Colostral immunoglobulin status

The role of colostrum in protecting the newborn calf from the effects of enteropathogens is well known. The failure of the newborn calf to ingest an adequate quantity of good-quality colostrum containing a high level of colostral immunoglobulins within a few hours after birth is a major risk factor contributing to acute undifferentiated diarrhea. Complete or partial failure of transfer of passive immunity is highly prevalent in diarrheic veal calves infected with cryptosporidia, coronavirus, and rotavirus.¹ Earlier work centered on the protective effect of colostrum against septicemic and enteric colibacillosis. More recently, the role of colostral and milk antibodies against rotavirus and coronavirus enteritis in newborn calves has been established. Specific protection against these viral diarrheas in the newborn calf depends on the level of antibody in the lumen of the intestine. While it is easy to state that calves should receive liberal quantities of colostrum, the veterinarian in the field who encounters an outbreak of acute diarrhea in beef calves, for example, cannot usually easily determine whether in fact the calves possess protective levels of immunoglobulins.

Lactose intolerance as a cause of diarrhea has been reported in a calf but its occurrence is rare.²

Cases of diarrhea due to specific nutritional deficiencies are reported rarely and not well documented. However, field observations indicate that outbreaks of diarrhea in sucking beef calves may have been associated with specific nutrient deficiencies such as copper or selenium. These are not documented but should be considered in certain situations where these deficiencies are known to be present in the herd. An epidemic of intractable diarrhea in 2-month-old beef calves was associated with deficient tissue and plasma levels of vitamin E in the affected calves, which also had lesions of skeletal and myocardial muscular dystrophy with adequate levels of selenium.³ A combination of low vitamin E status and low immunoglobulin status may be a contributing factor in neonatal diarrhea of calves,⁴ but this is not well documented.

Environmental and management risk factors

Veterinarians have commonly observed a relationship between adverse climatic conditions and epidemics of diarrhea in calves. During inclement weather, such as a snowstorm, a common practice in beef herds is to confine the calving cows in a small area where they can be fed and watered, and observed, more easily. The overcrowding may be followed by an outbreak of calf diarrhea. Cold, wet, windy weather during the winter months in temperate climates and hot humid weather during the summer months may be associated with an increased incidence of dairy calf mortality due to diarrhea. Changes in weather and wet, windy and cold weather are commonly associated with subsequent outbreaks of the disease in beef calves raised outdoors. Increases in population density in calf houses, and on calving grounds, resulting in highly contaminated calving grounds, are important risk factors. In beef herds in the USA and Canada, the risk factors that are associated with an increase in calf mortality from diarrhea include:

Some studies have shown that the major contributing factor to dairy calf mortality is the care provided by the calf attendant. Not infrequently, however, outbreaks can occur in herds in which the management is excellent and not uncommonly an etiological diagnosis cannot be made.

Certain herd characteristics and herd management practices are associated with an increased incidence of diarrhea in dairy herds.⁷ Larger herd size is associated with an increased incidence of diarrhea.⁷ The greater disease rate may be associated with a greater possibility of a large epidemic in a larger population, and cows in larger herds may be more densely housed. In dairy herds with 10–49 cows, the number of young stock at the end of the month, the incidence density of respiratory disease in the calves per herd per calf-month, and the cumulative incidence of vaccinations for calves given to prevent diarrhea may be associated with increased incidence of diarrhea. As the incidence of respiratory disease increases in large dairy herds, the incidence of diarrhea in the calves increases, especially in large herds of over 200 cows.⁷ Reduction of the incidence of respiratory disease in calves will result in a decrease in the incidence of diarrheal disease. The use of individual maternity stalls and regular removal of bedding between calvings is associated with a decrease in the incidence of neonatal calf diarrhea. As the number of calves in the herd increases, calf attendants may become too busy to perform duties of calf care thoroughly. Overcrowding may also be a risk factor.

Dairy calves fed milk replacers may develop diarrhea because of the inferior quality of some milk replacers.⁸

Epidemics of diarrhea in piglets are commonly associated with inadequate sanitation and hygiene in the farrowing rooms, which may be under continuous use without sufficient time for cleaning and disinfection between farrowings. Certain management procedures may be associated with differences in the prevalence of enteropathogens.

In dairy calves fed on nipple feeders there may be increased probability of the calves shedding detectable fecal levels of Salmonella, E. coli, rotavirus, or coronavirus. The use of group pens has been associated with increased odds of encountering Campylobacter jejuni, the presence of which is of uncertain significance. Calves with diarrhea on these farms tend to have increased odds of shedding rotavirus and K99⁺ E. coli.

Calves

The distribution and occurrence of enteropathogens in the feces of diarrheic and normal healthy calves varies depending on the geographical location, the farm, the age and type of calves being examined, and the extent to which the diagnostic laboratory is capable of isolating or demonstrating the pathogens.⁹ Rotavirus, Cryptosporidium spp., coronavirus, and enterotoxigenic E. coli, collectively, are responsible for 75–95% of infections in neonatal calves worldwide. The relative frequencies of each of the four differ between locations and between seasons and years.¹⁰ Any one of the common pathogens may predominate or be absent in a certain group of animals.¹¹ Mixed infections are common.¹² Rotavirus will be most common in some groups, especially housed calves.¹² Coronavirus may predominate in beef calves in some countries and not in others, and Cryptosporidium may occur in 30–50% of diarrheic calves on a worldwide basis.¹³ A survey of veal calf farms revealed that Cryptosporidium infection is an important cause of transient diarrhea and that there was no association between diarrhea and infection with either Salmonella spp., enterotoxigenic E. coli, rotaviruses, or verocytogenic E. coli in the population examined.¹⁴ Cryptosporidia, rotavirus, and coronavirus are the most commonly identified enteropathogens in intensively reared veal calves.¹ In dairy calves, the prevalences of giardiosis and cryptosporidiosis may be high and both parasites may be associated with diarrhea. Cryptosporidium parvum is an important pathogen in calves under 1 month of age, but Giardia duodenalis may be more important when calves are older. Calves may clear C. parvum infections within 2 weeks; however, G. duodenalis infections may become chronic in the same calves.¹⁵ The combination of Cryptosporidium sp. and rotavirus may predominate in some situations. Cryptosporidium spp. were the second most commonly detected pathogens next to rotavirus, and case-control studies indicated a highly significant association with diarrhea. Enteropathogens may not be detectable in up to 30% of diarrheic calves. Eimeria spp. can cause coccidiosis in calves any time after about 21 days after birth but the disease is more common in calves several months old.

In some countries, enterotoxigenic K99⁺ E. coli may occur in 30–40% of diarrheic calves, while in others the incidence may be as low as 3–6%. Attaching and effacing E. coli that cause hemorrhagic colitis and blood in the feces of diarrheic calves about 2 weeks of age are being recognized with increasing frequency. They may occur concurrently with other enteropathogens (cryptosporidia, rotavirus, coronavirus, enterotoxigenic E. coli, bovine virus diarrhea virus (BVDV), and coccidia). Some isolates of attaching and effacing E. coli produce verocytotoxin.

The age occurrence of the common enteropathogens associated with diarrhea in calves is shown in Table 18.2. Case-control studies of diarrheic and healthy calves from the same groups indicate that the enteropathogens commonly found in diarrheic calves can also be found in healthy calves but at a lower frequency, with the exception of rotavirus, which may be excreted by up to 50% of healthy calves. The prevalence of enteropathogens in healthy calves on farms where there is no recent history of diarrhea indicates an absence of Salmonella spp., enterotoxigenic E. coli, Cryptosporidium, and coronavirus, but the presence of rotavirus in some calves. It appears that healthy calves may be infected more often with enterotoxigenic E. coli, Cryptosporidium, coronavirus, and rotavirus in herds in which some calves have or recently have had enteric disease than in herds free from major enteric disease.

Table 18.2 Age occurrence of the common enteropathogens in calves

Campylobacter spp. and Yersinia spp. are well adapted to the bovine host and can be found in the feces of diarrheic and healthy calves at a similar prevalence.¹⁶ Their significance as pathogens in newborn calves is questionable. They are probably part of the normal enteric flora of ruminants. However, as they represent a source of gastrointestinal infections in humans, management factors limiting intestinal colonization of these bacteria should be considered in beef cow/calf herds.¹⁶

Rotavirus and coronavirus occur with almost equal frequency in the intestinal tracts of normal and diarrheic calves of some studies. Intestinal lesions compatible with the viral infections are found in about 70% of diarrheic calves. Thus, these viruses are widespread in the bovine population and only under some circumstances will the infection be severe enough to cause lesions and diarrhea. Other viruses, such as parvovirus, astrovirus, Breda virus, and calici-like virus, have been isolated from the feces of diarrheic calves but their role in the etiology is yet to be defined.

A necrotizing enteritis of suckled beef calves 7–10 weeks of age on pasture in Scotland has been reported.¹⁷ Fever, acute diarrhea and dysentery, and a case fatality rate of 25% are characteristic. No etiological agent has been identified.

Lambs and goat kids

The E. coli strains isolated from diarrheic lambs and goat kids on Spanish farms are not generally toxigenic and belong to a large number of O serogroups.¹⁸

Piglets

In outbreaks of diarrhea in neonatal piglets during the first 5 days of life, the enteropathogens that are commonly present in the feces include the transmissible gastroenteritis (TGE) virus, enterotoxigenic E. coli, Isospora spp., rotavirus, C. perfringens type C, and adenovirus. Clostridium difficile has emerged as an important pathogen causing enteritis in suckling piglets.^19,20 The TGE virus causes diarrhea in piglets under 15 days of age, enterotoxigenic E. coli under 5 days of age, Isospora sp. between 5 and 15 days of age, and rotavirus in piglets over 10 days of age. During the second and third weeks of life, Isospora suis is the most common pathogen in outbreaks of diarrhea in litters of piglets. While individual piglets may be infected by a single pathogen, it is common for more than one pathogen to be present in the litter. This stresses the importance of submitting to the diagnostic laboratory some piglets that are representative of the problem. A seasonal occurrence of the common enteropathogens has also been observed. The prevalence of the TGE virus may be highest during the fall, winter, and spring months, and the coccidia and E. coli are more common during the summer, fall, and early winter, with the lowest prevalence in the spring.

Foals

Diarrhea in foals is common but most cases are mild, transient and not associated with infectious agents. Diarrhea is the most commonly reported disease in foals under 7 days of age.²¹ The most common occurrence is associated with ‘foal heat’ in the mare. Group A rotavirus is the most common cause of epidemics of diarrhea in foals.²² Enterotoxigenic E. coli has been isolated from a diarrheic foal. Other pathogens that have been isolated from foals with diarrhea include C. jejuni, C. perfringens, and Rhodococcus equi.

Calves

The prevalence of enterotoxigenic E. coli in diarrheic calves varies widely geographically, between herds and depending on the age of the animals. The prevalence can be as high as 50–60% in diarrheic calves under 3 days of age and only 5–10% in diarrheic calves 8 days of age. In some countries the prevalence is only 5–8% in diarrheic calves under 3 days of age. Thus enterotoxigenic colibacillosis is a major cause of diarrhea in calves less than 3 days of age and is not associated with outbreaks of diarrhea in calves older than 3 days. Enterotoxigenic E. coli infection in calves older than 2–3 days will in most cases be associated with a virus infection. The prevalence of the organism is also very low or not present in clinically normal calves in herds that have not had a problem with diarrhea. In some beef herds affected with diarrhea in young calves there may be little evidence of infection with enterotoxigenic E. coli, and other factors need to be examined.

Piglets

The prevalence of enterotoxigenic E. coli in diarrheic piglets also varies geographically and with herds. In some areas the K99⁺ pilus was found more frequently than the K88⁺ or 987P, whereas in other regions the K88⁺ is more common.

Calves

In dairy calves raised under intensive and poorly managed conditions the morbidity rate may reach 75% but is usually about 30%. Case fatality rates vary from 10–50% depending on the level of clinical management.

In beef calves the morbidity rates vary from 10–50% and the case fatality rates from 5–25% or even higher in some years. The population mortality rate in both beef and dairy calves can vary from a low of 3% in well-managed herds to a high of 60% in problem herds in certain years.

Piglets

In piglets the morbidity rate of preweaning diarrhea varies widely between herds but averages about 6% of litters, mostly in the first week of life. The morbidity rates increase with increased litter size and decrease with increasing parity of the sow. Losses due to stillbirths, traumatic injuries, starvation, and undersize at birth account for a much greater combined total preweaning loss but colibacillosis accounts for approximately 50% of the gastroenteropathies encountered during the preweaning period.

Foals and lambs

Incidence rates are not readily available for foals and lambs, but E. coli is the most commonly isolated organism from the tissues and blood of septic foals.

Animal risk factors

Environmental and management risk factors

Pathogen risk factors

Enterotoxigenic colibacillosis

Enterotoxigenic strains of E. coli colonize and proliferate in the upper small intestine and produce enterotoxins, which cause an increase in net secretion of fluid and electrolytes from the systemic circulation. The adhesion of E. coli to the intestinal epithelial cells is mediated by bacterial pili. The enterotoxigenic form of colibacillosis occurs most commonly in calves and piglets and less commonly in foals and lambs.

The factors that allow or control the colonization and proliferation of these strains and their production of enterotoxin are not well understood. The bacterial fimbriae attach to specific receptor sites on villous epithelial cells, following which the bacteria multiply and form microcolonies that cover the surface of the villi. The capsular polysaccharide of E. coli may also be involved in adhesion and colonization. The fimbriae of E. coli are strongly immunogenic, a factor that is utilized in the production of vaccines. Experimentally, the intestinal fluid from the most proximal small intestine of calves is more suppressive of K99⁺ pilus expression than fluid from more distal segments of the small intestine. Thus any factor that results in an increase of the pH in the lumen may allow the proliferation of the organism and, conversely, lowering the pH may reduce the severity of colibacillosis.

Diarrhea, dehydration, metabolic acidosis and electrolyte imbalance

The production of the enterotoxin results in net secretion of fluid and electrolytes from the systemic circulation into the lumen of the intestine, resulting in varying degrees of diarrhea, dehydration, electrolyte imbalances, acidosis, hyperkalemia when the acidosis is severe, circulatory failure, shock and death. The hyperkalemia in calves with neonatal diarrhea and acidosis has been associated with cardiac rate and rhythm abnormalities, including bradycardia and atrial standstill.⁴⁰ The response to the enterotoxin in calves and piglets is similar to cholera enterotoxin in humans and takes place through an intact mucosa. Enterotoxin stimulates mucosal adenylcyclase activity, leading to an increase in cyclic adenosine monophosphate (AMP), which increases intestinal fluid secretion. The secretion originates primarily in the intestinal crypts but the villous epithelium also has a secretory function. The fluids secreted are alkaline and, in comparison to serum, isotonic, low in protein and high in sodium and bicarbonate ions. Distension of the right abdomen of diarrheic calves may occur, which may be associated with fluid distension of the abomasum and the intestines.⁴¹

When the disease is confined to the intestine, it responds reasonably well to treatment in the early stages. If death occurs, it is due to acidosis, electrolyte imbalance, and dehydration. The acid–base and electrolyte changes in piglets 1–3 days of age infected naturally and experimentally with enterotoxigenic E. coli reveal a severe dehydration and metabolic acidosis.

Severe metabolic changes can occur in calves with diarrhea. If the disease is progressive, the acidosis becomes more severe, lactic acidosis develops because of a reduced ability to utilize lactic acid, and severe hypoglycemia may occur because of a reduced rate of conversion of lactic acid to glucose. If extensive fluids are lost, hypovolemia and shock occur. The blood oxygen-binding capacity may be impaired in calves with diarrhea and severe dehydration.³⁹

The most commonly accepted explanations of the metabolic acidosis in diarrheic calves are fecal bicarbonate loss and l-lactic acidosis. l-lactic acidosis is thought to occur from inadequate perfusion because of dehydration or endotoxemia, with subsequent anaerobic glycolysis and decreased clearance of l-lactate. However, neonatal diarrheic calves have high serum concentrations of d-lactate, with relatively low serum l-lactate concentrations.^42-44 d-lactic acidosis is a contributory factor in calves with high anion gap acidosis. The anion gap significantly correlates with d- and total dl-lactate concentrations in serum but not with serum l-lactate concentrations.⁴⁴ The elevated serum and urine concentrations of serum d-lactate are probably gastrointestinal in origin and the rumen and colon in diarrheic calves are considered as sites for the sources of d-lactate.⁴³

The anion gap is defined as the sum of the major cations minus the sum of the major anions and is a measure of ‘unmeasured anions’:

Anion gap = [Na⁺] + [K⁺] − [Cl⁻] − [HCO₃⁻].

The anion gap may be changed in numerous disturbances, including those of acid–base imbalance, and is useful in the interpretation of acid–base findings, but is of no value in assessing the intrinsic severity of acid–base disturbances.

Diarrheic calves are generally hyperkalemic with a high serum anion gap, a depressed serum bicarbonate and a low blood pH.

The severity and nature of the acidosis in diarrheic calves varies with the age of the calf. Diarrheic calves under 1 week of age often have a lactic acidosis, while those over 1 week of age have a nonlactic acidosis. Younger calves tend to dehydrate more rapidly and severely than older calves, which may be related to the greater incidence of enterotoxigenic colibacillosis in the young age group. The severity of dehydration, hypothermia, and metabolic acidosis is associated with the level of mental depression.⁴⁵ The clinical signs and age of the calf can be used to predict the severity of acidosis; the more severe the acidosis, the greater the depression.

Metabolic acidosis without clinical evidence of dehydration occurs in some calves that have had a history of diarrhea in the previous several days.⁴⁶ A similar syndrome has been described in goat kids. The pathogenesis is unclear but is thought to be associated with diarrhea in the preceding several days. One possibility is an inadequate amount of bicarbonate in the fluids and electrolytes used for the treatment of the dehydration.

Hyperkalemia in the diarrheic calf

Hyperkalemia is most common in dehydrated diarrheic calves that are severely acidotic. The potassium moves out of the intracellular space into the extracellular space, resulting in hyperkalemia. The predominant clinical finding is bradycardia (heart rate < 90 bpm) in a dehydrated diarrheic calf. However, hypoglycemia and hypothermia may be associated with bradycardia in a similar calf.

Hypernatremia in the diarrheic calf

Acute hypernatremia may occur in diarrheic calves.^46,47 Clinical findings are nonspecific and include depression, weakness, dehydration, and diarrhea. Laboratory determination of serum electrolytes is necessary to make the diagnosis. The serum sodium concentration is above 160 mEq/L. It is presumed that mixing errors in the preparation of oral electrolyte solutions for diarrhea is the cause. The experimental oral administration of 1 L of electrolyte concentrate containing 2750 mEq sodium found that calves would willingly consume the solution mixed with milk and developed signs of hypernatremia within 6 hours of administration.

Hypernatremia (serum sodium > 153 mEq/L) in neonatal elk calves has been described.⁴⁸ The most common clinical findings were diarrhea, dehydration, depression, and anorexia.

Effect of colostral immunoglobulin status

An adequate level of serum immunoglobulins protects calves from death due to the effects of diarrhea, but not necessarily from diarrhea. The best protection is provided if both the immunoglobulin levels in the serum and the levels in the colostrum and milk during the first week after birth are high. The immunoglobulin subclasses in the plasma of calves that have received sufficient colostrum are IgG, IgM (and IgM is probably the more important of the two for the prevention of septicemia) and IgA. The serum IgG concentrations of calves under 3 weeks of age dying from infectious disease were much lower than in normal calves. Of the dead calves 50% had serum IgG levels that were more than 2 standard deviations below the normal mean, and an additional 35% had concentrations greater than 1 standard deviation below the normal mean. In the intestine, no single subclass of immunoglobulin is known to be responsible for protection against the fatal effects of diarrhea. Individually, each immunoglobulin subclass can prevent death from diarrhea even though calves may be affected with varying degrees of diarrhea. In contrast to the situation in the pig, IgA appears to be least effective.

In pigs, IgA becomes the dominant immunoglobulin in sow colostrum after the first few days of lactation, and this is the immunoglobulin that is not absorbed but is retained in, and reaches a high level in, the gut and plays a major role in providing local protection against enteric colibacillosis in piglets. Porcine colostral IgA is more resistant to gastrointestinal proteolytic enzymes than IgG₂ and IgM. On the other hand, IgG is at a peak concentration in colostrum in the first day after parturition, is readily absorbed by the newborn piglet and is vital in providing protection against septicemia. Lysozyme in sows’ milk may assist in the control of the bacterial population in the gut of the unweaned piglet.

Intestinal mucosa

In general, the enterotoxigenic E. coli exert their effects by the enterotoxin causing hypersecretion through an intact intestinal epithelium. However, the intraluminal exposure of the jejunum of 3-week-old pigs to sterile crude culture filtrates from strains of E. coli known to produce two types of heat-stable enterotoxin will induce microscopic alterations of the villous epithelium. Focal emigration of neutrophils, especially through the epithelium above aggregated lymphatic follicles, stunting of jejunal and ileal villi, and adherence of bacteria to jejunal and ileal mucosae are the most consistent finding. These changes are useful in making the diagnosis of enterotoxigenic colibacillosis in calves. While enterotoxigenic strains are considered to be noninvasive this does not preclude the possibility that invasion into the systemic circulation may occur, resulting in septicemia, or that septicemic strains may not also be present.

Enzyme histochemistry studies of the small intestinal mucosa in experimental infections of calves with rotavirus and enterotoxigenic E. coli indicate a marked decrease in enzyme activity in dual infections and a lesser decrease in monoinfections. Increased enzyme activity occurred in parts of the intestinal mucosa that were not affected or only slightly affected by the enteropathogens, which may be an adaptation of the mucosa to maintain absorptive function. Lactose digestion is slightly impaired in calves with mild diarrhea. Calves with acute diarrhea are in a catabolic state and respond with a larger increase of plasma glucose concentration to a given amount of absorbed glucose than do healthy calves.⁴⁹ Fat and carbohydrate malabsorption frequently occurs in diarrheic calves over 5 days of age and may contribute to the death of these animals in cold weather.

Attaching and effacing colibacillosis

Attaching and effacing enteropathogenic E. coli can cause naturally occurring diarrhea and dysentery in calves at 18–21 days.¹ They do not produce enterotoxin but adhere to the surface of the enterocytes of the large intestine. Affected calves pass bright red blood in the diarrheic feces. The lesions in experimentally infected calves are indistinguishable from those produced by some E. coli that are enteropathogenic for humans, rabbits, and pigs. They do not produce enterotoxin. The bovine O118:H16 enterohemorrhagic E. coli strain is able to colonize the intestine of newborn calves and to induce diarrhea 24 hours after challenge and to produce attaching and effacing lesions in the small and large intestines.⁵⁰

Synergism between enteropathogens

Enterotoxigenic colibacillosis occurs naturally and can be reproduced experimentally using enterotoxigenic E. coli in calves under 2 days of age but not in calves 1 week of age. Diarrheic calves older than 3 days of age may be infected with enterotoxigenic K99⁺ E. coli and rotavirus. There is evidence that prior or simultaneous infection of the intestine with rotavirus will enable the E. coli to colonize in older calves.¹ Thus, there may be synergism between rotavirus and enterotoxigenic E. coli in calves older than 2 days; this may explain the fatal diarrhea that can occur in calves at 1 week of age, which normally would not be fatal with a single infection. The rotavirus may enhance colonization of the E. coli. In the Kashmir valley of India, diarrheic lambs from birth to 3 months of age are commonly infected with group A rotavirus and pathogenic serogroups of E. coli O26, O113, O157, all of which are pathogenic to humans.⁵¹

The simultaneous experimental infection of neonatal gnotobiotic calves at 24 hours of age with rotavirus and enterotoxigenic E. coli results in a severe diarrheal disease. The same situation occurs in piglets. However, in both species the effect was considered to be additive rather than synergistic.

Coliform septicemia

This is most common in calves during the first 4 days of life. Most affected calves have low levels of serum colostral immunoglobulins because of inadequate transfer of colostral immunoglobulins.¹⁶ The illness is peracute, the course varying from 24–96 hours with a survival rate of less than 12%.¹⁶ There are no diagnostic clinical signs. Affected animals are depressed and weak, commonly recumbent, and dehydrated; tachycardia is present and, although the temperature may be high initially, it falls rapidly to subnormal levels when the calf becomes weak and moribund. The suck reflex is weak or absent, the oral mucous membranes are dry and cool, and the capillary refill time may be prolonged. Cold extremities, weak peripheral pulse and prolonged capillary refill time are common. Scleral injection is common. Diarrhea and dysentery may occur but are uncommon.

Multiple body system and organ involvement is characteristic of neonatal septicemia and careful clinical examination is required to detect abnormalities. If a calf survives the septicemic state, clinical evidence of postsepticemic localization may appear in about 1 week. This includes arthritis, meningitis, panophthalmitis and, less commonly, pneumonia. In a series of 32 cases of meningitis in neonatal calves, the most frequent clinical findings were lethargy, anorexia, recumbency, loss of the suck reflex, stupor, and coma.⁵² Opisthotonos, convulsions, tremors, and hyperesthesia were seen less frequently. The case fatality rate was 100% in spite of intensive therapy, and lesions of septicemia were present at necropsy.

Enteric colibacillosis

This is the most common form of colibacillosis in newborn calves, primarily 3–5 days of age. It may occur in calves as early as 1 day of age and only rarely up to 3 weeks. The clinical severity will vary depending upon the number and kind of organisms causing the disease. The presence of a single enterotoxigenic strain of E. coli may cause a state of collapse usually designated as enteric toxemia. In this form of the disease the outstanding clinical signs include severe weakness, coma, subnormal temperature, a cold clammy skin, pale mucosae, wetness around the mouth, collapse of superficial veins, slowness and irregularity of the heart, mild convulsive movements, and periodic apnea. Diarrhea is usually not evident, although the abdomen may be slightly distended and succussion and auscultation may reveal fluid-splashing sounds suggesting a fluid-filled intestine.⁴¹ The prognosis for these calves is poor and they commonly die 2–6 hours after the onset of signs.

In the more common form of the disease in calves, there is diarrhea in which the feces are profuse and watery to pasty, usually pale yellow to white in color, and occasionally streaked with blood flecks and very foul-smelling. The dry matter content of the feces is commonly below 10%. Defecation is frequent and effortless, and the tail and perineum are soiled with feces. The temperature is usually normal in the initial stages but becomes subnormal as the disease worsens. Affected calves may or may not suck or drink depending on the degrees of acidosis, dehydration, and weakness. Calves under 8 days of age may be weak, primarily from the effects of rapid and severe dehydration; in calves older than 8 days the acidosis may be more severe and makes a greater contribution to depression and weakness. In the early stages of the disease, the abdomen may be slightly distended as a result of distension of fluid-filled intestines, which can be detected by succussion and auscultation of the abdomen. In some of these calves the diarrhea is not obvious but is delayed for several hours, when it can be quite profuse. Mildly to moderately affected calves may be diarrheic for a few days and recover spontaneously with or without treatment. However, 15–20% of calves with enteric colibacillosis become progressively worse over a period of 3–5 days, gradually become more weak, lose the desire to suck and progressively appear more obviously clinically dehydrated.

Throughout the course of the diarrhea the degree of dehydration will vary from just barely detectable clinically (4–6% body weight (BW)) to up to 10–16% of body weight. It is best assessed by ‘tenting’ the skin of the lateral portion of the cervical region and measuring the time required for the skin fold to return to normal.⁵⁶ In calves with 8% of dehydration, 5–10 seconds will be required for the skin fold to return to normal; in 10–12% dehydration up to 30 seconds. Recession of the eyeball (enophthalmos) is a reliable and obvious indication of the degree of dehydration. Slight sinking of the eyeball without an obvious space between the eyeball and the orbit represents 6–8% dehydration; moderate separation of the eyeball from the orbit represents 9–12% dehydration; and marked separation of the eyeball from the orbit represents over 12% and up to 16% dehydration. A summary of the relationship between degree of dehydration (% BW), depth of enophthalmos (mm), cervical skin tent duration in seconds and the state of the mucous membranes and extremities in calves with experimentally induced diarrhea is set out in Table 18.4.⁵⁶

Table 18.4 Degree of dehydration in calves with experimentally induced diarrhea

Death usually occurs in 3–5 days. Affected calves can lose 10–16% of their original body weight during the first 24–48 hours of the diarrhea. The hyperkalemia in calves with neonatal diarrhea and acidosis has been associated with cardiac rate and rhythm abnormalities including bradycardia and atrial standstill. Herd outbreaks of the disease in beef calves may last for several weeks, during which time almost every calf will be affected within several days after birth.

Veal calf hemorrhagic enteritis is a fatal syndrome of veal calves characterized by anorexia, fever, diarrhea with mucus-containing feces which become bloody in the later stages, and hemorrhagic diathesis on the conjunctivae and mucous membranes of the mouth and nose. The etiology is unknown; the E. coli strains isolated from the feces of affected calves produced enterotoxins and verocytotoxins but their significance is uncertain.

In some calves between 10 and 20 days of age with a history of diarrhea in the previous several days, from which they have recovered, there will be metabolic acidosis without clinical signs of dehydration.⁴⁵ Affected calves are depressed, weak, ataxic and sometimes recumbent, and appear comatose. Affected calves respond quickly to treatment with intravenous sodium bicarbonate. A similar syndrome occurs in goat kids.

Coliform septicemia

This is uncommon but occurs in piglets within 24–48 hours of birth. Some are found dead without any premonitory signs. Usually more than one, and sometimes the entire litter, are affected. Severely affected piglets seen clinically are weak, almost comatose, appear cyanotic, and feel cold and clammy and have a subnormal temperature. Usually there is no diarrhea. The prognosis for these is poor and most will die in spite of therapy.

Enterotoxigenic colibacillosis (newborn piglet diarrhea)

This is the most common form of colibacillosis in piglets and occurs from 12 hours of age up to several days of age, with a peak incidence at 3 days of age. As with the septicemic form, usually more than one pig or the entire litter is affected. The first sign usually noticed is the fecal puddles on the floor. Affected piglets may still nurse in the early stages but gradually lose their appetite as the disease progresses. The feces vary from a pasty to watery consistency and are usually yellow to brown in color. When the diarrhea is profuse and watery there will be no obvious staining of the perineum and hindquarters with feces but the tails of the piglets will be straight and wet. The temperature is usually normal or subnormal. The disease is progressive; diarrhea and dehydration continues, the piglets become very weak and lie in lateral recumbency and make weak paddling movements. Within several hours they appear very dehydrated and shrunken, and commonly die within 24 hours after the onset of signs. In severe outbreaks the entire litter may be affected and die within a few hours of birth. The prognosis is favorable if treatment is started early before significant dehydration and acidosis occur.

Harleco apparatus

The acid–base status of individual calves can be determined in the field using a simple total carbon dioxide apparatus, which provides values close to blood bicarbonate concentrations.⁶⁰ Unlike the blood gas machines, the Harleco apparatus offers clinicians in practice an affordable means of diagnosing metabolic acidosis.⁶¹ The Harleco measures ‘total carbon dioxide’, the carbon dioxide liberated from a blood sample by adding strong acid. Nearly all the carbon dioxide is derived from bicarbonate rather than blood carbon dioxide. The CO₂ liberated is derived from bicarbonate and from dissolved CO₂, but there is so little dissolved CO₂ in blood that the Tco₂ may be taken for clinical purposes as a measurement of plasma bicarbonate. The level of acidosis in calves can be classified according to the Harleco readings (total carbon dioxide, mmol/L) as follows:

The strong ion difference method of evaluating acid–base balance in cattle has been described and justified to identify the mechanism for a change in acid–base balance and thereby to focus treatment on the inciting cause, and to quantify unmeasured strong anion concentration in plasma.⁶² A comparison of the measurement of total carbon dioxide and strong ion difference (SID) for the evaluation of metabolic acidosis in calves did not find any advantage to the SID.⁶¹

Portable pH meter

A portal pH meter has been evaluated to measure blood pH in neonatal calves.⁶³ Compared to a blood gas analyzer, the portable pH meter was more accurate in measuring urine pH and ruminal fluid pH in cows than blood pH in neonatal calves.

Coliform septicemia

Enteric colibacillosis

Categorizing diarrheic calves into treatment groups

Based on the history and clinical findings, affected calves can be divided into categories according to the type of therapy required and which is most economical.

The details for parenteral and oral fluid and electrolyte therapy are described here.

Parenteral fluid and electrolyte therapy

In severe dehydration and acidosis, solutions containing the bicarbonate ion are indicated.

Oral fluid and electrolyte therapy

Oral fluid and electrolyte therapy are indicated for calves in the early stages of diarrhea or after they have been successfully hydrated following parenteral fluid therapy. Severely dehydrated or moribund calves may not respond favorably to oral fluid therapy alone. Owners must be encouraged to provide oral fluid and electrolyte therapy to diarrheic neonatal farm animals as soon as possible after the onset of diarrhea. Almost all the information available on oral fluids has been developed following clinical studies in diarrheic calves, and the recommendations here reflect those studies.⁷²

In neonatal calves, the ideal oral electrolyte solution should: (a) supply sufficient sodium to facilitate normalization of extracellular fluid deficits; (b) provide two or more agents (such as glucose, acetate, propionate, or glycine) that facilitate intestinal absorption of sodium and water; (c) provide an alkalinizing agent (such as acetate, propionate, citrate, or bicarbonate) to treat the metabolic acidosis often present in dehydrated diarrheic calves; (d) not interfere with milk clotting in the abomasum; (e) provide sufficient energy, as these electrolyte solutions may be administered instead of milk or milk replacer for short periods of time; and (f) facilitate repair of damaged intestinal epithelium.⁷³

Acetate and propionate are the preferred alkalinizing agents for treating dehydrated calves with mild metabolic acidosis. Acetate and propionate have similar alkalinizing ability to bicarbonate on an equimolar basis, with the advantage that acetate and propionate produce energy. Acetate and propionate also stimulate sodium and water absorption in the calf small intestine, but in the jejunum to a greater extent than the ileum. Acetate and propionate are readily metabolized by both fed and fasted calves. Moreover, acetate and propionate are metabolized by peripheral tissues, are not produced endogenously in shock and dehydration (as is lactate) and do not have an unmetabolized isomer (d-lactate). Finally, acetate and propionate do not alkalinize the abomasum and intestine, whereas bicarbonate can permit bacteria to proliferate in an alkalinized abomasum, while also inhibiting the normal clotting of milk.

The ideal electrolyte concentration, osmolarity and energy source for an oral electrolyte solution to treat neonatal calf diarrhea remain controversial. The optimal solution should have a sodium concentration between 60 and 120 mmol/L, a potassium concentration between 10 and 20 mmol/L, a chloride concentration between 40 and 80 mmol/L, 40–80 mmol/L of metabolizable (nonbicarbonate) base, such as acetate or propionate, and glucose as an energy source. Some investigators have suggested that the optimal oral electrolyte solution required a higher sodium concentration (120–133 mmol/L) to rapidly correct extracellular electrolyte and fluid losses that typically develop in calves with diarrhea and dehydration. Combined administration of sodium and glucose is beneficial because glucose facilitates sodium absorption via the small intestinal sodium/glucose co-transport mechanism.

Oral fluid and electrolyte therapy is effective in enteric colibacillosis of neonatal farm animals because glucose continues to be absorbed by the small intestine by an active transport mechanism accompanied by glucose-coupled sodium absorption and absorption of water. In enterotoxigenic colibacillosis, while there is net hypersecretion caused by the enterotoxin, the intestinal mucosa is sufficiently intact so that water and sodium will be absorbed in the presence of glucose. Most oral electrolytes intended for the treatment of dehydration and acidosis in diarrheic calves contain sodium chloride and sodium bicarbonate with one or some of the following substances: glucose, acetates and citrates, phosphates, potassium salts, glycine and amino acids.

Several commercial oral preparations are available. The ideal solutions should be palatable and provide rapid rehydration and correction of acidosis. Diarrheic calves must receive sufficient fluid therapy to compensate for the fluid and electrolyte losses that occur and for maintenance and contemporary losses during the period of clinical diarrhea and convalescence. However, the calf must be returned to a milk diet within a few days in order to avoid the effects of malnutrition. Oral fluid and electrolyte formulas cannot provide the daily maintenance requirements of energy, protein, and fat. Some formulas contain a large quantity of glucose and are supplemented with glycine, sodium acetate, and citric acid, thus making a calorie-dense, partially nitrogen-balanced hyperosmolal solution. A glutamine-containing oral rehydration solution for the treatment of experimental E. coli calf diarrhea was more effective in correcting and sustaining plasma, extracellular fluid, and blood volume compared with standard World Health Organization solutions without the glutamine.⁷⁴

There are marked variations in the alkalinizing abilities of the oral electrolyte solutions that are available commercially.⁷² Those that contain at least 80 mmol/L of bicarbonate are much more effective for the rapid correction of acidosis and depression in diarrheic calves than administration of rehydrating electrolyte solutions alone. The bicarbonate-rich preparations have the best alkalinizing response when given to calves affected with acidosis from experimentally induced diarrhea. Oral electrolyte solutions containing acid phosphate salts are undesirable because they cause net acidification of the blood, with a fall in blood pH. One possible theoretical disadvantage of an alkaline electrolyte solution (containing sodium bicarbonate) is that it may prolong the clotting of milk by rennin in the abomasum, which could cause maldigestion and prolong the diarrhea, especially if the oral fluids and electrolytes are diluted 1:1 with whole milk.

Several commercially available oral electrolyte solutions contain metabolizable bases such as acetate instead of bicarbonate and are formulated to correct and maintain acid–base imbalance and maintain normal milk digestion.⁷² Those preparations that contain either bicarbonate or acetate provide good therapeutic results and have been evaluated in experimentally induced diarrheic calves, which were also offered milk at the rate of 5% of their body weight twice daily.⁷² Offering milk along with the oral electrolytes is effective in maintaining body weight in experimental calves. No scientific information is available on the effects of milk and oral electrolytes in naturally occurring cases. Those preparations not containing any alkalinizing agent do not correct the acid–base imbalance and result in poor recovery rates in experimental calves.

Ideally, the oral fluids and electrolytes should not be mixed with milk but fed or offered between feedings of milk. The oral fluids should be given by nipple bottle if the animal will suck but the use of a stomach tube or esophageal feeder is satisfactory. Fluids can be administered to the dehydrated calf using an esophageal feeder, even though the reticular groove does not close. At least 2 L of fluid should be given, which results in a transfer of fluids from the rumen to the abomasum. If larger doses are required they should be divided and given at intervals of 2 hours or more to avoid abdominal discomfort from distension.

Fecal consistency is not a highly reliable criterion of success in the evaluation of oral fluid therapy for calf diarrhea. Calves with the greatest improvement in fecal consistency had no greater increase in plasma volume than calves that did not improve their fecal consistency. Thus improvement of the feces may not offer a reliable guide to the rehydration of the calf.

Response to therapy

Calves that respond and recover usually show marked improvement from intravenous and/or oral fluid therapy within 24–36 hours. Calves that respond to the hydration therapy begin to urinate within an hour after fluid administration is begun, and usually maintain hydration thereafter. Calves that do not respond will not hydrate normally; they may not begin to urinate because of irreversible renal failure, their feces remain watery, they remain depressed and not strong enough to suck or drink, and continued fluid therapy beyond 3 days is usually futile. Those calves that respond favorably to the initial fluid therapy and then become depressed 12–18 hours later may have returned to a state of metabolic acidosis. Mental depression, the loss of the suck reflex and muscular weakness are indications of acidosis, and affected calves will require retreatment with sodium bicarbonate.

Overhydration and overinfusion at too rapid an administration rate increases intravascular pressure, resulting in pulmonary edema characterized by hyperpnea, tachypnea, tachycardia, and death.

Change in small intestinal bacterial flora in calves with diarrhea

There has been a paradigm shift in the last 40 years towards categorizing an episode of calf diarrhea as being due to a specific etiologic agent, such as rotavirus, coronavirus, cryptosporidia, Salmonella spp., or enterotoxigenic E. coli. While the etiologic approach has correctly focused attention on preventive programs, including vaccination and optimizing transfer of colostral immunity, the approach has diverted attention from the universal finding of all studies, that calves with diarrhea have coliform bacterial overgrowth of the small intestine.⁷⁵

Studies completed more than 70 years ago documented increased numbers of E. coli bacteria in the abomasum, duodenum, and jejunum of scouring calves. Moreover, calves severely affected with diarrhea had increased numbers of E. coli bacteria in the anterior portion of their intestinal tracts compared to mildly affected animals. More recent studies have consistently documented the fact that calves with naturally acquired diarrhea, regardless of age and the etiological cause for the diarrhea, have altered small intestinal bacterial flora. Specifically, E. coli bacterial numbers are increased 5–10 000-fold in the duodenum, jejunum, and ileum of calves with naturally acquired diarrhea, even when the diarrhea was not due to enterotoxigenic strains of E. coli, and where rotavirus and coronavirus were identified in the feces. The largest increase in E. coli bacterial numbers occurs in the distal jejunum and ileum, whereas the E. coli or coliform bacterial numbers in the colon and feces are similar or higher for calves with diarrhea than calves without diarrhea, with E. coli being more numerous in the feces of colostrum-deprived than colostrum-fed calves. Small intestinal overgrowth with coliform bacteria can persist after departure of the initiating enteric pathogen.

In calves with naturally acquired diarrhea, increased small-intestinal colonization with E. coli has been associated with impaired glucose, xylose, and fat absorption.

Mixed infections with enteric pathogens are commonly diagnosed in calves with naturally acquired diarrhea, and the clinical signs and pathological damage associated with rotavirus infection are more severe when E. coli is present than when it is absent. Primary viral morphologic damage to the small intestine also facilitates systemic invasion by normal intestinal flora, including E. coli.

In calves with experimentally induced enterotoxigenic E. coli diarrhea, colonization of the small intestine by E. coli has been associated with impaired glucose and lactose absorption, decreased serum glucose concentration and possibly increased susceptibility to cryptosporidial infection.

In summary, calves with diarrhea have increased coliform bacterial numbers in the small intestine, regardless of etiology, and this colonization is associated with altered small-intestinal function, morphological damage, and increased susceptibility to bacteremia. It therefore follows that administration of antimicrobials that decrease small intestinal coliform bacterial numbers in calves with diarrhea may prevent the development of bacteremia, decrease mortality, and decrease morphological damage to the small intestine, thereby facilitating digestion and absorption and increasing growth rate.

Incidence of bacteremia in calves with diarrhea

Calves with diarrhea are more likely to have failure of transfer of passive immunity or partial failure of transfer of passive immunity, and this group of calves, in turn, is more likely to be bacteremic. This is an additional reason that antimicrobials may be indicated in the treatment of calf diarrhea. Colostrum-deprived calves that subsequently developed diarrhea were frequently bacteremic, whereas bacteremia occurred much less frequently in colostrum-fed calves that developed diarrhea.

Based on field studies of diarrheic calves, it can be assumed that, on average, 30% of severely ill calves with diarrhea are bacteremic, that the risk of bacteremia is higher in calves with failure of transfer of passive immunity than in calves with adequate transfer of passive immunity, and that the risk of bacteremia is higher in calves 5 days of age or older. The frequency of bacteremia is sufficiently high that treatment of calves with diarrhea that are severely ill (as manifest by reduced suck reflex, > 6% dehydration, weakness, inability to stand or clinical depression) should include routine treatment against bacteremia, with emphasis on treating potential E. coli bacteremia. Veterinarians should also assume that 8–18% of diarrheic calves with adequate transfer of passive immunity and systemic illness are bacteremic. The prevalence of bacteremia is sufficiently high in systemically ill calves that effective antimicrobial treatment for potential bacteremia should be routinely instituted, regardless of transfer of passive immunity status and treatment cost. Withholding an effective treatment for a life-threatening condition, such as bacteremia in calves with diarrhea, cannot be condoned on animal welfare grounds.

Antimicrobial susceptibility of fecal Escherichia coli isolates

The most important determinant of antimicrobial efficacy in calf diarrhea is obtaining an effective antimicrobial concentration against bacteria at the sites of infection (small intestine and blood). The results of fecal antimicrobial susceptibility testing have traditionally been used to guide treatment decisions; however, susceptibility testing in calf diarrhea probably has clinical relevance only when applied to fecal isolates of enterotoxigenic strains of E. coli or pathogenic Salmonella spp., and blood culture isolates from calves with bacteremia. Validation of susceptibility testing as being predictive of treatment outcome for calves with diarrhea is currently lacking.

The ability of fecal bacterial culture and antimicrobial susceptibility testing using the Kirby Bauer technique to guide treatment in calf diarrhea is questionable when applied to fecal E. coli isolates that have not been identified as enterotoxigenic.⁷⁵ There do not appear to be any data demonstrating that fecal bacterial flora is representative of the bacterial flora of the small intestine, which is the physiologically important site of infection in calf diarrhea. Marked changes in small-intestinal bacterial populations can occur without changes in fecal bacterial populations, and the predominant strain of E. coli in the feces of a diarrheic calf can change several times during the diarrhea episode. Furthermore, and most importantly, 45% of calves with diarrhea had different strains of E. coli isolated from the upper and lower small intestine, indicating that fecal E. coli strains are not always representative of small-intestinal E. coli strains.

An additional bias present in most antimicrobial susceptibility studies conducted on fecal E. coli isolates is that data are frequently obtained from dead calves, which are likely to be treatment failures. The time since death is also likely to be an important determinant of the value of fecal culture, because such a rapid proliferation of bacteria occurs in the alimentary tract after death that the results of examinations made on dead calves received at the laboratory can have little significance. Calves that die from diarrhea are likely to have received multiple antimicrobial treatments, and preferential growth of antimicrobial-resistant E. coli strains starts within 3 hours of antimicrobial administration. A further concern with fecal susceptibility testing is that the Kirby Bauer breakpoints (minimum inhibitory concentration (MIC)) are not based on typical antimicrobial concentrations in the small intestine and blood of calves. What are urgently needed are studies documenting the antimicrobial susceptibility of E. coli isolates from the small intestine of untreated calves, based on achievable drug concentrations and dosage regimens. Until these data are available, it appears that antimicrobial efficacy is best evaluated by the clinical response to treatment, rather than the results of in vitro antimicrobial susceptibility testing performed on fecal E. coli isolates.

A comparison of antibiotic resistance for E. coli populations isolated from groups of diarrheic and control calves in the UK found a higher incidence of antibiotic-resistant E. coli in samples obtained from farms with calf diarrhea than from farms without the disease.⁷⁶ Considering all samples, bacterial colonies in 84% were resistant to ampicillin, in 13% to apramycin and in 6% to nalidixic acid. Antibiotic resistance among enterotoxigenic E. coli from piglets and calves with diarrhea in a diagnostic laboratory survey of one geographic area in Canada over a 13-year period found that least resistance occurred against ceftiofur for all, followed by apramycin and gentamicin for porcine and florfenicol for bovine isolates.⁷⁷

Passive surveillance for antimicrobial resistance in E. coli isolates from diarrheic food animals continues to indicate that resistance to many of the common antimicrobials is increasing. In a UK study over a 5-year period, E. coli isolates from calves with diarrhea become more resistant to furazolidone, trimethoprim-potentiated sulfonamide, clavulanic-acid-potentiated amoxicillin, and tetracycline.⁷⁸ E. coli strains from outbreaks of diarrhea in lambs in Spain became increasingly resistant to nalidixic acid, enoxacin, and enrofloxacin on the basis of the National Committee on Clinical Laboratory Standards (NCCL) breakpoints for human and animal isolates.^79,80 Among the fluoroquinolones used for treatment of domestic animals in Spain, enrofloxacin is approved for use for the treatment of colibacillosis and diarrhea in lambs. However, the bovine and ovine strains of E. coli that possess potential virulence factors were more sensitive to quinolones than those that do not express those factors. Some antimicrobial-resistant E. coli strains from diarrheic calves in the USA may possess a chromosomal flo gene that specifies cross-resistance to both florfenicol and chloramphenicol, and its presence among E. coli isolates of diverse genetic backgrounds indicates a distribution much wider than previously thought.⁸¹ In Spain, 5.9% of E. coli strains from cattle were resistant to nalidixic acid and 4.9% were resistant to enrofloxacin and ciprofloxacin.⁸² In sheep and goats only 0.5% and 1.4%, respectively, of the strains were resistant to nalidixic acid and none to fluoroquinolones. Most of the quinolone-resistant strains were nonpathogenic strains isolated from cattle.

The CTX-M-14-like enzyme has been detected in E. coli recovered from the feces of diarrheic dairy calves in Wales.⁷⁴ The enzyme is an extended-spectrum beta-lactamase (ESBL), which confers resistance to a wide range of beta-lactam (penicillin and cephalosporin) compounds. Organisms possessing ESBLs are considered to be resistant to second-, third-, and fourth-generation cephalosporins, and in vitro resistance to amoxicillin/clavulanate among producers is variable, reflecting the amount of beta-lactamase produced. In addition to this enzyme, the isolates produced a TEM-35 (IRT-4) beta-lactamase that conferred resistance to the amoxicillin/clavulanate combination. These two enzymes confer resistance to all the beta-lactamase compounds approved for veterinary use in the UK. Thus their occurrence in animals may be an important development for both animal and public health. ESBLs in human infections have emerged as a significant and developing problem, occurring in patients in the community as well as in those with recent hospital contact. Spread of this form of resistance in bacteria affecting the animal population could have serious implications for animal health, rendering many therapeutic options redundant.

Antibiotic resistance to intestinal bacteria also occurs in dairy calves fed milk from cows treated with antibiotics.⁸³ The resistance increases with higher concentrations of antibiotics in the milk.

Antimicrobial susceptibility of blood Escherichia coli isolates

The Kirby Bauer technique for antimicrobial susceptibility testing has more clinical relevance for predicting the clinical response to antimicrobial treatment when applied to blood isolates rather than fecal isolates.⁷⁵ This is because the Kirby Bauer breakpoints (MICs) are based on achievable antimicrobial concentrations in human plasma and MIC₉₀ values for human E. coli isolates, which provide a reasonable approximation to achievable MIC values in calf plasma and MIC₉₀ values for bovine E. coli isolates. Unfortunately, susceptibility results are not available for at least 48 hours, and very few studies have documented the antimicrobial susceptibility of blood isolates in calves with diarrhea. In a 1997 study of dairy calves in California, the antimicrobial susceptibility of isolates from the blood of calves with severe diarrhea or illness produced the following results; ceftiofur, 19/25 (76%) sensitive; potentiated sulfonamides, 14/25 (56%) sensitive; gentamicin, 12/25 (48%) sensitive; ampicillin, 11/25 (44%) sensitive; tetracycline, 3/25 (12%) sensitive, although there was a clinically significant year-to-year difference in the results of susceptibility testing that may have reflected different antimicrobial administration protocols on the farm.⁷⁵

Evidence-based recommendations for antimicrobial treatment of diarrheic calves

The four critical measures of success of antimicrobial therapy in calf diarrhea are (in decreasing order of importance): (1) mortality rate, (2) growth rate in survivors, (3) severity of diarrhea in survivors, and (4) duration of diarrhea in survivors.

Success of antimicrobial therapy can vary with the route of administration and whether the antimicrobial is dissolved in milk, oral electrolyte solutions, or water. Oral antimicrobials administered as a bolus or contained in a gelatin capsule are deposited into the rumen and therefore have a different serum concentration–time profile from antimicrobials dissolved in milk replacer that are suckled by the calf or administered as an oral drench at the back of the pharynx. Antimicrobials that bypass the rumen are not thought to alter rumen microflora, potentially permitting bacterial recolonization of the small intestine from the rumen. Finally, when oral antimicrobials are administered to calves with diarrhea, the antimicrobial concentration in the lumen of the small intestine is lower and the rate of antimicrobial elimination faster than in healthy calves.

Orally administered amoxicillin, chlortetracycline, neomycin, oxytetracycline, streptomycin, sulfachloropyridazine, sulfamethazine, and tetracycline are currently labeled in the USA for the treatment of calf diarrhea. No parenteral antimicrobials have a label claim in the USA for treating calf diarrhea.

In his review of the literature on the use of antibiotics for the treatment of calf diarrhea, Constable⁷⁵ concluded that recommendations by some veterinarians that oral or parenteral antimicrobials should not be used were not supported by a critical evidence-based review of the literature. The arguments used to support a nonantimicrobial treatment approach have included:

Constable⁷⁵ concluded that antimicrobial treatment of diarrheic calves should be practiced and focused against E. coli in the small intestine and blood, as these constitute the two sites of infection. Fecal bacterial culture and antimicrobial susceptibility testing is not recommended in calves with diarrhea, because fecal bacterial populations do not accurately reflect small-intestinal or blood bacterial populations and the breakpoints for susceptibility test results have not been validated. Antimicrobial efficacy is therefore best evaluated by the clinical response to treatment.

In the USA parenterally administered oxytetracycline and sulfachloropyridiazine and orally administered amoxicillin, chlortetracycline, neomycin, oxytetracycline, streptomycin, sulfachloropyridazine, sulfamethazine, and tetracycline are currently labeled for the treatment of calf diarrhea. Unfortunately, there is little published data supporting their efficacy in treating calves with diarrhea.⁷⁵ Extralabel antimicrobial use (excluding prohibited antimicrobials) is therefore justified in treating calf diarrhea because of the apparent lack of published studies documenting clinical efficacy of antimicrobials with a label claim, and because the health of the animal is threatened and suffering or death may result from failure to treat systemically ill calves.

Because the two sites of infection in calf diarrhea are the small intestine and blood, administered antimicrobials should have both local (small intestinal) and systemic effects. In addition, the antimicrobial must reach therapeutic concentrations at the site of infection for a long enough period and, ideally, have only a narrow Gram-negative spectrum of activity in order to minimize collateral damage to other enteric bacteria. In general, oral and parenteral administration of broad-spectrum beta-lactam and fluoroquinolone antimicrobials have proven efficacy in treating naturally acquired and experimentally induced diarrhea, parenteral administration of trimethoprim–sulfadiazine has proven efficacy in treating experimentally induced Salmonella enterica serotype Dublin (although efficacy has only been demonstrated when antimicrobial administration starts before diarrhea is present), and oral administration of the predominantly Gram-negative-spectrum antimicrobial apramycin has proven efficacy in treating naturally acquired diarrhea. Because use of fluoroquinolone antimicrobials in an extralabel manner is illegal in the USA, and apramycin is an aminocyclitol antimicrobial that is poorly absorbed after oral administration (oral bioavailability <15%) and has relatively high MIC values against Salmonella spp. and E. coli (MIC₉₀ > 3 μg/mL) in the calf, treatment recommendations will focus on the use of broad-spectrum beta-lactam antimicrobials such as amoxicillin, ampicillin, ceftiofur, and potentiated sulfonamides (trimethoprim– sulfadiazine).

Dairy calves

These comments are directed particularly at calves born indoors, where contamination is higher than outdoors.

Beef calves

These are usually born on pasture or on confined calving grounds.

In beef herds, breeding plans should insure that heifers calve at least 2 weeks before the mature cows. Limiting the breeding and therefore the calving season to 45 days or less for heifers also offers several advantages. A short calving season allows the producer to more effectively and economically concentrate personnel resources to the calving management compared to a longer calving season. Calving heifers earlier allows them additional time required before the next breeding season to be on an increasing plane of nutrition necessary to maintain a high conception rate. The earlier calving of heifers also provides less exposure of their calves to infection pressure from the mature animals in the herd.

The incidence and severity of neonatal disease will typically increase, and the age at disease onset will decrease as the calving season progresses. This phenomenon is common in beef herds because of the effect of the calf as a biological amplifier. The more the calving season is shortened, the more the biological amplification effect is negated.

For beef herds, it is necessary to have a plan for cattle movement throughout the calving season. This requires a minimum of four or five separate pastures. These include a gestation pasture, a calving pasture and a series of nursery pastures. To ensure that beef calves are born in a sanitary environment, the herd should be moved from the gestation pasture to the calving pasture 1–2 weeks before calving. One day after birth, the cow or heifer and her calf should be moved to a nursery pasture. Cow–calf pairs should be added to a single nursery pasture until the appropriate number of pairs has been reached. Thereafter, cow–calf pairs can be added to a second nursery pasture. The difference in age between the oldest and youngest calf in a nursery pasture should never exceed 30 days, and smaller differences are preferable. This negates the biological amplification effect. The longer the calving season, the greater the need for a large number of nursery pastures. Calves that develop diarrhea should be removed immediately to an area away from healthy calves, treated and not returned until all calves in the group are at low risk for developing diarrhea (> 30 days of age).

The nutrition of the pregnant cows, and particularly the first-calf heifers, must be monitored through gestation to insure an adequate body condition and sufficient resources to provide an adequate supply of good-quality colostrum.

Veal calves

These calves are usually obtained from several different sources and 25–30% or higher may be deficient in serum immunoglobulin.

Lambs and kids

The principles described above for calves apply to lambs and kids. Lambing sheds can be a source of heavy contamination and must be managed accordingly to reduce infection pressure on newborn lambs.

Piglets

Piglets born in a total-confinement system may be exposed to a high infection rate.

Colostrum management

The next most important control measure is to insure that liberal quantities of good-quality colostrum are available and ingested within minutes and no later than a few hours after birth. While the optimum amount of colostrum that should be ingested by a certain time after birth is well known, the major difficulty with all species under practical conditions is to know how much colostrum a particular neonate has ingested. Because modern livestock production has become so intensive, it is imperative that the animal attendants make every effort to insure that sufficient colostrum is ingested by that particular species. In one study, in large dairy herds, 42% of calves left with their dams for 1 day following birth had failed either to suck sufficient colostrum or to absorb sufficient colostral immunoglobulins.

Failure of transfer of passive immunity, as determined by calf serum immunoglobulin IgG₁ concentration below 10 mg/mL at 48 hours of age, occurred in 61.4% of calves from a dairy in which calves were nursed by their dams, 19.3% of calves from a dairy using nipple-bottle feeding and 10.8% of calves from a dairy using tube feeding.⁸ A higher prevalence of failure of transfer of passive immunity in dairy calves can occur because an insufficient volume of colostrum is ingested by the calf. When artificial feeding is used, inadequate immunoglobulin concentration in the colostrum fed is the most important factor resulting in failure of transfer of passive immunity.⁸ The prevalence of failure of transfer of passive immunity in dairy herds can be minimized by artificially feeding all newborn calves large volumes (3–4 L) of fresh or refrigerated first-milking colostrum from cows that had nonlactating intervals of normal duration. This volume is considerably greater than the intake that Holstein calves usually achieve by sucking, and also exceeds the voluntary intake of most calves fed colostrum by nipple bottle.

Calves need to ingest at least 100 g IgG₁ in the first colostrum feeding to insure adequate transfer of passive immunity. Thus the routine force-feeding of a sufficient amount of pooled colostrum immediately after birth results in high serum levels of colostrum immunoglobulins in dairy calves and is becoming a common practice in dairy herds.

Encouraging and assisting the calf to suck within 1 hour after birth is also effective. The provision of early assisted sucking of colostrum to satiation within 1 hour after birth will result in high concentrations of absorbed immunoglobulins in the majority of calves. The ingestion of 100 g or more of colostral immunoglobulins within a few hours after birth is more effective in achieving high levels of colostral immunoglobulins in calves than either leaving the calf with the cow for the next 12–24 hours or encouraging the calf to suck again at 12 hours, which will not result in a significant increase in absorbed immunoglobulins.

Despite early assisted sucking, a small proportion of calves will remain hypogammaglobulinemic because of low concentrations of immunoglobulins in their dams’ colostrum, usually associated with leakage of colostrum from the udder before calving.

In large herds where economics permit, a laboratory surveillance system may be used on batches of calves to determine the serum levels of immunoglobulin acquired. An accurate analysis may be done by electrophoresis or an estimation using the zinc sulfate turbidity test. Blood should be collected from calves at 24 hours of age. Samples taken a few days later may not be a true reflection of the original serum immunoglobulin levels. The information obtained from determination of serum immunoglobulins in calves at 24 hours of age can be used to improve management practices, particularly the early ingestion of colostrum.

Quality of colostrum

Colostrum supplements and replacers

Some colostrum-derived oral supplements containing immunoglobulin are available for newborn calves in which colostrum intake is suspected or known to be inadequate. Colostrum supplement products have been developed to provide exogenous IgG to calves when the dam’s fresh colostrum is of low IgG concentration. Many producers also use these products to replace colostrum when it is unavailable as a result of maternal agalactia, acute mastitis or other causes of inadequate colostrum supply. However, they contain low immunoglobulin concentrations compared to those found in high-quality first-milking colostrum. Most colostral supplements provide only 25–45 g IgG/dose of 454 g, which is reconstituted in 2 L of water. Feeding one or even two doses of such supplements is insufficient to provide a mass of 100 g of IgG within the first 12 hours after birth. Colostrum replacers are intended to provide the sole source of IgG and thus must provide at least 100 g IgG. Newborn colostrum-deprived dairy calves fed spray-dried colostrum containing 126 g of immunoglobulin in 3 L of water as their sole source of Ig achieved normal mean serum immunoglobulin concentrations.⁹¹ Whey protein concentrate as a colostrum substitute, administered to calves as a single feeding, was ineffective in preventing neonatal morbidity and mortality compared with a single feeding of pooled colostrum.⁹²

The IgG derived from bovine serum or Ig concentrates from bovine plasma are well absorbed by neonatal calves when given in adequate amounts.^93,94 The serum concentration of IgG in calves at 2 days of age force-fed a colostrum supplement containing spray-dried serum (total of 90 g immunoglobulin protein) within 3 hours after birth was much lower than in calves fed 4 L of fresh colostrum.⁹⁵ The mass of IgG and the method of processing are critical. Products providing less than 100 g of IgG/dose should not be used to replace colostrum.

To be successful, colostral supplements and replacers must provide enough IgG mass, which results in 24-hour calf serum IgG concentrations of more than 10 g/L.

Purified bovine immunoglobulin

The administration of purified bovine gammaglobulin to calves that are deficient appears to be a logical approach but the results have been unsuccessful. Large doses (30–50 g) of gammaglobulin given intravenously would be required to increase the level of serum gammaglobulin from 0.5 g/dL to 1.5 g/dL of serum, which is considered an adequate level. The cost would be prohibitive. The administration of gammaglobulins by any parenteral route other than the intravenous route does not result in a significant increase in serum levels of the immunoglobulin.

To be effective, infusion of immunoglobulin derived from blood must increase serum IgG concentrations and reduce morbidity and mortality prior to weaning without affecting later production. Parenteral infusions of immunoglobulins will increase the concentrations of serum IgG in calves but may not necessarily have an effect on morbidity or mortality.⁹⁶ High levels of specific circulating Ig can serve as a reservoir of antibody to move into the intestine and prevent enteric infection.⁹⁷ Thus immunoglobulin sources other than colostrum may not provide Ig that are specific for antigens present in the environment or might be insufficient when calves are exposed to a heavy infection pressure.

The other factors that influence the ingestion of colostrum and the absorption of immunoglobulin are presented in the section on the epidemiology of colibacillosis, above, and in Chapter 3.

Guidelines to insure transfer of colostral immunoglobulins

Some guidelines for insuring that newborn farm animals ingest sufficient quantities of colostrum and absorb adequate amounts of colostral immunoglobulins are summarized here. Additional details are presented in Chapter 3.

Calves

Vaccination of pregnant cattle with either purified E. coli K99⁺ pili or a whole-cell preparation containing sufficient K99⁺ antigen can significantly reduce the incidence of enterotoxigenic colibacillosis in calves. Good protection is also possible when the dams are vaccinated with a four-strain E. coli whole-cell bacterin containing sufficient K99⁺ pilus antigen and the polysaccharide capsular K antigen. Colostral antibodies specific for K99⁺ pilus antigen and the polysaccharide capsular K antigen on the surface of the challenge exposure strain of enterotoxigenic E. coli are protective. There is a highly significant correlation between the lacteal immunity to the K99⁺ antigen and the prevention of severe diarrhea or death in calves challenged with enterotoxigenic E. coli. The colostral levels of K99⁺ antibody are highest during the first 2 days after parturition, which is the most susceptible period for enterotoxigenic colibacillosis to occur in the newborn calf. The continuous presence of the K99⁺ antibody in the lumen of the intestine prevents adherence of the bacteria to the intestinal epithelium. The K99⁺ antibody is also absorbed during the period of immunoglobulin absorption and may be excreted into the intestine during diarrhea. This may be one of the reasons that mortality is inversely proportional to serum immunoglobulin levels. The pregnant dams are vaccinated twice in the first year, 6 and 2 weeks prior to parturition. Each year thereafter they are given a single booster vaccination. An oil emulsion E. coli K99⁺ bacterin given once or twice to pregnant beef cows 6 weeks before calving elicited high levels of serum antibody that provided protection against experimental infection of newborn calves for up to 87 weeks after vaccination.

Vaccines containing both the K99⁺ antigen of enterotoxigenic E. coli and rotavirus, and in some cases coronavirus, have been evaluated with variable results. The colostral antibodies to the K99⁺ antigen are higher in vaccinated than unvaccinated dams but the colostral antibodies to rotavirus and coronavirus may not be significantly different between vaccinated and unvaccinated dams. In these field trials vaccination had no effect on the prevalence of diarrhea, calf mortality or the presence of the three enteropathogens. In other field trials the combined vaccine did provide some protection against outbreaks of calf diarrhea. The use of an inactivated oil-adjuvanted rotavirus E. coli vaccine given to beef cows in the last trimester of pregnancy decreases the mortality from diarrhea and has a positive influence on the average weight gains of the calves at weaning. To be effective the rotavirus and coronavirus antibodies must be present in the postcolostral milk for several days after parturition during the period when calves are most susceptible to the viral infection. Vaccination of pregnant cows twice during the dry period at intervals of 4 weeks can increase the colostral antibody levels to E. coli K99⁺ by 26 times on day 1 compared to controls. Much lower increases occur at the levels of coronavirus and rotavirus.

A commercially inactivated vaccine containing bovine rotavirus (serotype G6 P5), bovine coronavirus (originally isolated from a calf with diarrhea) and purified cell-free E. coli FS (K99) (adsorbed on to aluminum hydroxide gel), formulated as an emulsion in a light mineral oil has been evaluated in a herd of Ayrshire/Friesian cows vaccinated once 31 days before the first expected calving date.¹⁰³ Compared to control cows, a significant increase in the mean specific antibody titer against all three antigens occurred in the serum of vaccinated animals (even in the presence of pre-existing antibody), which was accompanied by increased levels of protective antibody to rotavirus, coronavirus, and E. coli F5 (K99) in their colostrum and milk for at least 28 days.

Because naturally acquired antibodies to the J5 antigen may have an important role in the control of neonatal disease caused by bacterial infections with associated pathogens that share antigens with E. coli (J5 strain), vaccination of calves with an E. coli O111:B4(J5) vaccine at 1–3 days of age and 2 weeks later has been evaluated to control morbidity and mortality in dairy calves up to 60 days of age.²⁹ The use of either a killed E. coli O111:B4(J5) bacterin or a modified live, genetically altered (aro-) Salmonella dublin vaccine, or both in neonatal calves was effective in reducing mortality due to colibacillosis and salmonellosis.¹⁰⁴ Such a vaccine may be beneficial in controlling mortality in well-managed herds but is contraindicated in poorly managed herds.

Passive immunotherapy of calves under 2 days of age with J5 E. coli hyperimmune plasma given subcutaneously at a dose of 5 mL/kg BW has been examined.¹⁰⁵ The plasma was safe and potent. It was not superior to control plasma or to no treatment for calf morbidity and mortality.

The oral administration of a K99⁺-specific monoclonal antibody to calves during the first 12 hours after birth may be an effective method of reducing the incidence of fatal enterotoxigenic colibacillosis, particularly when outbreaks of the disease occur in unvaccinated herds. Clinical trials indicate that the severity of dehydration, depression, weight loss and duration of diarrhea were significantly reduced in calves that had received the K99⁺-specific monoclonal antibody. In experimentally challenged calves the mortality was 29% in the treated calves and 82% in the control calves.

The decision to vaccinate in any particular year will depend on the recognition of risk factors. Such risk factors include:

Piglets

Piglets born from gilts are more susceptible than those from mature sows, which suggests that immunity improves with parity. On a practical basis this suggests that gilts should be mixed with older sows that have been resident on the premises for some time. The length of time required for such natural immunization to occur is uncertain, but 1 month during late gestation seems logical.

Naturally occurring enteric colibacillosis in newborn piglets can be effectively controlled by vaccination of the pregnant dam. Field trials in large-scale farm conditions indicate that the vaccines are efficacious.¹⁰⁶ Partial budget analysis of vaccinating pregnant sows with E. coli vaccines revealed an economic return on investment of 124% because of the decrease in morbidity and mortality due to diarrhea in piglets 1–2 weeks of age.¹⁰⁷ Three antigen types of pili, designated K88⁺, K99⁺, and 987P, are now implicated in colonization of the small intestine of newborn piglets by enterotoxigenic E. coli. The vaccination of pregnant sows with oral or parenteral vaccines containing these antigens will provide protection against enterotoxigenic colibacillosis associated with E. coli bearing pili homologous to those in the vaccines. The parenteral vaccines are cell-free preparations of pili, and the oral vaccines contain live enteropathogenic E. coli. The oral vaccine is given 2 weeks before farrowing and is administered in the feed daily for 3 days as 200 mL per day of a broth culture containing 10¹¹ E. coli. A simple and effective method of immunization of pregnant sows is to feed live cultures of enterotoxigenic E. coli isolated from piglets affected with neonatal colibacillosis on the same farm. The oral vaccine can be given in the feed, beginning about 8 weeks after breeding and continued to parturition. The oral vaccine results in the stimulation of IgA antibody in the intestinal tract, which is then transferred to the mammary gland and into the colostrum. A combination of oral and parenteral vaccination is superior to either route alone. The parenteral vaccine is given about 2 weeks after breeding and repeated 2–4 weeks before parturition. The parenteral vaccination results in the production of high levels of IgM antibody for protection against both experimental and naturally occurring enterotoxigenic colibacillosis. This vaccination also reduces the number of E. coli excreted in the feces of vaccinated sows, which are major sources of the organism. Immunization of pregnant sows with an E. coli bacterin enriched with the K88⁺ antigen results in the secretion of milk capable of preventing adhesion of K88⁺ E. coli to the gut for at least 5 weeks after birth, at which time the piglet becomes naturally resistant to adhesion by the organism.

The possibility of selecting and breeding from pigs that may be resistant genetically to the disease is being explored. The highest incidence of diarrhea occurs in progeny of resistant dams and sired by susceptible sires. The homozygous dominants (SS) and the heterozygotes (Ss) possess the receptor and are susceptible whereas in the homozygous recessives (ss) it is absent and the pigs are resistant. Sows that are genetically resistant may not be able to mount an immune response to the K88⁺ antigen because of the inability of the organism to colonize the intestinal tract.

Lambs and kids

Vaccination of pregnant ewes with K99⁺ antigen will confer colostral immunity to lambs challenged with homologous enteropathogenic E. coli. The pregnant ewes are vaccinated twice in the first year, at 8–10 weeks and 2–4 weeks before lambing, and in the second year one vaccination 2–4 weeks before lambing is adequate.

Immunization of pregnant goats has been used to stimulate the development of lacteal immunity against naturally occurring colibacillosis in kids.¹⁰⁹ Vaccination of pregnant does 1 month before parturition with a subunit vaccine containing K88, K9, and 987P fimbrial antigens of E. coli, and C. perfringens types B, C, and D toxins in an aluminum hydroxide adjuvant, along with improved management conditions, was highly successful in reducing neonatal morbidity and mortality due to diarrhea.⁴⁰ Compared to two control groups, one in which no improvement in management was made and the second in which improvements were made without vaccination, in the vaccinated group with improved management conditions, neonatal morbidity and mortality were both reduced by a factor of 3 in Group 1 and by factors of 9.5 and 12.5 in Group 3. Also, the duration of diarrhea was 3.7 and 12 times shorter in the kids of Groups 2 and 3, respectively.