in the microbiological quality of beef.88 There is considerable evidence to support the effectiveness of in-plant application of multiple decontamination technologies (hurdle technology). Reductions were achieved from 43% of lots sampled pre-evisceration as positive for E. coli O157:H7 to 1.9% remaining positive postprocessing after multiple decontamination methods on the slaughter floor.19
In February 2005, the beef industry welcomed news from the USDA-FSIS showing a significant drop in E. coli O157:H7 prevalence in 2004, as compared to 2003. The FSIS data showed that the percentage of E. coli-O157:H7-positive ground beef samples collected in 2004 fell by 43.3% compared with the previous year. The data showed that, between 2000 and 2004, the percentage of positive samples of E. coli O157:H7 had declined by more than 80%. FSIS also reported that there were six recalls related to E. coli O157:H7 in 2004 compared to 12 in 2003 and 21 in 2002.
This is very good news for consumers and all sectors of the beef industry and represents the coordinated efforts to reduce this pathogen throughout the beef production chain, from farm to kitchen. The Beef Industry Food Safety Council (BIFSCo), which is funded by beef producers with checkoff dollars, directs a broad effort to solve the E. coli O157:H7 problem, focusing on research, consumer education, and public policy. BIFSCo has been working toward compiling best practices from across the beef industry, which includes sharing strategies among competitors. The program is coordinated on behalf of the Cattlemen’s Beef Board and state beef councils by the National Cattlemen’s Beef Association (NCBA). The NCBA serves as one of the Beef Board’s contractors for checkoff-funded programs.
Following the E. coli summit in January 2003, BIFSCo worked to compile best practices from across the beef industry. Through this effort, all sectors of the beef industry have worked collectively toward the goal of improving safety – from cow–calf producers and feedlot operators to packers and processors, to retailers and foodservice providers. The best practices were completed and distributed throughout the industry.
In addition, checkoff-funded research continues to show promise for interventions such as thermal pasteurization, feed additives, and animal and carcass washes that eliminate or reduce pathogen presence.
Irradiation of beef in the postharvest stage is a process which could be used to inactivate pathogens. At the present time, the percentage of beef being irradiated is very small. Constraints include reluctant consumer acceptance of radiation-treated food, increased price of production, and the irradiation’s negative effect on odor and flavor.
To prevent infection with E. coli O157:H7, consumers must be encouraged to follow four simple steps: Chill promptly; Clean hand and kitchen surfaces; Separate, don’t cross-contaminate; and Cook thoroughly.
Farm animals and the farm environment present a variety of possible sources of infection with E. coli O157:H7. Farm visits are popular among city families for holidays and family gatherings, and schools in urban areas frequently promote educational farm visits for their students. The consumption of unpasteurized milk by visiting children and close physical contact with animals have been documented as most likely sources of infection in some outbreaks of E. coli O157:H7 infection. Farm animals and the farm environment present a variety of possible sources of infection. Visitors to animal farms, especially groups such as schoolchildren, must avoid petting animals whose hair coats and skin may harbor E. coli O157:H7.69 Verotoxin-producing E. coli of bovine origin can infect humans in the farm environment. Many dairy-farm residents regularly consume unpasteurized milk, a potential source of E. coli O157:H7.
Using geographical information system mapping and modeling, studies in Canada have clearly identified a relationship between an increased risk of verotoxin-producing E. coli infections and disease among rural populations and cattle density.69
Johnson RP, Wilson JB, Michel P, et al. Human infection with verocytotoxigenic Escherichia coli associated with exposure to farms and rural environments. In: Stewart CS, Flint HJ, editors. Escherichia coli O157 in farm animals. Wallingford, Oxfordshire: CAB International; 1999:147-168.
Stewart CS, Flint HJ. Escherichia coli O157 in farm animals. Wallingford, Oxfordshire: CAB International, 1999.
Russell JB, Diez-Gonzalez F, Jarvis GN. Invited review: effects of diet shifts on Escherichia coli. J Dairy Sci. 2000;83:863-873.
Hancock D, Besser T, LeJeune J, et al. The control of VTEC in the animal reservoir. Int J Food Microbiol. 2001;66:71-78.
Meyer-Broseta S, Bastian SN, Arne PD, et al. Review of epidemiological surveys on the prevalence of contamination of healthy cattle with Escherichia coli serogroup O157:H7. Int J Hyg Environ Health. 2001;203:347-361.
Bach SJ, McAllister TA, Veira DM, et al. Transmission and control of Escherichia coli O157H:7: a review. Can J Anim Sci. 2002;82:475-490.
Huffman RD. Current and future technologies for the decontamination of carcasses and fresh meat. Meat Sci. 2002;62:285-294.
Renter DG, Sargeant JM. Enterohemorrhagic Escherichia coli O157H:7: epidemiology and ecology in bovine production environments. Anim Health Res Rev. 2002;3:83-94.
Sanchez S, Lee MD, Harmon BG, et al. Animal issues associated with Escherichia coli O157H:7. J Am Vet Med Assoc. 2002;221:1122-1126.
Bettelheim KA. Non-O157 verotoxin-producing Escherichia coli: a problem, paradox, and paradigm. Exp Biol Med. 2003;228:333-344.
Callaway TR, Elder RO, Keen JE, et al. Forage feeding to reduce preharvest Escherichia coli populations in cattle: a review. J Dairy Sci. 2003;86:852-860.
Moxley RA. Detection and diagnosis of Escherichia coli O157:H7 in food-producing animals. In: Torrence ME, Isaacson RE, editors. Microbial food safety in animal agriculture: current topics. Ames, IA: Iowa University Press; 2003:143-154.
Callaway TR, Anderson RC, Edrington TS, et al. Recent pre-harvest supplementation strategies to reduce carriage and shedding of zoonotic enteric bacterial pathogens in food animals. Anim Health Res Rev. 2004;5:35-47.
Moxley RA. Escherichia coli O157H:7: an update on intestinal colonization and virulence mechanisms. Anim Health Res Rev. 2004;5:15-33.
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Escherichia coli infections in weaned pigs
Diarrhea is most frequent when pigs are exposed to pathogenic E. coli strains.1 The effect of weaning is to produce a marked decrease in the diversity of coliforms in the individual piglet.2 Different strains of E. coli were predominant in different animals,2 which may in turn facilitate the spread of pathogenic strains.
Many strains are nonpathogenic. Pathogenic E. coli can be divided into a variety of pathotypes but there are three major types. These are enterotoxigenic E. coli, verotoxin-producing E. coli and attaching and effacing E. coli. There are two major complicating factors in pigs: one of these is that the intestine has different distributions of receptors with changing age.3 The other is that nearly all isolates (94.8%) that carry the enterotoxin genes also carry genes for one of the fimbrial adhesins.4 The two most prominent genotypes are K88, LT1, STb and F18, STa, STb, SLT.
These have two major virulence factors: (a) adhesins or fimbriae and (b) enterotoxins. The adhesins promote or control the adherence to small intestinal epithelial cells and include K88, K99, F41, 987P, and F18. Only F18 and K88 are frequently associated with disease in weaned pigs. Only young pigs are susceptible to K99 or 987P. Age-related susceptibility is related to the presence or absence of the appropriate receptors in the small intestine. The enterotoxins belong to two groups; heat-labile enterotoxin (LT) and the heat-stable enterotoxins STa or STb. Only weaned pigs are susceptible to F18 and resistance develops by 8 weeks of age, as the binding appears to be blocked. The receptor for F18 has not yet been identified but it is a glycoconjugate in which the attached carbohydrate acts as a target for the fimbriae. The F18 adhesin occurs in two forms: ab (found in verotoxin-producing E. coli) and ac (found in enterotoxigenic E. coli).5 There is also an F4 fimbrial antigen which is on a different chromosome from F18. F4 enterotoxigenic E. coli cause problems in the first week after weaning but F18 verotoxin-producing E. coli cause problems 1–2 weeks after weaning.6
These strains produce Shiga toxin or Shiga-like toxins (verocytotoxins) and cause edema disease. Swine verotoxin-producing E. coli colonize the intestine via the F18 pilus, as do some swine enterotoxigenic E. coli. It is not uncommon to find F18-positive strains that produce enterotoxins and verotoxins and can cause both diarrhea and edema disease.
These bacteria possess the eae gene, which encodes for intimin, which is an adhesin factor that facilitates the attachment of bacteria to intestinal epithelial cells.7 It is on a plasmid that is distinct from the K88-encoding plasmid8 in enteropathogenic E. coli and enterotoxigenic E. coli strains that produce attaching and effacing lesions.
The following strains are found:
• F18ab (more associated with edema disease); F18ac
• F41, usually associated with K99 fimbriae
• Strains containing LT, STa, STb, Shiga-like toxin 2e(Stx2e), and possibly enteroaggregative E. coli.9
Twenty years ago most of the pig strains were 987p- or K99-positive. The genes for Stx2e and F18 were rare then but are common now.
In a recent survey in Sweden only two O157:H7-positive and four O157:H7-negative strains were found.10 The O157:H7-positive strains have been described.11 Pathogenicity is indicated by genes encoding for one or more of the Shiga toxins but several other factors may also be necessary. Most strains do not possess Shiga toxins but do carry the F4 or F18 fimbrial adhesins. A third of strains produced STa or STb but less than a third produce STx and half the eae gene.
Postweaning diarrhea and edema disease are two common E. coli infections of weaned pigs. In postweaning diarrhea, there is diarrhea, dehydration, and often death. In edema disease or enterotoxemia there is subcutaneous edema of the forehead and eyelids, and neurological clinical signs such as ataxia, convulsions, recumbency, and death. Enterotoxigenic E. coli strains isolated from cases of postweaning diarrhea mainly belong to O groups O8, O141, O147, O149, and O157. Strains associated with edema disease predominantly have O groups O138, O139, or O141. Postweaning diarrhea is a significant cause of mortality between weaning and marketing in some herds. Although the clinical signs in these two diseases are different, they occur in similar age groups and the same type of management change may precede their occurrence. Weaning and weaning age are both associated with significant effects on the microbial populations.12 In postweaning diarrhea the bacteria disappear more quickly, usually by about 7 days postinfection, but in edema disease may still be there 9 days postinfection but with a slower buildup to a peak within 3–5 days postinfection.13 A typical scenario would be severe diarrhea occurring 4–5 days postweaning followed by clinical edema disease with mortality reaching as high as 50%. Both are associated with the proliferation of predominantly hemolytic serotypes of E. coli within the small intestine. However, it is rare to encounter both diseases concurrently on the same farm. In postweaning diarrhea the serotypes are enterotoxigenic and the major manifestation is diarrhea resulting from enterotoxin activity at the time of proliferation. In edema disease non-enterotoxigenic strains produce a verotoxin that, after a period of time, indirectly produces the neurological syndrome characteristic of this disease.
One of the features of virulence in E. coli is the presence of mobile genetic elements such as plasmids, bacteriophages, and pathogenicity islands. A pathogenicity island coding for F18-positive fimbriae has been found.14 Cytolethal distending toxins have also been described.15 In many countries the prevalence of edema disease has decreased markedly during the past decade, whereas that of coliform gastroenteritis has increased. It is possible that this change reflects the trend towards earlier weaning of pigs, although the emergence and spread of new enterotoxigenic strains may also be a factor. More recently a third disease, cerebrospinal angiopathy, has been attributed to the effects of infection with E. coli. Although there are some similarities in the etiology and epidemiology of these diseases they are sufficiently different to warrant a separate description. One of the major features in common is the process of weaning, probably the most serious disturbance a young piglet may face. It alters immune functions and produces stress.16 It also profoundly alters the intestinal microflora17,18 particularly the coliform flora.19 Some strains may increase but others may decrease.
Edema disease occurs in weaner and grower pigs and is characterized by subcutaneous and subserosal edema, progressive ataxia, recumbency, and death.
Etiology Escherichia coli strains producing verocytotoxin and Shiga-like toxin
Epidemiology In rapidly growing weaner pigs between 4 and 12 weeks of age following change in diet or feeding practices. Outbreaks occur
Sign sudden death. Incoordination, falling, edema of eyelids and face; piglets die in 6–36 hours
Clinical pathology Culture E. coli from feces
Lesions Facial edema, full stomach and mesenteric edema
Edema disease is associated with E. coli strains producing a verocytotoxin type II variant (VT2e), and Shiga-like toxin (SLT2e), and belonging to one of the three serogroups that cause edema disease: O138, O139, or O141.20 The biochemical phenotypes were studied in Sweden and each of the O138, O139, and O141 serotypes is dominated by one phenotypic type even though others do occur within the serotype.21 The entire pathogenicity island known as ETT2 is necessary for the edema disease virulence factors in O138, O139, or O141.22 Isolates of E. coli have been found in which the toxin or F18 fimbrial types were not related to selected electrophoretic types which suggests that toxin and F18 genes in the isolates from pigs with postweaning diarrhea or edema disease occur in a variety of chromosomal backgrounds.23 The bacteria colonize the small intestine without causing significant changes by means of the adherence factor F18 (F107).24 The E. coli strains with the highest mucin-binding capacity belonged to potential ST toxin producers, whereas strains without genes encoding for toxin production displayed much weaker binding to mucin capacity.25 In a recent outbreak in Denmark, where edema disease had not previously been observed, most isolates were of serotype O139 but a few isolates could not be typed by O serotyping.26 All the isolates from the Danish pigs with edema disease grouped together in one cluster, in contrast to isolates from other countries, which did not form any clusters. In Denmark 563 isolates were serotyped27 and O149 was found in 49.9% of the isolates, O138 in 14.9%, O139 in 6.9%, O141 in 4.1%, and O8 in 3.7%. The virulence genes were examined and they fell into six pathotypes, which contained 65.7% of all isolates.27 The F107 fimbriae are a major colonization factor in E. coli that cause edema disease.28,29 The serotypes of E. coli isolated from piglets with diarrhea, piglets with edema disease, and healthy piglets have been reported.30,31
The inheritance of susceptibility to edema disease has been examined.32,33 Inheritance of resistance to intestinal colonization with E. coli causing edema disease is thought to be under the control of one locus consisting of two alleles with susceptibility (S)-dominating resistance(s).32 Genetic susceptibility to edema disease is due to the ability of F107-expressing E. coli to adhere to and colonize intestinal brush border cells, and not due to toxin susceptibility. There is a high correlation between intestinal F18 receptor genotype and susceptibility to disease34 but pigs with resistant F18 receptor genotypes were not entirely protected against colonization by E. coli.
The specific serotypes of E. coli that are capable of causing the disease are introduced into a piggery and become part of the normal intestinal flora. They may not cause disease until a particular set of environmental conditions arise, when they proliferate excessively within the intestine to produce toxin. The disease occurs predominantly in pigs between 4 and 12 weeks of age. It may occur sporadically but more commonly occurs as an outbreak affecting up to 50% of the pigs within the group. Characteristically the larger and faster-growing pigs within the group are affected. The disease is not common in runt or poorly thriving pigs. Age at weaning, diet, overcrowding, chilling, transportation and other factors influence the susceptibility of pigs to E. coli producing SLTIIe and could determine whether subclinical or clinical edema disease occurs following infection. Piglets fed high-protein diets are more susceptible to experimental clinical edema disease than piglets fed low-protein diets.35 The disease frequently occurs within 1 week following a change in diet or ad libitum feeding but may also follow such factors as weaning, vaccination, pen change, and regrouping. A study even found verotoxin-producing E. coli O139 in water storage tanks and drinking water.36
One of the observations on F18 fimbriae is that they have increased greatly since 1997 from 10% to 70% and this may be tied into the genetic selection of the stress gene.37
The outbreak is sudden in onset but short-lived, averaging 8 days and seldom exceeding 15 days. The epidemiology of the disease in affected herds is not characteristic of a highly contagious disease and it does not usually spread to involve other pens of pigs on the same farm.
The disease follows proliferation of the relevant serotypes within the intestine. Serotypes of E. coli associated with gut edema may be isolated from the feces of healthy pigs. The factors initiating proliferation are unknown but changes in the composition or amount of diet commonly precipitate the onset. Management factors that potentiate oral–fecal cycling of these organisms are likely to be of importance in spread within the group.
It is a simple progression. The intestine of a susceptible pig, which is usually fast-growing and without maternal antibody, has receptors for F18 pili. This appears to be the major factor, and then colonization by E. coli occurs with toxin production, absorption of toxin, and damage to vascular epithelium. The endothelium appears to have a specific toxin receptor for Stx2e38 and finally edema develops in target tissues.
Nutritional factors and gastrointestinal stasis result in proliferation of the E. coli strains in the small intestine and toxin production. There is generally a delay between the initial period of maximal intestinal proliferation of the organism and the onset of clinical signs. In the experimental disease, clinical signs occur 5–7 days following initial oral challenge with bacteria and up to 36 hours following intravenous inoculation with toxin. The delay appears to be related to the development of vascular lesions, with increased vascular permeability leading to edema formation and encephalomalacia. The experimental oral inoculation of the edema-disease-producing E. coli results in colonization of the organisms in the small intestine, and lesions of the vessels of the intestinal mucosa are detectable as early as 2 days after infection. An experimental model for subclinical edema disease in weaned pigs has been described.39 Microscopic vascular lesions were found in pigs 14 days after oral inoculation with a SLT2-positive strain of E. coli. Once postweaning diarrhea occurs then there is an increased intestinal permeability that predisposes to edema disease and once edema disease has developed the influx of SLT toxin into the bloodstream is facilitated further, thus precipitating the disease.40 Edema disease is associated with metabolic acidosis, which might be explained by endogenous acid production, and small-intestinal acidosis. Intestinal acidosis is known to cause mucosal hyperexcitability.
The disease occurs sporadically and unexpectedly in a group, often affecting a number of pigs within a few hours, and shows no tendency to spread from group to group. The thriftiest pigs are most likely to be affected and, once the diagnosis is made, all pigs in the pen should be examined in an attempt to detect other animals in the early stages of the disease. The incidence in a litter will vary up to 50% or more.
The earliest and most obvious sign is incoordination of the hindlimbs, although this may be preceded by an attack of diarrhea. The pig has difficulty in standing and sways and sags in the hindquarters. There is difficulty in getting up and in getting the legs past each other when walking because of a stiff, stringhalt-like action affecting either the forelegs or hindlegs. In some cases there are obvious signs of nervous irritation manifested by muscle tremor, aimless wandering, and clonic convulsions. Complete flaccid paralysis follows.
On close examination, edema of the eyelids and conjunctiva may be visible. This may also involve the front of the face and ears but cannot usually be seen until necropsy. The voice is often hoarse and may become almost inaudible. Blindness may be apparent. The feces are usually firm and rectal temperatures are almost always below normal. The course of the disease may be very short, with some pigs being found dead without signs having been observed. In most cases, illness is observed for 6–36 hours, with a few cases being more prolonged. Recovery does sometimes occur but some degree of incoordination may persist.
As an aid to diagnosis, while affected animals are still alive, fecal samples should be cultured to determine the presence of hemolytic E. coli. Knowledge of the drug sensitivity of the organism may be important in prescribing control measures. The edema disease principle is cytotoxic to Vero cells and may be useful in an assay system for diagnosis. The toxin Stx2e has been detected in the peripheral blood of pigs with clinical disease, which not only shows that toxin is transported but may eventually lead to a technique for the detection of early cases.41
The pig is well grown for its age, the stomach is full of feed and the feces are usually normal. Edema of the eyelids, forehead, belly, elbow and hock joints, throat, and ears is accompanied by edema of the stomach wall and mesocolon in classical cases. Excess pleural, peritoneal, and pericardial fluid is also characteristic and the skeletal muscles are pale. The edema may often be slight and quite localized, so examination of suspected areas should be carried out carefully, using multiple incisions, especially along the greater curvature of the stomach near the cardia. Hemolytic E. coli can be recovered in almost pure culture from the intestine, particularly the colon and the rectum, and in some cases from the mesenteric lymph nodes. Polyclonal antisera directed against serotypes of E. coli associated with edema disease are used to confirm the diagnosis via an agglutination test. Histologically, the important lesions are mural edema, hyaline degeneration and fibrinoid necrosis in arteries and arterioles. In subacute to chronic cases this angiopathy may result in focal brain hemorrhages and encephalomalacia.
• Bacteriology – ileum and colon (CULT). The differentiation of pathogenic and nonpathogenic E. coli can be achieved through PCR.42,43 A multiplex PCR has been developed for STa, STb, K99, 987P, and F41.44 The tests tell you that the gene is present but not whether it is actually encoding for the proteins
• Histology – formalin-fixed colon, ileum, jejunum, gastric fundus, brain, mesenteric lymph node (LM).
Although there are a number of diseases of pigs in the susceptible age group in which nervous signs predominate, gut edema is usually easy to diagnose because of the rapidity with which the disease strikes, the number of pigs affected at one time, the short duration of the outbreak, and the obvious edema of tissues. Affected pigs are usually in prime condition. The ataxia and recumbency must be differentiated from diseases of the nervous system of pigs which cause ataxia and recumbency. These include pseudorabies, viral encephalomyelitis (Teschen’s disease), encephalomyocarditis, streptococcal meningitis, salt poisoning, and organic arsenic poisoning. Mulberry heart disease and encephalomyocarditis can produce similar signs, and differentiation on necropsy findings and histopathology is nesessary. In poisoning by Amaranthus spp. and Chenopodium album the signs may be roughly similar but the edema is limited to the perirenal tissues.
Treatment is ineffective. Elimination of the toxin-producing bacteria may be attempted by use of antimicrobials in the feed or water supplies. The choice of antimicrobial will vary depending on area variations of the drug sensitivities of E. coli. The feed consumption of the unaffected pigs in the group should be reduced immediately and then gradually returned to previous levels over a period of a few days.
Pigs should be kept on the same creep feed for at least 2 weeks after weaning, and the change in feed should be made gradually over a 3–5-day period. Feed restriction through the critical period is frequently practiced and may reduce the occurrence of gut edema. Similarly an increase in crude fiber and decrease in nutrient quality of the diet through this period may reduce the incidence. However, it is evident that a severe restriction and marked decrease in nutrient quality is required to fully achieve this effect and this is not compatible with the purpose of growing pigs. It is essential that pigs on restricted intakes be provided with adequate trough space to allow an even intake of food among the group. For similar reasons, litters of pigs that are batched at or after weaning should be divided into groups of approximately equal body weight.
The strategic incorporation of an antimicrobial into the feed during the risk period may be necessary on some farms. A reduction in the potential for oral–fecal cycling of organisms in the group may reduce the incidence of gut edema. A reduction in the age of weaning may also reduce the incidence. Both organic acids and medication with 50 ppm of enrofloxacin are useful in controlling and/or preventing postweaning edema disease.45
Treatment with anti-VT2E serum can provide protective immunity against edema disease in pigs.46
No vaccine is available. Only vaccines with the preformed fimbriae induce protection and this is limited to the homologous variant47 but, experimentally, vaccination of piglets with a genetically modified Shiga-like toxin 2e prevents edema disease following challenge with the Shiga-like toxin after weaning.35 The concentration of protein in the diet also influenced susceptibility to edema disease. Pigs fed a low-protein diet and not vaccinated developed subclinical edema disease. Pigs fed a high-protein diet and not vaccinated developed clinical edema disease. Pigs fed a high-protein diet and vaccinated had a reduction in the incidence of subclinical edema and did not develop clinical edema disease.
Mainil J. Shiga/verocytotoxins and Shiga verocytotoxic E. coli in animals. Vet Res. 1999;30:235-257.
Moxley RA. Edema disease. Vet Clin North Am Food Anim Pract. 2000;16:175-185.
Moxley RA. Prevalence of serogroups and virulence groups in E. coli associated with postweaning diarrhea and edema disease in pigs and a comparison of diagnostic approaches. Vet Microbiol. 2002;85:169-185.
1 Melin L. J Vet Med B. 2004;51:12.
2 Melin L. J Vet Med B. 2000;47:663.
3 Fang L, et al. Infect Immun. 2000;68:564.
4 Post K, et al. J Swine Health Prod. 2000;8:119.
5 Fekete PZ, et al. Vet Microbiol. 2002;85:275.
6 Van der Broek W, et al. Infect Immun. 1999;67:520.
7 Oswald E, et al. Infect Immun. 2000;68:64.
8 Yamamoto T, Nakazawa M. J Clin Microbiol. 1997;35:223.
9 Choi C, et al. Vet Microbiol. 2001;81:65.
10 Eriksson E, et al. Vet Rec. 2002;152:712.
11 Osek J. Vet Rec. 2002;150:689.
12 Franklin MA, et al. J Anim Sci. 2002;80:2904.
13 Verdonck F, et al. Vaccine. 2002;20:2995.
14 Fekete PZ, et al. Int J Med Microbiol. 2003;293:287.
15 Toth I, et al. J Clin Microbiol. 2003;41:4285.
16 Waitrang E, et al. J Vet Med B. 1998;45:7.
17 Katouli M, et al. J Appl Microbiol. 1997;82:511.
18 Katouli M, et al. J Appl Microbiol. 1999;87:564.
19 Melin L, et al. Vet Microbiol. 1997;54:287.
20 Aarestrup FM, et al. Vet Rec. 1996;139:373.
21 Matsson S, Wallgren P. In: Proceedings of the 16th International Pig Veterinary Society Congress 2000:51.
22 Prager R, et al. Vet Microbiol. 2004;99:287.
23 Nagy B, et al. J Clin Microbiol. 1999;37:1642.
24 Rippinger P, et al. Vet Microbiol. 1995;45:281.
25 Styriak I, et al. Dtsch Tierärztl Wochenschr. 2001;108:454.
26 Moller F, et al. J Clin Microbiol. 1997;35:20.
27 Frydendahl K. Vet Microbiol. 2002;85:169.
28 Imberechts H, et al. Vet Microbiol. 1994;40:219.
29 Rippinger P, et al. Vet Microbiol. 1995;45:281.
30 Garabal JI, et al. Vet Microbiol. 1995;47:17.
31 Garabal JI, et al. Vet Microbiol. 1996;48:113.
32 Bertschinger HU, et al. Vet Microbiol. 1993;35:79.
33 Bosworth BT, et al. J Swine Health Prod. 1994;2:19.
34 Frydendahl F, et al. Vet Microbiol. 2003;93:39.
35 Bosworth BT, et al. Infect Immun. 1996;64:55.
36 Hada M, et al. Jpn J Vet Med Assoc. 1998;51:659.
37 Holtcamp A. Proc Am Assoc Swine Vet. 2000:337.
38 Ha S-K, et al. J Vet Diagn Invest. 2003;15:378.
39 Kausche FM, et al. Am J Vet Res. 1992;53:281.
40 Narbuurs JA, et al. Res Vet Sci. 2001;70:247.
41 Cornick NA, et al. J Infect Dis. 2000;181:242.
42 Johnson WM, et al. J Clin Microbiol. 1990;28:2351.
43 Pollard DR, et al. J Clin Microbiol. 1990;28:540.
44 Rouillard T, et al. Rev Med Vet. 2002;153:261.
45 Tsiloyiannis VK, et al. Res Vet Sci. 2001;70:281.
Postweaning diarrhea occurs commonly within several days after weaning and is characterized by reduced growth rate associated with alterations in the mucosa of the small intestine and, in some pigs, by acute coliform gastroenteritis characterized by sudden death, or severe diarrhea, dehydration, and toxemia. It is a major cause of economic loss from both mortality and inferior growth rate for several days to 2 weeks following weaning. The etiology, epidemiology, and pathogenesis are multifactorial and complex because of the several weaning-associated factors that may interact. In some instances the postweaning diarrhea may be followed by edema disease.
Etiology Specific serotype of enterotoxigenic E. coli
Epidemiology 3–10 days post weaning; high morbidity and case-fatality rates. Stressors of weaning are risk factors (change of feed, loss of maternal contact and maternal antibody, mixing litters, environmental changes)
Sign Some pigs found dead. Outbreaks of severe diarrhea a few days post weaning. Fever, dehydration, anorexia, loss of weight, death in a few days
Clinical pathology Culture organism from feces and intestinal contents
Lesions Dehydration, serofibrinous peritonitis, fluid-filled intestines, mesenteric edema
Diagnostic confirmation Isolate specific serotypes of E. coli
Treatment Antimicrobials in water supply
Control Minimize stress at weaning, antimicrobials in feed and water post weaning. Intestinal acidification. Zine oxide in diet post-weaning
The disease is associated with enterotoxigenic strains of E. coli that produce adhesion factors that allow colonization of the intestine and enterotoxins that induce the intact intestinal mucosa to secrete fluid. A summary would be that postweaning diarrhea is associated with fimbrial types F4 and F18 and carrying the genes for the Shiga-like toxin 2(SLT-2E), labile toxin (LT) and/or Shiga toxin A and B (STa or STb). Toxin and F18 fimbrial genes in E. coli isolated from pigs with postweaning diarrhea or edema disease occur in a variety of chromosomal backgrounds.1
Most commonly, O groups 8, 138, 139, 141, and 149 are associated with the disease.2,3 Most appear to be O149: STaSTbLT:F4(K88). The strains of O149 isolated in recent years from weaned pigs with diarrhea possess the gene for an additional enterotoxin STa, which older strains lack. Of the new strains, which correspond to O149 H10, 92% code for this gene.4 This enteroaggregative E. coli heat-stable enterotoxin 1 (EAST 1) gene is found in isolates from weaned pigs that have diarrhea or edema disease.5 The F107 fimbriae can be found in association with postweaning diarrhea isolates6,7 and other adhesive fimbriae such as Av24 and 2134P have been described.8 Many enterotoxigenic E. coli isolates colonize the small intestine of weaned pigs but lack known colonization factors.9,10 The serogroups of E. coli isolated from pigs with postweaning diarrhea in piggeries in Spain include strains that produce the enterotoxigenic E. coli and verotoxin-producing E. coli and cytotoxic necrotizing factor toxins.11,12 The disease can be reproduced consistently in weaned pigs, provided the pig’s epithelial cell brush borders are susceptible to the adhesin of strains of E. coli with fimbrial antigen F4 (K88).13 The DNA sequences coding for the F18 fimbrial antigens and AIDA adhesin are on the same plasmid in E. coli isolated from the cecum.14 Usually they were LTSTb or STb(13%) and 12% were hemolytic and F18-positive. The remainder were nonhemolytic, belonging to the K48 serogroup.
Although there is an etiological similarity between postweaning diarrhea and neonatal enteric colibacillosis in sucking piglets, the relationship is not exact. Strains associated with neonatal enteric colibacillosis may not have the ability to produce postweaning diarrhea and many strains isolated from coliform gastroenteritis lack K88+ antigen.
Cytotoxic necrotizing factor (CNF) strains of E. coli have been isolated from weaner pigs with necrotic enteritis in South Africa.15
Some non-enterotoxigenic O45 isolates of E. coli associated with postweaning diarrhea produce attaching and effacing lesions,16 and their proliferation may be associated with diet.17 Dual infection with attaching and effacing and enterotoxigenic E. coli may also be associated with postweaning diarrhea.18
Infection with rotavirus may be an etiological factor. The rotavirus may infect and destroy villous epithelial cells of the small intestine, which may allow colonization of the E. coli. Experimentally, a high nutrient intake fed three times daily to piglets weaned at 3 weeks of age produced the most prolonged diarrhea, colonization of the intestine by hemolytic enteropathogenic E. coli and persistent shedding of rotavirus. However, other observations cast doubt on the importance of rotaviruses as a cause of the diarrhea because rotaviruses may be found in the feces of pigs without diarrhea a few days after weaning. The acute disease can be reproduced using K88 E. coli strains without concomitant infection with rotavirus.
Postweaning diarrhea occurs predominantly in pigs 3–10 days after they are weaned. There is considerable variation in the morbidity and mortality between groups, rooms of pigs and buildings. Most outbreaks are in early weaned pigs. Most commonly, pigs are first observed sick or dead on the fourth or fifth day. The spread within affected groups is rapid and a morbidity rate of 80–90% of the group within 2–3 days is not uncommon. Frequently, other pens of susceptible pigs within the same area will also develop the disease within a short period of the initial outbreak. The problem may persist within a herd, affecting successive groups of weaned pigs over a period of weeks or months. The onset of the problem may be associated with the introduction of a different batch or formulation of the creep feed. The case fatality rate may be as high as 30% and survivors may subsequently show a reduced growth rate. The weaning of piglets at 3 weeks of age is commonly followed in a few days by a postweaning reduction in growth rate, variations in total dietary intake and the development of diarrhea. Piglets weaned at 3–4 weeks of age into an uncomfortable unsanitary environment appear especially susceptible.
The proliferation of E. coli in the intestine following weaning appears secondary to some underlying gastrointestinal disturbance. After weaning there is a progressive increase in viscosity of the intestinal contents, which alters the intestinal structure and growth and stimulates the proliferation of enterotoxigenic E. coli in newly weaned pigs.19 In all groups of pigs examined the number of serotypes or diversity of intestinal flora was reduced in the first week after weaning.20 The disease is associated with an earlier, more prolonged and greater proliferation of enterotoxigenic E. coli in the small intestines than occurs in healthy pigs after weaning. Some studies have shown that susceptibility to adhesion with K88+ enterotoxigenic E. coli is a requirement for the production of the disease. Experimentally, pigs that did not have the adhesin receptor did not develop diarrhea when challenged with K88+ E. coli and when in the same environment as the adhesin-positive pigs.
It is believed that several factors commonly associated with weaning predispose pigs to postweaning diarrhea associated with enterotoxigenic E. coli. Some of these risk factors include:
• Stress from loss of maternal contact
• Introduction to strange pens and penmates
• Inadequate ventilation in the weaning pens
• Reduction in ambient temperature
• Cessation of lactation immunoglobulins
• Decreased gastric bactericidal activity attributable to a temporary increase in gastric pH
• Preweaning exposure (creep feeding) to the dietary antigens fed after weaning.21
Hand-washing and donning clean outerwear did not prevent the transmission of E. coli but showering and donning clean outerwear did prevent transmission.22
Experimentally, there is some evidence that the stress of cold ambient temperature (15°C) can result in a greater incidence of diarrhea in weaned pigs than in those housed at 30°C.
The nature and the amount of the diet that the piglet consumes before and after weaning may be a predisposing factor. One hypothesis suggests that a transient hypersensitivity of the intestine may occur if piglets are primed by small amounts of dietary antigen before weaning, by creep feeding, followed by ingestion of greater quantities of the diet after weaning. Pigs that develop diarrhea tend to be those that consume more food after weaning than their contemporaries.
In general, weaning at 3 weeks of age is associated with alterations in the villous epithelium of the small intestine that result in varying degrees of malabsorption and a reduction in daily growth rate that may last for 2 weeks. There are large rapid reductions in intestinal lactase activity, which coincide with reductions in growth rate and a reduced ability to absorb xylose. There is a reduction in villus height and an increase in crypt depth in the small intestine but these alterations are not necessarily associated with the consumption of creep feed before weaning, which does not support the hypothesis that hypersensitivity to a dietary antigen caused by priming prior to weaning is a factor. There is now considerable doubt about the validity of the intestinal hypersensitivity hypothesis. Recent experimental work indicates that creep feeding is not required for the production of the diarrhea and does not induce morphological changes characteristic of an allergic reaction in the small intestine.23 The presence of nondigested food in the gut lumen favors proliferation of enterotoxigenic E. coli. Proteins of animal origin may provide some protection.24
Dietary manipulation can modify several changes that normally occur in the small intestine of the piglet after weaning.25 Feeding a sow milk replacer or a diet based on hydrolyzed casein reduces the increases in crypt depth and the reductions in brush border enzymes. The use of an antibiotic to suppress the microbial activity does not alter the changes in the mucosa after weaning.
The ecology of E. coli and rotavirus in the stomach and intestines of healthy unweaned pigs and pigs after weaning has been examined. Gastric pH is higher in weaned pigs and may not reach a level sufficient to prevent significant numbers gaining access to the small intestine. This factor can be of importance in pigs weaned in pens where oral–fecal cycling of E. coli may provide a massive challenge. After weaning, the hemolytic enterotoxigenic E. coli serotype O149:K91, K88a,c (Abbotstown strain) commonly colonizes the rostral small intestine from lower down the intestinal tract. This serotype was also never found in the gastric contents of weaned pigs. When this serotype is present it tends to dominate the E. coli flora at all levels of the intestine. While rotaviruses are common in the intestinal contents of weaned pigs, the presence of the virus is not necessary for production of the disease.
The loss of lactogenic immunity at weaning may be a risk factor. Milk from sows whose progeny develop postweaning diarrhea contain antibodies capable of neutralizing the enterotoxigenic effect of the homologous E. coli. This suggests that the presence of antibody-mediated activity against enteropathogenic E. coli may be important in preventing the disease during the nursing period. At weaning this protection is removed and the piglet is unable to produce its own antibodies rapidly enough to prevent the disease. The stress of weaning does not appear to affect the immune mechanisms of the pig.
The weaning of piglets at birth or 1 day old is associated with a high mortality rate due to diarrhea and septicemia. The high mortality rate is associated with a lack of colostral antibodies and the strict hygienic conditions required for the artificial rearing of pigs weaned at birth.
The colonization and proliferation of E. coli in the small intestine originates from organisms in the lower part of the intestinal tract. Serotypes of E. coli associated with postweaning diarrhea may be found in the feces of healthy pigs. The virulence factors for postweaning diarrhea are associated with the F4 and F18 fimbrial antigens that carry the genes for the production of toxins (STa, STb, LT, SLT2a, SLX2e). The F18ab variant is expressed by E. coli O139 strain producing Shiga-like toxin and causing edema disease. The F18ac fimbrial E. coli strains often relating to serogroup O141 or O157 and cause diarrhea by expressing enterotoxins (STa or STb) either together or with or without Shiga-like toxins.26 Following weaning, their numbers in feces normally increase markedly even in pigs that remain healthy. The E. coli proliferate in the small intestine and produce an enterotoxin that causes a net loss of fluid and electrolytes to the lumen and subsequent diarrhea. After weaning the net absorption of fluid and electrolytes in the small intestine of pigs is temporarily decreased.25
The number of hemolytic E. coli present in the proximal portion of the jejunum may be 103–105 times higher in affected pigs than in weaned pigs of the same age that do not show signs of disease.27 The susceptibility of the small intestine to the enterotoxin varies according to the area: the upper small intestine is highly susceptible and susceptibility decreases through the more distal portions. Unlike many other species, the weanling pig depends largely on its large intestine for absorption of fluid and electrolytes with only small changes in net fluid movement occurring along the jejunal and ileal segments. In fatal cases, death results from the combined effects of dehydration and acidosis resulting from fluid and electrolyte losses. In the peracute and acute forms of the disease, there is a shock-like syndrome with marked gastric and enteric congestion, hemorrhagic enteritis, and death.28
The experimental model of the disease is characterized by the three syndromes:
• Moderate diarrhea of 3–4 days’ duration, accompanied by fecal shedding of the inoculated organism and reduced body weight gain
• Fecal shedding of the organism with reduced weight gain but without diarrhea.
The role of the rotavirus in the pathogenesis of postweaning diarrhea is uncertain.29 The rotavirus can be found in the feces of healthy unweaned and weaned pigs. The virus is capable of infecting and destroying villous epithelial cells which could contribute to the partial villous atrophy, loss of digestive enzyme activity, malabsorption and reduced growth rate. Experimental inoculation of an enteropathogenic E. coli and the rotavirus causes a more severe disease than either agent does alone. The effects of experimentally induced villus atrophy in weaned pigs infected with the transmissible gastroenteritis virus and a K88+ enterotoxigenic E. coli have been examined.
Changes in the mucosa of the small intestine of recently weaned pigs have been observed and are the subject of much controversy. There is a reduction in the length of the villi, a marked reduction in intestinal disaccharidase activity and an increase in the depth and activity of the intestinal crypts. These changes are maximal at 3–7 days following weaning, persisting until the second week and coinciding with the reduced growth rate.
The postweaning reduction in growth rate may affect 50–100% of the pigs within a few days after weaning and persist for up to 2 weeks. In some situations diarrhea may not develop in any of the pigs in the group. A reduction in feed intake, gaunt abdomens, and lusterless hair coats are characteristic findings of piglets with postweaning ‘check’. They may appear unthrifty for 10 days to 2 weeks, by which time they will improve remarkably.
Most commonly one or two pigs, in good nutritional condition, are found dead with little having been seen in the way of premonitory signs. At this time the others within the group may appear normal but closer examination will reveal several pigs showing mild depression and moderate pyrexia. A postmortem examination of dead pigs should be conducted early in the examination. A proportion of the group will develop diarrhea within 6–24 hours and by 3 days after the initial onset the morbidity may approach 100%. Feed consumption falls precipitously at the early stages of the outbreak but affected pigs will still drink. Affected pigs may show a pink discoloration of the skin of the ears, ventral neck, and belly in the terminal stages. Diarrhea is the cardinal sign – the feces are very watery and yellow in color but may be passed without staining of the buttocks and tail. Pyrexia is not a feature in individual pigs once diarrhea is evident. Affected pigs show a dramatic loss of condition and luster and become progressively dehydrated. Voice changes and staggering, incoordinated movements may be observed in the terminal stage in some pigs. The course of an outbreak within a group is generally 7–10 days and the majority of pigs that die do so within the initial 5 days. Surviving pigs show poor growth rate for a further 2–3 weeks and some individuals show permanent retardation in growth. In outbreaks in early weaned pigs diarrhea is usually evident before death occurs. There is some evidence to show that the postweaning diarrhea may be activated by porcine reproductive and respiratory syndrome.30
Culture of the feces and intestinal contents for enterotoxigenic strains of E. coli is indicated.
Pigs dying early in the course of the outbreak are in good nutritional condition but those dying later show weight loss and dehydration. Mild skin discoloration of the ears and ventral areas of the head, neck, and abdomen is usually present. In acute cases there is a moderate increase in peritoneal fluid and barely perceptible fibrinous tags between loops of the small intestine may be present. The vessels of the mesentery are congested and occasionally petechial hemorrhages and edema are present. The gastric mucosa is congested and an infarct is usually present along the greater curvature. The small intestines are dilated and contain yellow mucoid liquid or occasionally bloodstained material. The mucosa of the small intestine is congested and sometimes there are hemorrhagic areas. The content of the large intestine is fluid to porridge-like in consistency and the mucosa may be congested. In some cases mild mesocolonic edema is visible. Hemolytic E. coli can be isolated in large numbers from the small intestine and mesenteric lymph nodes. Polyclonal antisera directed against known pathogenic serotypes are usually employed to test the isolate but a negative result does not preclude the strain from being an enteropathogenic organism.
Microscopically, there is usually bacterial adherence to intestinal villi. Other changes are those commonly associated with endotoxemia, especially microvascular thrombosis in a variety of organs.
• Bacteriology – mesenteric lymph node, segment of ileum, colon (CULT)
• Histology – formalin-fixed stomach, several segments of small intestine, colon, liver, lung, spleen (LM).
Postweaning diarrhea is the prime consideration in pigs that are scouring or dying within a 3–10-day period of some feed or management change.
Swine dysentery and salmonellosis are manifested by diarrhea and death but they are not necessarily related to weaning or feed change, and both are more common in older growing pigs. Salmonellosis poses the greatest difficulty in initial diagnosis from coliform gastroenteritis. In salmonellosis, the feces are generally more fetid with more mucus, mucosal shreds, and occasionally blood, and the skin discoloration is more dramatic. On necropsy examination enlarged hemorrhagic peripheral and abdominal lymph nodes and an enlarged pulpy spleen are more suggestive of salmonellosis; however, cultural differentiation is frequently required. If there is doubt the pigs should be treated to cover both conditions until a final decision is obtained. The onset of swine dysentery is comparatively more insidious than that of postweaning diarrhea; the characteristic feces, clinical and epidemiological pattern and postmortem lesions differentiate these two conditions.
Swine fever should always be a consideration in outbreaks in pigs manifested by diarrhea and death. However, the epidemiological and postmortem features are different.
Other common causes of acute death in growing pigs such as erysipelas, pasteurellosis, and Actinobacillus pleuropneumoniae infection are easily differentiated on necropsy examination.
Edema disease occurs under similar circumstances to coliform gastroenteritis but the clinical manifestation and postmortem findings are entirely different.
A Swedish study31 showed that, except for resistance to tetracyclines, sulfamethoxazole, and streptomycin, antibiotic resistance is not unduly spread across E. coli isolates. A Spanish study32 was similar. Tetracyclines should not be the first choice of treatment because of the rapid acquisition of resistance. Nearly all isolates are highly susceptible to enrofloxacin, gentamicin, and neomycin.
It is imperative that treatment of all pigs within the group be instigated at the initial signs of the onset of postweaning diarrhea, even though at that time the majority of pigs may appear clinically normal. Delay will result in high mortality rates. Any pig within the group that shows fever, depression, or diarrhea should be initially treated individually both parenterally and orally and the whole group should then be placed on oral antibacterial medication. Water medication is preferable to medication through the feed as it is easier to institute and affected pigs will generally drink even if they do not eat. Neomycin, tetracyclines, sulfonamides, or trimethoprim-potentiated sulfonamides and ampicillin are the usual drugs of choice. Danofloxacin is safe and highly effective.33 Experimental infection with K88-positive E. coli was controlled by ceftiofur sodium given intramuscularly daily for three consecutive days.34 When pulse dosing is used there appears to be less resistance.35 In herds with postweaning problems, prior sensitivity testing will guide the choice of the antibacterial to be used. Antibiotic medication should be continued for a further 2 days after diarrhea is no longer evident and is generally required for a period of 5–7 days.
Consideration should be given to the medication of at-risk equivalent groups of pigs within the same environment. Intraperitoneal fluid and electrolyte replacement for severely dehydrated pigs and electrolytes in the drinking water should also be considered.
Recommendations for effective and economical control of postweaning reduced growth rate and postweaning diarrhea in pigs weaned at 3 weeks of age are difficult because the etiology and pathogenesis of this complex disease are not well understood. Epidemiologically, the disease is associated with weaning and the effects of the diet consumed before and after weaning. In all cases the piglet should be 4.5 kg (10 lb) and preferably 5.5 kg (12 lb) at weaning.
A whole variety of techniques, including intestinal acidification, antimicrobial medication in water or feed, environmental modifications, competitive exclusion, feeding probiotics, binding agents such as eggs, milk, or bacterial byproducts (most of these studies show they do not work), zinc oxide, or vaccination of sows and piglets with toxoids have been tried. The use of dietary egg yolk antibodies also does not appear to be effective.36
Intestinal acidification reduces the binding of the E. coli to the epithelial surface and a pH of 3.5–4.0 at the trough or nipple drinker is best. Citric acid, formic acid, propionic acid, or a citric acid/copper sulfate mixture can be used.
Zinc oxide in particular stabilizes the intestinal flora. Piglets given lactose and fiber were least affected and the next least affected were animals that received zinc oxide.37 Pigs fed dietary antibiotic growth promoters and zinc oxide had lower counts of anaerobic bacteria in their feces than control piglets. The removal of these ingredients from the diet will increase days to slaughter.38
It has been traditionally accepted, without reliable evidence, that the sudden transition in diet at weaning is the major predisposing factor. However, as presented under Epidemiology, the experimental observations are conflicting. One set of observations indicates that, if pigs eat a small quantity of creep feed before weaning, they are then ‘primed’ and develop an intestinal hypersensitivity that, following the ingestion of the same diet after weaning, results in the disease. It has been suggested that piglets should consume at least 600 g of creep feed before weaning in order to develop a mature digestive system. Another set of observations indicates that those pigs that consumed an excessive quantity of feed after weaning developed the disease.
The complete withholding of creep feed followed by abrupt weaning at 3 weeks of age seemed to have a protective effect, possibly associated with a low dietary intake. Farms with lower rates of postweaning diarrhea used their first piglet ration (phase 1 feeding) for much longer and also changed over to the second ration over a much longer period. Competitive exclusion has been shown to be of benefit.39-41
The recommendations set out here are based on the hypothesis that the consumption of adequate quantities of creep feed prior to weaning is the most effective and economical practice.
Every effort should be made to minimize the stress associated with weaning. Stressors influence the fecal shedding of enterotoxigenic E. coli by young piglets by a mechanism that may not involve modulation of the immune response.42 In order to avoid a sudden transition in diet at weaning, creep feed should be introduced to the suckling piglets by at least 10 days of age. It is important that the creep feed and feeder area be kept fresh to maintain palatability. The same feed should be fed for at least 2 weeks following weaning and all subsequent feed changes should be made gradually over a 3–5-day period. Feed restriction in the immediate 2-week period following weaning may reduce the incidence but generally is not successful. It is a common field observation that the incidence of diarrhea varies with different sources of feed but experimental studies to confirm this relationship are not available.
Where possible, at weaning the sow should be removed and the pigs should be kept as single litters in the same pen for the immediate postweaning period. If grouping of litters is practiced at this time, or later, the pigs should be grouped in equivalent sizes. Multiple suckling in the preweaning period may reduce stress associated with grouping of part-weaned pigs. With all pigs, but especially those weaned earlier than 6 weeks, the pen construction should be such as to encourage proper eliminative patterns by the pigs and good pen hygiene (see Salmonellosis) so as to minimize oral–fecal cycling of hemolytic E. coli. The environment also appears especially important in this group and draft-free pen construction should be such as to encourage proper ventilation. It is preferable to wean pigs on weight rather than age and in many piggeries a weaning weight of less than 6 kg is associated with a high incidence of postweaning enteric disease.
The inclusion of an antimicrobial in the feed or water to cover the critical period of susceptibility (generally for 7–10 days after weaning) can be used as a preventive measure. Apramycin at the rate of 150 g/tonne of feed for 2 weeks after weaning may be associated with improved growth rates and a reduction in mortality. The high incidence of drug resistance in isolates of E. coli43 makes prior sensitivity testing mandatory and the antibiotic may need to be changed if new strains gain access to the herd. The routine use of prophylactic antibiotics for this purpose needs to be considered in relation to the problem of genetically transmitted drug resistance, however it is currently often necessary for short-term control of a problem.
Vaccination may offer an alternative method of control. However, currently there are no vaccines available for the control of colibacillosis in weaned pigs. Oral inoculation with 5 × 108–109 of nontoxigenic strains can be followed with K88 at day 1 of move, K88/F18ab at day 7 and F18 at days 13–15. Only the oral vaccines with the preformed fimbriae appear to produce any protection to homologous fimbrial variant.44 The results vary and some authors think that the prolonged transit time in the stomach after weaning may deactivate the F4 fimbriae when this has been used as a fimbrial vaccine.45 Microencapsulated enterotoxigenic E. coli and detached fimbriae have been used for peroral vaccination in pigs.46,47 Parenteral vaccination for the control of coliform gastroenteritis has proved of variable value, probably because parenterally administered antigens do not usually stimulate the production of IgA antibodies and intestinal immunity. Oral immunization by the incorporation of E. coli antigens into creep feed has been shown to reduce the incidence and severity of postweaning diarrhea. A live avirulent E. coli vaccine for K88+, LT+ enterotoxigenic colibacillosis in weaned pigs provided protection experimentally. Rearing early-weaned piglets artificially for the purpose of increasing the efficiency of the sow is an attractive management concept. However, high death losses from diarrhea have slowed progress in this new development. The incorporation of antibodies in the diet of such piglets as a prophylactic measure should be possible and is being explored.
The ultimate control is by removing the receptor gene in the population and although this has been done experimentally these animals are not yet available in large numbers commercially.
Mainil J. Shiga/verocytotoxins and Shiga verotoxigenic E. coli in animals. Vet Res. 1999;30:235-257.
Moxley RA, Duhamel GE. Comparative pathogenesis of bacterial enteric disease of swine. Adv Exp Med Biol. 1999;473:83-101.
Nagy B, Fekete PZ. Enterotoxigenic E. coli (ETEC) in farm animals. Vet Res. 1999;30:259-284.
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Cerebrospinal angiopathy is a sporadic disease of recently weaned pigs manifested primarily by neurological signs. In some areas the disease is the more common cause of central nervous system disorders in this age group of pigs.1 The disease affects only one or a few pigs within a litter of a group occurring up to 5 weeks after weaning.1,2 although a similar condition has been reported in fattening and adult pigs.3 The disease is characterized by the variety of neurological signs that it presents. Incoordination and a decreased central awareness are common presenting signs but abnormal head position, aimless wandering and persistent circling may also be observed. There is usually apparent impairment of vision. Fever is not a feature and the clinical course may last for several days. Affected animals may die but more commonly are destroyed because of their emaciated condition.2,4 Wasting without neurological disorder may also occur.5 They are also prone to savaging by unaffected penmates.
Histologically the disease is characterized by an angiopathy that is not restricted to the central nervous system. The similarity of the angiopathy to that seen in chronic edema disease6 has led to postulation1,2,4 that this disease is a sequel to subclinical edema disease. The disease has been reported occurring in pigs 15–27 days after experimental E. coli infection.5 The characteristic feature is the presence of perivascular eosinophilic droplets.7
The main differential diagnosis is that of spinal or brain abscess and the porcine viral encephalomyelitides. Affected pigs should be housed separately as soon as clinical signs are observed. In view of the nature of the lesion, therapy is unlikely to be of value; however, recovery following treatment with oxytetracycline has been reported.1
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5 Bertschinger HU, Pohlenz J. Schweiz Arch Tierheilkd. 1974;116:543.