ESCHERICHIA COLI

E. coli are part of the normal flora of the bovine gastrointestinal tract. Pathogenic strains of E. coli possess virulence attributes that are involved in the pathogenesis of disease. Virulence attributes include adhesins, enterotoxins, and cytotoxins. Pathogenic strains of E. coli may be shed by adult cattle with transmission to neonates by the fecal-oral route. Sick neonates amplify environmental contamination via prolific fecal shedding.

ENTEROTOXIGENIC E. COLI

ETEC possess two virulence factors: fimbriae (pili) and enterotoxins. F5 (K99) and/or F41 fimbriae mediate adherence, and thermolabile (LT) and thermostable (STa and STb) enterotoxins stimulate a secretory response by intestinal crypt cells. Although some bovine-origin ETEC produces LT, most strains that cause diarrhea in neonatal calves produce STa heat-stable enterotoxin.¹⁰² The STa enterotoxin and F5 antigen are plasmid-mediated virulence factors. Susceptibility to ETEC is age dependent according to the binding specificity of pili antigens to immature enterocytes.¹⁰³ Disease is typically observed in calves less than 3 days of age; however, concurrent infection with rotavirus may extend this window to 7 to 14 days of age.^104,105 Intestinal cells of calves older than 2 days of age acquire natural resistance to F5 adhesion.¹⁰³ Despite this, F5-positive E. coli organisms have been isolated from healthy 4- to 12-week-old calves and F5-positive ETEC organisms are shed in feces for several weeks after experimental infection of newborn calves.¹⁰⁶

ATTACHING AND EFFACING E. COLI AND SHIGA TOXIN–PRODUCING E. COLI

Attaching and effacing E. coli (AEEC) and Shiga toxin—producing E. coli (STEC) have been identified as causes of diarrhea and dysentery in calves.^107,108 Disease is mediated by cytotoxic damage to the intestinal mucosa. Lesions may be observed in the ileum, cecum, and colon.¹⁰⁹ AEEC (Vero or HeLa toxin—producing) induces a mucohemorrhagic colitis, with petechial or ecchymotic hemorrhages in the wall of the colon and rectum.^110-112 E. coli organisms that carry this toxin often belong to O serogroups 5, 26, 111, and 118.^111,113 Naturally occurring outbreaks have been reported in 2-day- to 4-week-old calves.¹¹⁴ The most common clinical sign is diarrhea, but dysentery, abdominal pain manifested by bruxism, and dehydration are seen in some cases.

STEC serotypes associated with dysentery in calves include O5:H−, O26:H11, O111:H−, O113:H21.¹¹⁵ These serotypes may produce Shiga toxins—those that are immunologically similar to the Shiga toxin produced by Shigella dysenteriae (Stx1) and those that are immunologically distinct from S. dysenteriae Shiga toxin (Stx2).¹¹⁶ Bovine STEC produces STx1, STx2, or both.¹¹⁷ AEEC, which causes disease and does not produce enterotoxins or Shiga toxin, is referred to as enteropathogenic E. coli (EPEC).

The prevalence of AEEC and STEC in calves and the incidence of disease caused by these strains are not clearly defined, as most diagnostic laboratories do not routinely screen for AEEC and STEC. In a study aimed at determining the clinical significance and prevalence of AEEC in Swiss cattle, fecal swabs of 93 cattle from two farms with calf diarrhea and of 54 cattle from two similar farms without clinical problems were screened for AEEC by PCR assay and colony-blot hybridization. On average, 21% of all cows were positive for AEEC by PCR, without differences between farms with and without diarrhea problems. By contrast, AEEC was detected by PCR in 60% of animals younger than 2 years from farms with diarrhea problems, whereas only 32% of comparable control animals from farms without clinical problems had AEEC.

SALMONELLA

There are over 2200 reported serotypes of Salmonella, yet fewer than 2% of these account for approximately 80% of the disease reported in livestock.¹¹⁸ In cattle, over 95% of Salmonella associated with disease is in serogroups B, C, D, and E. Salmonella induces a wide spectrum of disease in cattle of all ages ranging from inapparent subclinical infections to acute fulminant bacteremia, endotoxemia, and death. The variable manifestations of disease reflect the tissue trophisms of different Salmonella serotypes and the influence of challenge dose and host immunity. Common clinical signs associated with salmonellosis include fever, diarrhea, anorexia, depressed mentation, and dehydration. Many of the clinical signs are associated with endotoxemia. Systemic signs of endotoxemia include, fever, tachypnea, tachycardia, scleral injection, leukopenia or leukocytosis, and weakness. Some serotypes, particularly Salmonella typhimurium, have a tendency to induce severe inflammation of the bowel mucosa, resulting in dysentery and passage of fibrin and mucosal casts. Fluid, electrolyte, and protein loss may progress rapidly and become life-threatening if not corrected. With severe disease animals rapidly become emaciated because of the catabolic state induced by release of tumor necrosis factor alpha (TNF-α). Sequelae occasionally observed after invasive Salmonella infections in neonates include septic osteoarthritis and meningitis.

Immunity to Salmonella changes rapidly during the first 3 months of life. At 2 weeks of age the LD₅₀ for some virulent strains is 10^5,119 at 6 to 7 weeks is 10⁷, and at 12 to 14 weeks is 10.^10,120 In contrast, administration of 10¹⁰Salmonella to 24- to 28-week-old calves failed to induce clinical signs of disease.¹²⁰ The numbers cited reflect the influence of age on immunity but should not be interpreted as absolute. Different age predilections, manifestations of disease, and virulence are observed among Salmonella serotypes and among different strains of the same serotype.^121,122 Although adults may serve as carriers and a source of infection of Salmonella Dublin infection in neonates, disease in adults is less common in mature cattle compared with calves. In contrast, S. typhimurium tends to manifest disease in an epidemic manner, causing illness in all age groups.

Calves on endemically infected farms are commonly exposed to Salmonella in the first few days of life.¹²³Salmonella exposure may occur via contaminated colostrum or milk; surface contamination of teats and udder, personnel, or equipment; or the environment. Chronically infected carriers may shed 2.5 × 10⁸Salmonella organisms in milk per day (25 kg of milk containing 10⁵Salmonella per milliliter).¹²⁴ Feeding utensils and personnel often play a significant role in transmitting Salmonella between calves.¹²⁵ Salmonella infects the salivary glands and is shed in saliva and nasal secretions.^126,127 Adequate cleaning and disinfection of feeding and medicating utensils is necessary to remove Salmonella contamination. Salmonella is sensitive to most disinfectants, but removal of contaminating organic debris is imperative, as the activity of disinfectants is reduced by the presence of organic matter.¹²⁸

CLOSTRIDIA

Although clostridia are not commonly considered a major pathogen causing neonatal calf diarrhea, a number of reports associate clostridial infections with enteritis and abomasitis.

C. perfringens is the most important cause of clostridial enteric disease in calves. Some types of C. perfringens (mainly type A) are consistently recovered from the intestinal tracts of animals and from the environment, whereas others (types B, C, D, and E) are less common in the intestinal tracts of animals and can occasionally be found in the environment in areas where disease produced by these organisms is enzootic.¹²⁹ Disease is usually precipitated by management factors that lead to the proliferation of the organism within the gastrointestinal tract or attenuated digestion of clostridial toxins within the lumen of the alimentary tract.

C. perfringens type A has been associated with acute hemorrhagic abomasitis in neonatal calves. Clinical signs include acute abdominal distention, colic, depression, and sudden death. Onset of clinical signs is rapid; affected animals become anorectic, depressed, or restless. Signs of abdominal discomfort are observed in approximately half of the cases and include treading on the spot and kicking at the abdomen. On physical examination splashing and metallic sounds are heard on succussion of the distended abdomen; passage of a stomach tube fails to relieve the distention. Fecal output is reduced, and melena may be observed. Gross pathology may include abomasal ulcers, abomasitis, and abomasal tympany.^45,48 Trace mineral deficiencies of copper and/or selenium may also be involved in the pathogenesis of the condition.⁴⁴ A decreased prevalence of abomasal tympany and ulceration was reported in neonatal calves from herds having a history of these problems after implementation of a C. perfringens vaccination program.^44,52 Enterotoxemia caused by C. perfringens type A has been described in 2- to 4-month-old calves, with the condition observed more often in beef calves than in dairy calves.¹³⁰ The disease is characterized by a high case fatality rate, sudden deaths, lesions of necrotic and hemorrhagic enteritis of the small intestine, and, most often, an absence of other clinical signs.¹³¹

C. perfringens type B is not commonly associated with neonatal diarrhea in calves. C. perfringens type C infections are most frequently observed in neonates less than 10 days of age.¹³² Newborn animals are typically most susceptible, perhaps because of ready colonization of the gut by C. perfringens in the absence of well-established normal intestinal flora.¹²⁹ Alteration of the flora by sudden dietary changes may also be an inciting factor in type C infections. Vigorous, healthy calves develop hemorrhagic, necrotic enteritis and enterotoxemia, often accompanied by evidence of abdominal pain and neurologic signs that may include frenzied bellowing, aimless running, tetany, and opisthotonus. Death may be peracute, occasionally without other clinical signs, but may also follow a clinical course of several days.

CAMPYLOBACTER SPECIES

The clinical significance of Campylobacter species in calf scours is inconclusive. Campylobacter species are part of the normal intestinal flora. Experimental challenge studies have demonstrated the capacity of Campylobacter jejuni to cause enteritis in calves.^99,133-135 However, there is a paucity of convincing reports that demonstrate a causal association in naturally occurring cases.

ROTAVIRUS

Rotaviruses are the most common cause of neonatal diarrhea in calves.^136,137 Affected calves are generally 5 days to 2 weeks of age, although disease can occur at 24 hours, particularly in colostrum- deprived calves (see Fig. 20-2).^138,139 This age predilection is thought to occur because many cows secrete antirotavirus antibody in their colostrum, which confers local protection against rotavirus attack until antibody levels in milk decline 48 to 72 hours postpartum.^140,141 Resistance to infection is not age dependant, but age-dependant resistance to clinical disease has been demonstrated.¹⁴² Several possible mechanisms are associated involved in age dependence. Age restriction may be related to immunity, as neutralizing antibodies increase with age and virus exposure. The expression of intestinal mucins and the rate of epithelial cell replacement and fluid absorption are also age dependent and have been shown to affect rotavirus infection and disease expression.¹⁴³

Rotavirus invades small intestinal villous epithelial cells; the attack is usually self-limiting because of destruction of target cells.⁸² Enterocytes are lost to the gut faster than they can be replaced from the crypts. The shrunken villi are initially covered by squamous and cuboidal cells from the crypts; the villi gradually regenerate as these differentiate into absorptive columnar epithelium.^82,144 Intestinal secretions are increased owing to the compensatory hyperplasia of crypt cells and enterotoxigenic activity of the viral nonstructural protein NSP4.^85,86 Both increased secretory load and impaired absorption resulting from villous hypoplasia contribute to the diarrhea. It is thought that virulent strains replicate more quickly and infect a larger area of epithelium. Difference in rotavirus replication rates in the gut and age-dependent differences in the rate of enterocyte loss and natural replacement rate may explain the differences in clinical outcome. Concurrent infection with ETEC has also been shown to cause clinical signs at a later age than with a single infection of either agent alone.¹⁴⁵

Rotavirus of calves, lambs, kids, pigs, foals, mice, and children is morphologically identical. Infections are classified by the antigenic properties and/or sequence of the genes encoding the viral capsid proteins. Viral protein (VP) 6 is used to separate them into seven antigenically distinct serogroups, A through F. Rotaviruses from serogroups A, B, and C have been isolated from cattle, and serogroup A is the most common cause of diarrhea in calves. Group B rotaviruses have been isolated from calves and adult cattle; however, there is less information regarding their significance and prevalence in cattle.^146-150 Group B rotavirus is more common in lambs than in calves.¹⁵¹ Group C rotavirus has been isolated only from adult cattle.¹⁴⁹ The serotype and/or genotype of capsid proteins VP7 and VP4 are also used to differentiate the viruses into a number of G-types (glycoprotein) and P-types (protease sensitive protien).¹⁵² A range of both serotypic and genotypic diversity and virulence has been reported within serogroup A.^142,153-155 Rotavirus is shed in the feces of infected animals, and transmission is primarily fecal-oral. Clinical signs occur 1 to 3 days after infection and last for 5 to 9 days. Virus excretion commences with the onset of clinical signs and continues for 3 to 7 days.^142,156 Adult cows can be subclinically infected and intermittently shed the virus during pregnancy and especially at parturition.^157-159 It is likely that this is the most common source of infection, with carrier cows infecting their calves and then these calves infecting other calves.¹⁶⁰ Calves from carrier cows have a significantly higher risk of clinical disease, and the birth of calves from known carrier cows has been associated with the beginning of an outbreak. Recovered calves can become reinfected and shed virus.¹⁶¹

The environment may be an important source of infection. Rotaviruses can survive in fresh water for more than 2 weeks at 23° C and for months in water or soil <5° C.¹⁶² They are also stable in feces and effluent for up to 9 months and therefore are likely to remain in calving areas from year to year.¹⁶³

CORONAVIRUS

Bovine coronavirus commonly causes diarrhea in calves 5 days to 1 month of age.^{63,68,164,165} Disease can occur within 24 hours in colostrum-deprived calves and has also been recorded in calves up to 5 months of age.¹⁶⁶ Respiratory infections are common in older calves and may be important in the epizootiology of enteritis.¹⁶⁶

Calves may be infected with coronavirus by the oral or respiratory route.¹⁵⁶ Fecal shedding commences 3 days after infection and persists for up to a week; nasal shedding can be detected 2 days after infection and persists for 2 weeks. Once infected, calves initially excrete high levels of virus and are potent sources of contamination. Infection persists for weeks in apparently recovered calves, and these excrete low levels of virus for weeks.¹⁶⁷ Subclinical infection is common. Disease is more common in the winter months, and coronavirus survives in the environment from year to year.

Calves may be infected by virus shed by persistently infected cows.¹⁶⁸ Coronavirus has been detected in the feces of more than 70% of clinically normal cows.¹⁵⁹ The rate of virus excretion increases at parturition and in the winter months.^157,169 Calves born to carrier animals are at a significantly increased risk for developing diarrhea.¹⁵⁷

All BCV isolates are believed to belong to a single serotype.^169a Differences in hemagglutination-inhibition characteristics have been used to classify strains as types 1 through 3.^169b

The pathology of coronavirus is often more severe than that of rotavirus, resulting in a mucohemorrhagic enterocolitis. The virus infects both the small and large intestine. In the spiral colon there is widespread destruction of the cells of the colonic ridges.^79,81 Virus replication occurs in the surface epithelium, especially in the distal half of the villi, resulting in stunting and fusion of the villi. Immature cells replace epithelial cells, and in severe infection there can be areas of complete desquamation. Intestinal secretions continue, and absorption is impaired by reduced surface area. Undigested lactose accumulates in the intestinal lumen, often resulting in a secondary bacterial overgrowth, fermentation, lactate production, and an osmotic imbalance that draws fluid into the intestinal lumen. Most infections are self-limiting because the virus rarely attacks crypt epithelial cells.¹⁶⁸ In response to infection the mitotic rate of crypt cells increases, producing immature cells that are more resistant to virus infection and that migrate up the villi to replace the damaged cells.

In experimental challenge studies, diarrhea develops 48 hours after infection. Calves are initially depressed and anorectic for the acute phase and may become dehydrated and pyrexic in a severe infection.¹⁶⁸ Severe infections can result in death from dehydration, acidosis, shock, and cardiac failure. Respiratory signs are generally mild. Rhinitis, sneezing, and coughing may occur. Lesions may be found in the lungs, but clinical signs of pneumonia are rare, except when secondary infection occurs.

BOVINE VIRUS DIARRHEA VIRUS

BVD virus occasionally causes diarrhea and thrombocytopenia in young calves outside the confines of the persistently infected disease model.^70,71 Colostral antibodies generally protect young calves from BVD infection, but disease may occur as a result of FPT or the introduction of novel BVD strains with new cattle or viral mutation in persistently infected home-grown cattle. BVD is also thought to exacerbate infections caused by other pathogens.¹⁷⁰ It has also been implicated in necrotic enteritis, an acute enteritis of 7- to 12-week-old beef calves reported in the United Kingdom.¹⁷¹ Affected calves usually show oral ulcerations, particularly on the hard and soft palates. The buccal papillae are often blunted, and the tips may be ulcerated.¹⁷² Some variants of the virus produce intestinal bleeding, petechiation, ecchymosis, or prolonged bleeding from venipuncture sites secondary to thrombocytopenia.^70,173-175 Hematologic findings often include leukopenia and thrombocytopenia. The disease must be differentiated from other causes of enteritis that are complicated by bovine papular stomatitis infection. Bovine papular stomatitis is common in neonatal calves. It produces oral lesions that are hyperemic and red, with a central white area of necrosis and often a raised rim of proliferating epithelial cells. These lesions often involve the mucosa around the molars. They are usually of little consequence, and their importance lies in the fact that they may be confused with BVD. One feature that helps identify BVD ulcers is that they lack the zones of epithelial proliferation seen in bovine papular stomatitis.

BOVINE TOROVIRUS (BREDA VIRUS)

Bovine torovirus has been detected worldwide^176-179 and has recently been implicated as an important cause of calf diarrhea.^66,67 Initially known as Breda virus, it is part of the Coronaviridae family. It has been relatively infrequently reported because it is difficult to recognize by electron microscopy and it cannot as yet be grown in cell culture, which has precluded the development of routine immunospecific diagnostic tests.⁶⁶ Laboratory studies using PCR testing have implicated it as the sole pathogen isolated in 25% to 30% of fecal samples from calves with diarrhea under 6 weeks of age.^66,67 It is also found in the feces and nasal secretions of asymptomatic animals,^66,68 suggesting that the epizootiology is likely to be similar to that of rotavirus and coronavirus, with asymptomatic carriers acting as reservoirs of infection within a herd.¹⁵⁷ It is mainly a disease of calves less than 3 weeks of age, with diarrhea commencing as early as 1 to 3 days after birth,^178,179 but clinical signs have been observed in animals up to 10 months of age.^67,180 Clinically it produces mild to moderate diarrhea in calves under both experimental and field conditions.^179,181 The virus infects the small and large intestines, affecting differentiating epithelial cells in the crypts of the intestinal villi.^179,180 Clinical signs develop 24 to 72 hours after experimental infection.¹⁷⁹ It has also been isolated from the respiratory tract of cattle and associated with respiratory signs in calves at 1 month and 4 to 6 months of age.¹⁸²

OTHER VIRUSES

Calicivirus, astrovirus, adenovirus, parvovirus and picobirnavirus have all been associated with neonatal calf diarrhea.^183-187 The pathogenicity and contribution of these viruses to field outbreaks is uncertain.

CRYPTOSPORIDIUM

Two species of Cryptosporidium have been identified in cattle: Cryptosporidium parvum in the intestine and Cryptosporidium andersoni in the abomasum.¹⁸⁸ The two species have morphologically distinct oocysts and differ genetically.¹⁸⁹C. andersoni is a parasite of calves postweaning and has not been associated with neonatal diarrhea.

There are several subgenotypes of C. parvum, many of which appear to be host-specific and could represent distinct species.^188,190 These genotypes include type 1, which is found in human sources, and type 2, which is considered to be zoonotic and can be isolated from cattle, sheep, and goats.¹⁹⁰ Calves generally become infected between 1 and 4 weeks of age and display clinical signs for 4 to 14 days. Animals of all ages can be infected, but diarrhea is mainly associated with calves preweaning.¹⁹¹ Cryptosporidial infections are asymptomatic in cattle older than 4 months of age. C. parvum mainly infects the distal small intestine, but lesions are also found in the cecum and colon and occasionally the duodenum.¹⁹² The parasite invades the superficial cells of the mucosa in the intestine but is surrounded by an invagination of the host cell membrane and remains extracytoplasmic. Parasitic invasion of the mucosa leads to epithelial destruction and mild to moderate villous atrophy, with microvillous shortening and destruction. This leads to impaired nutrient digestion and transport and a resulting malabsorption diarrhea.

Affected calves often show no sign other than diarrhea but can show depression, dehydration, and anorexia.¹⁹³ Pyrexia and tenesmus have been noted.^194,195 Variable levels of morbidity have been reported, and mortality is generally low.^193,194,196 Other pathogens can be involved and are likely to contribute to the severity of the disease. Affected calves can take 4 to 6 weeks to recover. Cryptosporidiosis occurs less frequently in suckler calves at pasture, but when these calves are affected outbreaks were reported to be more severe than found in dairy calves, with mortality rates up to 30%.¹⁸⁸ High mortality rates have been attributed to lack of herd immunity in seasonal calving herds in which the transmission cycle is broken. Neutralizing antibodies in colostrum and milk reduce infectivity by immobilizing the parasite, blocking invasion, inhibiting adhesion to host cells, or having direct cytotoxicity for Cryptosporidium sporozoites.¹⁹⁷ High mortality rates have also been associated with concurrent low levels of selenium, inadequate nutrition, presence of concurrent enteric infections, and specific management practices.¹⁸⁸

Transmission is fecal-oral, by ingestion of an encysted, sporulated oocyst. Transmission can be direct from host to host, by ingestion of contaminated food or water, and probably mechanically via flies.¹⁹⁸ A study of oocyst shedding in experimentally infected neonatal calves demonstrated prepatent and patent periods ranging from 3 to 6 and 4 to 13 days, respectively.¹⁹⁹ However, oocyst excretion has been described at as early as 2 days of age, which means that calves are susceptible to infection during or shortly after birth.²⁰⁰ The parasite is capable of autoinfection, sporulating within the intestine and immediately infecting adjacent cells. This can result in protracted clinical illness and relapses. The ability to autoinfect results in huge parasite burdens after very small infective doses. Oocyst excretion has been described at as early as 2 days of age, which means that calves are susceptible to infection during or shortly after birth.²⁰⁰ Calves aged 1 week to 4 months of age are most likely to be actively shedding significant numbers of oocysts, with peak shedding occurring at 1 to 3 weeks of age.^191,199-201 Infected calves can shed in excess of 10⁶ oocysts g⁻¹ of feces.^199,202C. parvum oocysts have also been isolated from adult cows, with herd prevalence ranging from 7%–100%.^191,203-205 Mean shedding intensity reported for adult cows has ranged from 3 to 900 oocysts g⁻¹ of feces.^205-207 It is likely that carrier cows are a source of infection of young calves.

The most critical factor affecting environmental oocyst survival is the temperature. Drying of oocysts has been shown to dramatically reduce their viability and infectivity in mice.^208,209 Oocysts can enter watercourses and ground water by direct contact with cows or from runoff of rain or irrigation water from pastures and manure storage areas.^188,210Cryptosporidium oocysts have been shown to survive in water for at least 12 weeks at 4° C.²¹¹ Oocysts are resistant to chlorination of water and most disinfectants.¹⁸⁸ They have also been shown to survive in silage.²¹² Wildlife may be a significant reservoir for C. parvum and may act as a method of amplification and infection in the environment.^203,213,214

Cryptosporidia cause diarrhea and sometimes death in 3- to 30-day-old lambs. Protracted infections and mortality are most common in lambs infected in the first few days of life, as age resistance is seen after about 3 weeks of age.^92,215-217 Cryptosporidiosis has also been described in goats; it affected 5- to 20-day-old kids, signs lasted from 3 to 7 days, relapses were not uncommon, and there was a moderate mortality rate.²¹⁸ Cryptosporidia are resistant to all commonly available antimicrobial and anticoccidial agents and most disinfectants and can survive for long periods in the environment.

People working with diarrheic neonates should be warned of the risk of zoonotic disease. An outbreak of cryptosporidiosis has been described in caregivers in a veterinary hospital treating diarrheic calves. Affected people suffered from watery diarrhea, cramping, flatulence, and headache.²¹⁹ One person became infected as a result of handling soiled clothing.

GIARDIA

Giardia is often found in diarrheic calves in association with other pathogens, but its relevance as a pathogen in its own right is unclear. Several authors have documented cases of diarrhea in which Giardia infection has been implicated as the causative agent either by itself or in conjunction with C. parvum and rotavirus.^220-222 Affected calves are at least 2 weeks old, and often older than 1 month of age, with infection often becoming chronic and lasting for several months.^{200,220,223-225}Giardia has a prepatent period of 7 to 8 days, and the delayed interval between birth and infection is likely to relate to high levels of colostral protection against Giardia but low protective levels in milk.²²⁶ Many calves were shown to have a poor specific immune response to the infection, accounting for the chronicity of the infection.

The significance of Giardia as a primary pathogen has been questioned by the observation of similar or lower rates of infection in calves with diarrhea compared with asymptomatic calves.^200,227 Treatment of affected calves with fenbendazole reduces the duration but not the number of diarrhea episodes.²²²

COCCIDIOSIS

Thirteen species of Eimeria have been reported in cattle.²²⁸ Eimeria bovis and Eimeria zuernii have historically been the most common pathogenic species; however, there are increasing reports of Eimeria alabamensis causing disease.^229-231 Transmission is fecal-oral. Infected animals pass unsporulated oocysts in their feces that sporulate and become infective. The sporulated oocysts are protected from the environment by a double cyst wall.²³² Moist, temperate, cool conditions favor sporulation, and oocysts can survive for several years. Sporulated oocysts can resist freezing to −8° C for several months but are destroyed by high temperatures and dry conditions within a few weeks.²³³ Under optimal conditions sporulation can occur within a few days. The prepatent period of the two main pathogenic species is 15 to 20 days, and the patent period is approximately 11 days. E. alabamensis has a prepatent period of only 8 days and a patent period of 5 days.

Calves start shedding at about 1 month of age and shed for 3 to 4 months. E. bovis and E. zuernii schizonts first reproduce in the lower small intestine, then produce second-generation schizonts and gamonts in the cecum and colon, where they attack crypt cells.²²⁸ These latter stages induce both local and more extensive lesions.

Outbreaks of disease in calves and lambs are often related to overcrowded and confined conditions. Up to 95% of infections are subclinical, causing decreased growth rates that are often unnoticed.²³⁴ Clinical disease can be chronic or acute and is generally found in calves aged 3 weeks to 6 months, although animals 2 years of age or older may be affected. In beef cattle the most common reports of clinical disease are associated with weaning stress.²³⁵ Clinical signs may include diarrhea, ill thrift, increased susceptibility to pneumonia, tenesmus, increased mucus in feces, and hematochezia. Pyrexia, dehydration, and anemia may also be observed. The disease is usually self-limiting without reinfection. Chronic disease is often underdiagnosed.²³⁴ Calves appear weak and listless, with pasty feces, drooping eyes, and a staring coat. Fecal oocyst count is low or negligible. Disease results from continual reinfection as a result of a heavily contaminated environment.

ESCHERICHIA COLI

E. coli is a normal inhabitant of the gastrointestinal tract. Isolation of E. coli from fecal samples or gut contents is therefore of no significance unless the isolates are demonstrated to possess virulent attributes that are consistent with the clinical and/or pathologic presentation. Virulence attributes include adhesins, enterotoxins, and cytotoxins. ETEC adheres to enterocytes in the jejunum and ileum.²⁴² On gross pathology, ETEC is associated with fluid-distended loops of bowel without enteritis.²⁴³ Calves infected with ETEC have a mild inflammatory reaction in the small intestinal wall and some villous atrophy. In fresh specimens, sheets of gram-negative bacilli can be seen adhering to the small intestinal wall.²⁴² Definitive diagnosis of enterotoxigenicity rests on demonstration of the ability of the E. coli to dilate intestinal loops.²⁴⁴ ETEC can also be identified by the presence of F5 (K99) using antigen-specific immunoassays including latex agglutination,²⁴⁵ ELISA,²⁴⁶ fluorescent antibody,²⁴⁷ slide agglutination,²⁴⁷ and rapid dipstick tests. A potential limitation of immunoassays is the specificity of the antibodies used; strains of ETEC using non-F5 fimbriae will not be detected by these tests.^108,248

AEEC and STEC mediate disease by cytotoxic damage to the intestinal mucosa. Diagnosis of E. coli infection may be achieved using phenotypic differentiation of pathogenic strains from nonpathogenic normal flora E. coli via bioassays or immunoassays for toxins and fimbriae. Immunoassays have been developed to identify the presence of Stx1 and Stx2 in feces as a presumptive test for the detection of STEC in cattle feces.^249-251 An alternative approach to identify and differentiate ETEC, AEEC, and STEC is to use PCR to identify virulence-associated genes commonly found in these E. coli strains (F5, F41, enterotoxin, intimin, Stx1, and Stx2).¹¹⁷ The significance of STEC, EPEC, and AEEC in bovine enteritis is unknown because of a lack of appropriate assays for routine detection and because of the widespread presence of verotoxin-producing E. coli strains in healthy cattle that complicate the interpretation of detecting fecal shedding in sick animals.^252-254 Demonstration of verotoxin in cultures from bovine enteritis is not sufficient to imply a causative association.

CLOSTRIDIUM SPECIES

C. perfringens has been associated with enterotoxemia and hemorrhagic abomasitis in calves.^129,131C. perfringens organisms are normal flora of the gastrointestinal tract; therefore isolation of C. perfringens from feces is not in itself diagnostic. Pathogenic strains of C. perfringens produce exotoxins; five of these (alpha-,beta-,epsilon-, and iota-toxins and enterotoxin) are involved in the pathogenesis of disease.¹²⁹ The complete pathogenesis of enterotoxemia and abomasitis has yet to be completely elucidated. Production of specific toxins can be demonstrated only in a proportion of cases.²⁵⁵ Isolation of toxin-positive C. perfringens from intestinal contents does not confirm a clinical diagnosis of bovine enterotoxemia, because almost as many C. perfringens isolates from normal calves produce toxin and because toxin production cannot be demonstrated in as many as 40% of affected calves.²⁵⁶

A fresh necropsy is required to definitively diagnose clostridial enteritis. Observing many gram-positive bacilli in the mucosa associated with hemorrhagic enteritis is suggestive of clostridial enterotoxemia. Quantitative bacterial counts of intestinal contents at the site of the lesion have proven to be one of the most reliable methods for diagnosing enterotoxemia.¹³¹ A C. perfringens count greater than 10⁶/mL of intestinal contents is consistent with a diagnosis of enterotoxemia.¹³¹ Demonstrating the presence of C. perfringens toxins or the capacity to produce toxins provides support for the diagnosis. Tests for detecting toxins or the bacteria’s capacity to produce toxins include bioassays, immunoassays, western blot, and PCR assay.²⁵⁷ The basis of the bioassay is to demonstrate protection of mice using antitoxin. C. perfringens enterotoxin is produced during sporulation. In vitro detection of enterotoxin-production capacity of a C. Perfringens isolate using western blot or immunoassays requires sporulation to occur. In vitro techniques to induce sporulation are not 100% efficient, so detection of enterotoxin using these methods is less sensitive than PCR is at detecting the genes required to produce enterotoxin.²⁵⁸

SALMONELLA SPECIES

Salmonellae are capable of causing disease in cattle of all ages. Neonatal infections are common. The classic pathologic lesion is fibrinous or fibrinonecrotic to ulcerative enteritis.²⁵⁹ The severity of lesions is usually greatest in the distal small intestine and proximal large bowel. Hypertrophy of the mesenteric lymph nodes is a common finding.²⁶⁰ Serosal hemorrhages may be observed in the small and large intestines. Septic infarcts in the kidneys and inflammation of the gall bladder are less common findings. Pneumonia is a common finding with Salmonella Dublin infections, and gangrenous necrosis of distal extremities may also be observed.²⁶¹ Bacteremia is a feature of neonatal salmonellosis and may manifest as osteomyelitis and/or meningitis.

Isolation of Salmonella from feces of calves with diarrhea is consistent with a diagnosis of salmonellosis but in itself does not necessarily establish causality, as Salmonella may be isolated from the feces of apparently healthy calves.²⁶² Isolation of Salmonella from tissues at necropsy is indicative of invasive salmonellosis. A definitive diagnosis of salmonellosis is based on the clinical presentation, pathologic lesions, and isolation of Salmonella from tissues at necropsy.

There are numerous methods for isolating and detecting the presence of Salmonella. These include direct culture, enrichment cultures, PCR, immunoseparation, and immunoassays.

The process of directly inoculating tissues or other samples onto plating media, except in the case of acute infections, is usually nonproductive. Typically, with subclinical infection the number of salmonellae shed in feces is low relative to the high number of other bacteria. Fecal samples should be inoculated into selective-enrichment media for optimal recovery of Salmonella. Selective-enrichment broths are formulated to selectively inhibit other bacteria while allowing Salmonella to multiply to levels that may be detected after plating. Internal organs that are normally sterile do not need to be inoculated onto selective media; rather, they should be inoculated onto nonselective (blood agar) or weakly selective (MacConkey agar) media.

Rapid detection methods have been developed to expedite the detection of salmonella. These methods include electrical conductance and impedance, immunologic techniques, nucleic-acid—based assays, and PCR assay. These methods generally take 24 to 52 hours to screen for or detect and identify salmonella organisms. Most of these tests, and particularly the enzyme-linked immunologic techniques, require 10⁵ cells per milliliter for reliable results. Accordingly all these tests involve a preenrichment stage, and some also involve a selective-enrichment culture.²⁶³ When Salmonella is causing disease, clinically affected calves may shed 10⁹ Salmonella organisms per gram of feces.²⁶⁴ Detection of Salmonella in clinical samples when it is the inciting cause of the disease process is not normally difficult when multiple samples are collected from a representative sample of the affected population.

CORONAVIRUS

Coronavirus replication occurs in the epithelial cells of the distal half of the villi of the lower small intestine and colon. Infected cells die, slough, and are replaced by immature cells. In the small intestine these changes result in stunting and fusion of adjacent villi, and in the large intestine they lead to atrophy of the colonic ridges. On histopathology the tall columnar epithelial cells are replaced by cuboidal and squamous epithelial cells, and in severe infections there may be areas of complete desquamation.²⁶⁵ Virus is shed in respiratory secretions and feces. There are several methods for detecting bovine coronavirus virus in feces. These include isolation of the virus in cell culture,²⁶⁶ electron microscopy,²⁶⁷ immunoelectron microscopy,¹⁶⁶ immunoassays^{159,246,268-271} and molecular techniques including dot blot hybridization assays²⁷² and RT-PCR assay.^273,274 Isolation of bovine coronavirus using cell culture techniques is not often performed in diagnostic laboratories, as the technique is difficult and requires viable virus (fresh samples or shipped on dry ice).²⁷⁵ Electron microscopy has been used as a standard diagnostic method for bovine coronavirus. Although the intact virion of bovine coronavirus is fairly characteristic in appearance, it is not uncommon for the identifying surface projections of the virus to be lost during sample preparation or storage, making it difficult to properly identify virus particles by electron microscopy.

Numerous ELISA assays have been described for the detection of BCV antigen in feces. Several companies have developed commercial kits using this technology. The use of monoclonal antibodies rather than polyclonal antibodies is reported to increase the sensitivity and specificity of bovine coronavirus ELISAs.²⁷¹ The limit of detection for ELISA assays ranges from 10⁴ to 10⁷ virions per milliliter of feces.

A one-step RT-PCR assay, targeting a 730-bp fragment of the nucleocapsid gene of bovine coronavirus, and a nested PCR assay, targeting a 407-bp fragment of the nucleocapsid gene have been developed to detect bovine coronavirus. Compared with an antigen capture ELISA, the limit of detection for the RT-PCR and nested PCR assays was 10⁵ virions/mL and 10³ virions/mL, respectively, compared with 10⁷ virions/mL for the ELISA.^276,277

ROTAVIRUS

Bovine rotavirus infects enterocytes of the intestinal villus. Infected cells are predominantly in the distal third to half of the villus. The age at the time of infection influences the distribution of the virus in the gastrointestinal tract and the number of virions shed in feces. In experimental challenge studies, infection of day-old calves resulted in a uniform distribution of virus throughout the small intestine.²⁷⁸ Challenge of 10-day-old calves led to a patchy distribution of the virus with maximal viral load observed in the mid small intestine.²⁷⁸ Villous stunting is more pronounced in young calves.

Methods for detection of rotavirus include cell culture, fluorescent antibody staining, electron microscopy, immunoelectron microscopy, immunoassays, electrophoretic procedures, and RT-PCR assay.^{146-150,245,246,269,279-281} Bovine rotavirus is difficult to isolate in cell cultures because of the cytotoxic nature of feces and fecal filtrates and because the virus is inconsistent in production of cytopathic effects.²⁸⁰ The fluorescent antibody technique is simple, rapid, and specific; however, rotaviral antigen is usually difficult to detect within 24 to 72 hours after the onset of diarrhea because rotavirus-infected epithelial cells are rapidly shed from the tips of the villi.²⁸² Comparative studies evaluating methods of detecting rotavirus in feces show comparable test results between antigen capture assays (ELISA, latex agglutination) and electron microscopy.^{245,269,280,283} Direct immunofluorescence testing of fecal samples corresponds well (90%) with electron microscopic examination for rotaviruses when samples are collected during the 24 hours after the onset of diarrhea,²⁸⁴ but have poor agreement (33%) for field specimens submitted to a diagnostic laboratory.²⁸⁰

BOVINE VIRUS DIARRHEA

BVD virus rarely causes diarrhea in neonatal calves.⁷¹ Sporadic disease may be observed in persistently infected calves. Pathologic lesions include ulceration of the oral cavity, particularly on the hard and soft palates, and blunting of the buccal papillae.¹⁷² Erosions may be observed in the esophagus, and necrosis of Peyer’s patches may be observed in the ileum. Thrombocytopenia has been observed with BVD type II infections. Outbreaks of neonatal disease have been observed with this strain. Petechial and ecchymotic hemorrhages are a feature of this condition.^70,174,175 Hematologic findings often include leukopenia and thrombocytopenia.

Several options are available for the detection of BVD; these include virus isolation,^285,286 RT-PCR assay,²⁸⁷ immunohistochemistry,²⁸⁸ and antigen capture ELISA.²⁸⁹ Assays; the latter two are used in most commercial laboratories. Maternal antibodies reduce the sensitivity of the ELISA assay in young calves.²⁸⁷

BOVINE TOROVIRUS (BREDA VIRUS)

Bovine torovirus produces cytolytic infections of villi and crypt enterocytes in the small and large intestine.²⁹⁰ Bovine torovirus does not grow in tissue culture, cell culture, or embryonated eggs.²⁹¹ Therefore the large-scale preparation of reference antisera and antigens for the development of diagnostic tests has been precluded. Torovirus is capable of causing diarrhea in cattle, with disease observed most frequently in calves less than 3 weeks of age.^{68,179,181,292-294} Like other enteric viruses, bovine torovirus has been detected in feces of normal calves; therefore detection of the virus in feces from diarrheic cattle cannot be interpreted as causal. The lack of diagnostic reagents has limited the study of bovine torovirus, leaving questions about its epidemiology and relative importance in calf diarrhea.⁶⁷ Diagnostic methods that have been used to detect bovine torovirus include electron microscopy, immunofluorescence, antigen capture ELISA, and RT-PCR assay.^67,179

EIMERIA SPECIES

Eimeria species are host-specific. E. bovis affects primarily the mucosa of the cecum and the proximal part of the large intestine, whereas E. zuernii affects the mucosa of the cecum as well as the entire large intestine, including sometimes the rectum.²⁹⁵ The clinical signs of bovine coccidiosis are associated with the final stages of the eimerian life-cycle and commence shortly before oocyst shedding. Gross lesions in the cecum and large intestine range from having semiliquid contents, little or no blood, and few areas of epithelial sloughing to having extensive hemorrhage and large areas of epithelial sloughing and necrosis of the mucosa.²⁹⁵ The serosal surface is often reddened opposite the affected mucosal area, and the submucosa and external muscular layers are often thickened by edema.

Oocysts usually can be recovered 2 to 4 days after the onset of diarrhea.²⁹⁶ Oocysts can be identified microscopically by direct smear, flotation, or centrifugation methods. The oocysts of E. alabamensis are smaller and less distinctive than oocysts of other coccidian but are approximately 4 times larger than cryptosporidia. Oocyst counts of 5000 or more per gram of feces are considered significant in cattle.²³² Identification of oocysts in feces is not diagnostic for clinical coccidiosis, because the parasite is frequently detected in small numbers in the feces of healthy cattle.²⁹⁷ When scour problems are being investigated, multiple samples should be collected for oocyst counts to provide an indication as to the level of infection within the group. The potential for discord between clinical signs and fecal shedding limits the diagnostic utility of a single sample from an individual animal.

GIARDIA

Giardia infection is not associated with changes in intestinal villous height or crypt depth. However, transmission electron microscopy has been used to demonstrate a reduction in microvillous surface area.²⁹⁸ Diagnostic methods for detection of Giardia include direct microscopy, immunomagnetic separation, fluorescent antibody staining,^221,299 ELISA,³⁰⁰ and PCR assay.³⁰¹ When direct microscopy is used, fecal samples should be examined within 24 hours of collection. Concentration of trophozoites and cysts via density gradient centrifugation or filtration followed by fluorescent antibody staining is the diagnostic method used in most veterinary epidemiologic studies of Giardia in calves.^200,302,303 Other immunoassays and PCR techniques are emerging in human diagnostic laboratories.^300,301

CRYPTOSPORIDIA

C. parvum infections are mainly concentrated in the distal small intestine, but lesions may also be found in the cecum and colon, and occasionally in the duodenum.³⁰⁴ The pathologic findings associated with Cryptosporidium are a mild to moderate villous atrophy, villous fusion, and changes in the surface epithelium with infiltration of mononuclear cells and neutrophils in the lamina propria.³⁰⁴

Calves infected with C. parvum usually develop diarrhea in 72 to 96 hours; diarrhea is observed for 8 to 23 days,³⁰⁵ during which oocysts are excreted in feces. Oocysts are stable in feces for many days at room temperature.³⁰⁶ Laboratory methods for the diagnosis of cryptosporidial infections include microscopic examination of fecal smears or fecal preparations, immunoassays, and PCR assay. Cryptosporidia oocysts are small (4 to 6 μm in diameter) and are easily missed on a fecal smear. Because fecal smears do not concentrate the oocysts, this technique is less sensitive than fecal flotation. Concentration of the protozoa is achieved by salt³⁰⁷ or sugar flotation. Special stains may be used to facilitate detection of cryptosporidia during microscopic examination. Differential staining techniques are useful to distinguish Cryptosporidium oocysts from other fecal components (especially some yeasts) of similar size and shape.^308-311

A number of immunoassays have been developed for the detection of cryptosporidia. The detection thresholds of the different methods have been reported to be 3 × 10⁵ oocysts/g for a monoclonal antibody—based antigen capture ELISA, compared with 1 × 10⁶ oocysts/g detected by examination of acid-fast stained fecal smears and 1 × 10³ oocysts/g detected by indirect immunofluorescence.³¹² The detection threshold may be further enhanced by using a combination of immunomagnetic separation coupled with immunofluorescent microscopy. With this combination it is possible to detect as few as 10 oocysts/g.³¹³ Several dipstick immunoassays have also been developed. The detection threshold for this technology is reported to be 1 × 10³ oocysts/g.³¹⁴ This technology offers the potential for rapid, cost effective detection of cryptosporidia in fecal specimens.

Molecular techniques have been described for detection and typing of cryptosporidia.^190,315 The capacity to differentiate the different genotypes makes this approach useful for epidemiologic studies of cryptosporidia.³¹⁵

MINIMIZING PATHOGEN EXPOSURE IN DAIRY HERDS

Strategies to prevent disease in dairy calves focus on calving cows in a clean environment, removing calves from cows at birth, feeding adequate good-quality colostrum, placing calves in a clean, dry environment separate from other stock, feeding good-quality milk or milk replacer, providing adequate shelter, and providing access to water and high-quality calf pellets. Microbial contamination is an important determinant in the quality of colostrum and milk. Good sanitary practices are required for the harvest, storage, and feeding of both products. Microbial contamination of colostrum compromises passive transfer and in the case of pathogens such as Salmonella may lead to a direct pathogen challenge. Similarly, feeding milk with a high bacteria count increases the risk of diarrhea.

In dairy systems, where calves are reared by hand, the number of young stock on the farm and the incidence of respiratory disease are positive predictors for calf diarrhea.³³³ Cleanliness of the calving area is important; bedding should be changed between each calving, and large numbers of cows should not be cycled through a few stalls.³³³ Before calving the udder and the perineum of the cow should be cleaned. The calf should receive adequate colostrum.³⁷³ In recent years the use of calf hutches has gained widespread acceptance for managing calves after they have been separated from their dam. This system provides individual isolated housing for each calf. Cleaning is facilitated because the hutches can be moved to new sites between calves. Keeping preweaned calves in groups larger than six puts them at increased risk for diarrhea.³³⁵

Cleaning and disinfection after each calf batch plays an important role in reducing contamination in housed calves. The key to decontamination is the physical cleaning. Physical removal of organic contamination through scrubbing is preferred to application of high-pressure sprays, which can aerosolize organisms, allowing dissemination. On smooth, ideal surfaces physical removal of visible contamination by thorough washing with soap and water removes 99% of the microbial load (two logs). However, on typical housing surfaces washing removes only 90% (one log). Application of disinfectant after washing is important to eliminate remaining pathogens and to prevent bacterial pathogens from proliferating. Physical cleaning cannot be replaced by applying disinfectants in larger quantities, as organic material neutralizes most disinfectants. Disinfectant solutions are applied following cleaning. With regard to disinfectants, pathogen elimination is time dependent.³⁸⁰ Other important variables that influence the effectiveness of disinfectants and rate of pathogen reduction include concentration, temperature, pH, and water hardness.

In addition to cleaning between batches, it is important to clean nipple buckets and other feeding utensils between each feed. Separate equipment should be used to administer oral electrolytes and colostrum. Salmonella and coronavirus are shed in saliva and can contaminate equipment used for oral medication. Washing with warm soapy water is required to remove the fat residue left by milk and colostrum handling equipment.

Several microbial characteristics should be considered when disinfecting equipment that comes into contact with calves. Rotavirus is susceptible to sodium hypochlorite but is relatively resistant to many common disinfectants, such as chlorhexidine. Because as a nonenveloped virus it is not affected by soaps, washing with soap alone may actually spread the virus around on the washed surface.³⁸¹ Coronavirus is an enveloped single-stranded RNA virus and is not as stable in the environment as rotavirus. Because of their envelope, these viruses retain infectiousness better at lower than at higher relative humidity³⁸² and are considerably more sensitive to soaps and common disinfectants than are nonenveloped viruses. Cryptosporidium can autoinfect the original host; consequently, the infectious dose can be exceedingly small. In the environment, cryptosporidia are extremely resistant to most veterinary disinfectants except 5% ammonia, 6% hydrogen peroxide, or 10% formalin.^208,383,384 They survive very well in water, requiring 4 to 11 weeks to decline by one log.³⁸⁵ On the other hand, cryptosporidia are susceptible to drying, with oocyst infectivity declining in 1 to 4 days.²⁰⁹

The most readily cleaned surfaces are made of smooth impervious materials such as plastic and varnished wood (Table 20-4). Usually buildings are cleaned and then either disinfected or fumigated.³⁸⁶ Many disinfectants are inactivated by organic matter; viruses, coccidia, and particularly cryptosporidia may be resistant to their action (Table 20-5). Disinfectants may also be toxic and are best applied by personnel wearing rubber gloves and respirators (if indoors). In general, potent phenolics such as cresol (cresylic acid) are very useful for disinfecting dirty surfaces because they are not inactivated by organic matter and are effective against gram-negative organisms and viruses. The phenolics are highly toxic and leave lingering odors. Hypochlorite solutions (5 g of available chlorine per liter) have a broad spectrum of action but are rapidly inactivated by organic matter. They would be useful as a final disinfectant on previously cleaned surfaces. Because hypochlorite is unstable, it is unlikely to leave toxic residues. Iodophors are not very effective against rotavirus, particularly if organic matter is present. Virkon, a newer disinfectant and cleaner containingpotassium monopersulfate as the active ingredient, is effective for all pathogens except cryptosporidia. Normally a 1% solution is used and is prepared by mixing 10 g of powder with 1 L of water. Contact time should be a minimum of 10 minutes. Virkon has the advantage of having a detergent action that facilitates cleaning.

Table 20-4 Ability of Bacteria to Persist on Various Types of Surfaces Found in Farm Buildings

Modified from Morgan-Jones SC: In Collins E, et al: Disinfectants: their use and evaluation of effectiveness, London, 1981, Academic, p 199.

Table 20-5 Efficacy of Disinfectants Against Enteropathogens

Formaldehyde is one of the few agents that is effective against cryptosporidia. It requires a long contact time and is highly toxic. It is usually used for terminal fumigation in buildings that can be tightly sealed. Formaldehyde gas is best generated by heating paraformaldehyde (5 g/cubic meter of building) in an electrically heated pan at 204° C. Some manufacturers of paraformaldehyde provide pans specially designed for this purpose. The pans should be placed no more than 30 m apart and arranged so that electricity to the pan can be controlled from outside the building. There must be a safety mechanism to ensure that the pan does not overheat and cause a fire. The building must be sealed for at least 24 hours and cannot be entered until it has been thoroughly ventilated. Formaldehyde gas can also be generated by boiling formalin or by adding potassium permanganate to formalin. The latter method generates a violent chemical reaction and carries risks of explosion. Formalin aerosol generators are ineffective.³⁸⁷ After cleaning and disinfection one should allow a rest period for the building to ventilate before reintroducing calves. It is very important that cleaning and disinfection be thorough. Attention should also be given to rodent control because rodents can be a reservoir for Salmonella.³⁸⁸

MINIMIZING PATHOGEN EXPOSURE IN BEEF HERDS

In beef herds, calving areas should be located to take advantage of natural shelter and drainage and rotated from year to year to avoid pathogen buildup.^{330,372,389,390} Pregnant cattle are moved into a clean paddock no more than 2 weeks before the start of calving. It is best that cows and heifers are managed separately until their calves are at least 1 month old. This gives the opportunity to provide better feed to the heifers and to minimize infection between the groups. Confined, wet, or muddy areas should be avoided for calving cows, and if they need to be used the stocking rate should be decreased. Feed-out areas should be rotated and separated from watering points to encourage dispersal. When pasture for calved cows is limited, supplementary feed should be fed to dry cows to ensure enough freshpasture for calving and nursing cows. Clean water should be available in a trough that is accessible to cows and calves. Chronically sick animals, weak calves, and cows with no milk should be removed from the calving paddock and kept isolated from the herd.

Grazing and reproductive management have a significant impact on pathogen load. Where appropriate to grazing management, calving paddocks should be left vacant during the summer. For producers that manage periparturient cows in smaller calving paddocks to facilitate supervision, the emphasis should be on minimizing the stocking density in the calving paddock by removing cows and calves shortly after parturition.^372,389 Alternatively, pregnant cows can be moved away from cows with calves every 1 to 2 weeks or more frequently in large herds. Young calves in the calving paddock will markedly increase the rate of pathogen buildup and the subsequent challenge to newborn calves. Moreover, the increased stocking rate will amplify stress, affecting both transfer of passive immunity and the ability of young calves to rest. Often, calving areas are small because of the perceived need to assist cows with dystocia.³⁹⁰ Well-grown and appropriately fed heifers mated to suitable sires can minimize this need. Beef cows with calves at foot should be moved from the calving paddock into nursing groups that have a maximum age range of 4 weeks and a low stocking rate. Groups should not be mixed until all calves are at least 4 weeks old.^372,389,390 Reproductive management influences pathogen load by determining the age spread of the calves. Sick calves amplify environmental contamination. A prolonged calving period leads to a buildup of contamination so that calves born later in the calving period experience an increased pathogen challenge. It is desirable to maintain a calving period that is less than 60 days.

ENTEROTOXIGENIC E. COLI

The protective efficacy of ETEC bacterins is well documented.^400-403 Because ETEC scours occurs during the first 3 days of life, the neonate does not have time to mount a protective immune response to vaccination. Protection is afforded by vaccinating cows in late gestation so as to ensure high concentrations of anti-K99 colostral antibodies. Good maternal management is required to ensure that the calf receives the maternal antibodies. Antipilus antibodies block the adhesion of the pathogen to enterocytes and subsequently prevent disease.⁴⁰¹ In general it is recommended that the vaccines are given 6 and 3 weeks before calving. Studies with some vaccines have shown that the vaccine is still effective if the priming dose is given 18 months before calving and boosting is carried out in the second half of gestation.⁴⁰⁴ Clinical experience in beef farms with severe outbreaks of E. coli F5 (K99) diarrhea indicates that vaccinating cows that are more than 10 days from parturition can give considerable protection against death from ETEC infection.

Products containing monoclonal antibodies against F5 (K99) antigen have been shown to reduce the severity of diarrhea when calves are experimentally challenged a few hours after receiving the product.⁴⁰⁵ In field situations, however, monoclonal products can have a low efficacy, presumably because a single dose provides only a short period of enteric protection. Antibody supplements are expensive, and vaccination of the dam to boost colostral immunity will usually be more cost-effective. On farms experiencing an outbreak of neonatal diarrhea caused by F5(K99) E. coli, there may be a place for the use of these products until vaccinated cows begin to calve. However, short-term administration (once a day for first 3 days of life) of an antibiotic to which the E. coli is susceptible is also highly effective in preventing diarrhea in herds experiencing outbreaks of ETEC.

SALMONELLA

The successful reduction of Salmonella prevalence in livestock on a national level via implementation of a Salmonella control program emphasizing immunoprophylaxis with modified live and killed Salmonella vaccines indicates the potential benefits that can be derived from the application of effective Salmonella vaccines.⁴⁰⁶ Salmonella vaccine studies in cattle have focused on Salmonella bacterins and attenuated modified live Salmonella.

There are conflicting reports regarding the efficacy of Salmonella bacterins. The reported efficacy of Salmonella bacterins ranges from good to ineffective.^407-415 The overall consensus of these reports is that vaccination of cattle with Salmonella bacterins provides partial protection against Salmonella challenge. In the only reported controlled field trial an autogenous Salmonella bacterin was not found to have any effectiveness.⁴¹⁵ Anaphylactic reactions are occasionally reported in cattle vaccinated with Salmonella bacterins. The cause of these reactions is unknown but has been suggested to be associated with the lipopolysaccharide content of these products. Similar allergic-type reactions in humans caused by Salmonella bacterin vaccination during typhoid outbreaks are well documented.⁴¹⁶

Several naturally occurring and genetically manipulated attenuated Salmonella strains have been used to immunize cattle against salmonellosis. The most widely tested genetically altered Salmonella mutant vaccines in cattle are the auxotrophic strains. Aromatic amino acid (aro) and purine (pur) auxotrophs of Salmonella are attenuated and have decreased virulence.^417-423 Comparative vaccine trials indicate that modified live attenuated Salmonella vaccines provide greater protection against virulent Salmonella challenge than Salmonella bacterins.^413,423,424 Vaccination with modified live Salmonella vaccines attenuates the severity of clinical signs and pathologic lesions and reduces Salmonella shedding and mortality.^406,419,425

Calves immunized with modified live Salmonella vaccines are protected from homologous and heterologous Salmonella serotypes when challenged within 3 weeks of vaccination.^426-428 Live Salmonella vaccines induce transitory T-cell independent nonspecific protection that disappears about 1 month after immunization following clearance of the organisms from the reticuloendothelial system. Thereafter, protection against oral challenge is species- and serotype-specific, with recall of immunity presumably involving specific antigen recognition.^429,430

The level of passive protection of calves achieved by feeding colostrum from vaccinated cows is questionable. Several reports suggest that immune colostrum provides passive protection, but others report no protective effect. In trials in which protection was achieved, calves were challenged at 1 week of age; trials in which no protection was observed involved challenging calves at 3 weeks of age, suggesting that the duration of passive immunity associated with colostral transfer is relatively short. Considering that many calves are exposed to Salmonella in the first week of life, colostral protection may be useful.

ROTAVIRUS AND CORONAVIRUS

Bovine coronavirus is associated with several diseases in cattle. All BCV isolates are believed to belong to a single serotype.^169a Differences in hemagglutination-inhibition characteristics have been used to classify strains as types 1 through 3.^169b There are seven serogroups of rotavirus, with group A accounting for the majority of pathogenic isolates. Members of the group A rotaviruses are further classified according to antigenic and genetic differences in their outer capsid proteins, G and P. Both of these proteins are involved in neutralization of infectivity in vitro and in vivo.⁴³³ In the United States eight G serotypes and genotypes and four P serotypes and genotypes have been identified in cattle isolates.¹⁵³ The genome of rotavirus is composed of 11 gene segments that can be exchanged among isolates when animals are infected by more than one virus at the same time.⁴³⁴ Genetic reassortment can generate new progeny viruses that can evade what was once a protective immune response, thus allowing persistence of rotavirus in susceptible populations.⁴³³

Two approaches have been taken with immunoprophylaxis against rotavirus and coronavirus infections in calves. The first approach involves oral vaccination of neonatal calves with a modified live vaccine. Calves begin producing detectable levels of local secretory IgM within 4 to 6 days of vaccination.⁴³⁵ Calves are resistant to challenge from the initial appearance of local IgM antibodies.⁴³⁵ In order to consistently elicit an effective immune response, the vaccine must be administered orally, immediately after birth, and before the calf has nursed because the colostrum of most cows contains virus-neutralizing antibodies that interfere with the vaccine.⁴³⁶ There are conflicting reports of efficacy with these type of vaccines. In double-blind field studies that include vaccinated and nonvaccinated calves the vaccine was not shown to be effective.⁴³⁷ Conversely, when all calves were either vaccinated or not vaccinated in sequential comparisons, morbidity and mortality were significantly reduced.⁴³⁷

The second approach involves intramuscular vaccination of pregnant cows with either modified live vaccine or inactivated viral vaccines to stimulate high levels of specific viral neutralizing antibodies in colostrum and milk during the first several days of the calf’s life. Infectious viral particles are neutralized within the gut lumen, preventing infection of intestinal villous enterocytes. One advantage of passive immunization is the fact that cross-protection between serotypes is less of a problem. This is because vaccination of a mature cow that has had natural rotavirus exposure leads to cross-serotype stimulation of heterotypic antibodies.⁴³⁸ Single serotype vaccination therefore stimulates antibody production to a wide range of rotavirus serotypes, negating the need for multivalent rotavirus vaccines. Passively absorbed anti-bovine rotavirus IgG1 antibodies are transferred to the small intestinal lumen, where they protect against experimental challenge.⁴³⁹ Antigen sensitized maternal lymphocytes also confer partial protection against challenge with virulent bovine rotavirus.⁴⁴⁰ Colostrum and milk with a high virus-neutralizing antibody titer are highly protective while being consumed by the calf. For example, administering 400 mL of immune colostrum daily to calves from days 2 to 12 reduced the incidence of diarrhea from 41% to 3% in one study.⁴⁴¹ The concentration of rotavirus- and coronavirus-neutralizing antibodies in milk of vaccinated cows falls below protective levels by 3 to 7 days after parturition.^442-444 Rather than complete protection, the manifestations of passive immunity to bovine rotavirus that are often noted are (1) a delay of a few days in the onset of clinical signs, (2)a reduced severity of clinical signs, and/or (3) a reduction in the length of the period of viral shedding associated with infection.⁴⁴⁵ Although there are reports of successful field trials involving bovine rotavirus— and bovine rotavirus-coronavirus—vaccinated cows,^402,446,447 negative esults have also been reported.⁴⁴⁸ A common problem with commercial vaccines on the market in the United States and Europe is a lack of vaccine-specific data supporting efficacy claims. Protection correlates with serum titers; independent studies have sometimes failed to demonstrate effective seroconversion with some products.⁴⁴⁹

DIET

The microbial quality of the diet is an important factor in preventing diarrhea. Calves that are fed milk from mastitic quarters or antibiotic-containing milk are at increased risk for diarrhea.³³⁵ After fresh colostrum feeding, young calves have less diarrhea if placed on whole cow’s milk rather than on other diets. Pasteurizing surplus colostrum and waste milk reduces the incidence of diarrhea in animals on these diets.⁴⁵⁰ It is also important to offer a good-quality calf starter from approximately 2 to 3 days of age; at first little will be ingested, so offer small amounts and keep it fresh.

Some producers routinely administer vitamin A to neonatal calves. Many, but not all, studies in children indicate that supplementation can reduce the incidence of diarrhea in areas in which clinical and subclinical vitamin A deficiency is endemic.⁴⁵¹ In cattle, vitamin A deficiency is most likely when a diet of unsupplemented straw and grain is fed. Calves born to cows fed good-quality green forage or cattle receiving a vitamin A supplement should not require supplementation—particularly if they received adequate colostrum. Enteric absorption of vitamin A is diminished in calves with cryptosporidiosis, so the systemic route should be used in calves with this type of infection.⁴⁵²

BIOSECURITY

Infectious diseases are often purchased with brought-in stock. Operations that buy calves for rearing purposes should be encouraged to buy from as few sources as possible. Direct purchase from one farm is best; assembling collections of calves through auction markets should be avoided if possible.

Examination

Physical examination of the diarrheic calf is the first step in establishing therapeutic needs. It is important to determine the presence of any intercurrent disease. Treatment of uncomplicated cases of diarrhea depends on the estimation of dehydration, severity of acidosis, likelihood of intercurrent infection, presence or absence of hypothermia, and hypoglycemia. The severity of dehydration is gauged from the eyeball position and skin tent (Table 20-6). In acute diarrhea the degree of enophthalmus is the most reliable indicator of dehydration, but because the position of the eye within the orbit is also dependent on body fat stores, the skin tent in the cervical region may be the most reliable in calves with chronic diarrhea or cachexia.⁴⁵³ Skin tent can be measured over the eyelids and neck. Best results are obtained when the neck is held straight and the skin of the midneck is tented in the direction of the long axis of the neck to avoid the natural skinfolds that run across the neck. Some have claimed a relationship exists between severity of dehydration and acidosis. However, this has not been borne out in studies of diarrheic calves.⁴⁵⁴ Instead, acidosis can be gauged from the calf’s sucking or drinking drive, the degree of weakness, and the age of the calf (Fig. 20-4).^454,455 Estimation of severity of acidosis either from laboratory or physical findings is very important to the successful therapy of severely depressed calves. Rectal temperature measurement will determine whether or not the calf is hypothermic.

Table 20-6 Guidelines for Assessing Dehydration in Neonatal Calves

Fig. 20-4 Prediction of severity of metabolic acidosis from body position, strength of suck reflex, and age.

Heart rate is variable in diarrheic calves. Bradycardia (<90 beats/min) is clinically important and may indicate the presence of hypothermia, hypoglycemia, or hyperkalemia. Cardiac arrhythmias occur⁴⁵⁵ and are usually a result of severe hyperkalemia (K⁺ above 8 mEq/L) (Fig. 20-5). Hyperkalemic arrhythmia can usually be differentiated from arrhythmia resulting from cardiomyopathy (selenium deficiency) because the heart rate is not elevated. The presence of bradycardia or arrhythmia indicates the need for immediate fluid therapy with bicarbonate-containing solutions to prevent death.

Fig. 20-5 Bradycardia and atrial standstill in a severely hyperkalemic diarrheic calf. Heart rate is 84 beats/min. There is bigeminy, and the T waves are abnormally large. There is only one P wave, and it is not conducted. Serum potassium was 8.9 mmol/L, and sodium 116 mmol/L.

Body condition is usually good at the start of an attack of diarrhea. Poor condition often indicates chronic infection, mismothering, or poor feeding programs, which may be exacerbated by milk withdrawal for therapeutic purposes.

It is important to check for intercurrent infections, which are easily missed even with careful examination. The lungs should be examined for signs of pneumonia; the navel palpated for pain, swelling, and wetness; and the joints checked for signs of distention and lameness. The boundary between calves in which the primary insult is septicemia with a secondary diarrhea and those in which primary diarrhea is complicated by septicemia is blurred. Calves that are recumbent, under a week of age, have lost their suck reflex, or have evidence of intercurrent infection are more likely to be septicemic and require concurrent antibiotic therapy.⁴⁵⁶ Calves in which septicemia has progressed to produce signs of meningitis (e.g., extended neck with reluctance to flex neck), joint involvement, or ophthalmic signs (congested scleral vessels withhypopyon or iridospasm) have a poor to very poor prognosis. It is best to identify these cases before instituting therapy so that the owner can decide whether treatment is economically feasible.

The laboratory is useful for quantifying metabolic disturbances in diarrheic calves. Blood gas analysis will accurately determine the degree of metabolic acidosis. This is not routinely available to most practitioners. However, it may be worthwhile to make special efforts to get measurements when setting up a fluid therapy protocol for your area or when dealing with cases that fail to respond to treatment. Blood can be collected into a heparinized syringe and the syringe capped, placed in a Styrofoam cup surrounded by ice, and transported 4 hours or more to a laboratory. Alternately, the practice laboratory may have a total CO₂ (Harleco) analyzer or access to serum bicarbonate estimation as part of the serum chemistry profile. Total CO₂ or bicarbonate is a useful index of the severity of metabolic acidosis. Blood glucose can be readily determined using a hand-held Glucometer.

Fluid Therapy

The most common causes of death in diarrheic calves are dehydration and acidosis.⁴⁵⁷ The immediate objective in treating depressed diarrheic calves is to restore them to a normal systemic state. In some calves it may also be necessary to correct hypoglycemia or hypothermia, restrict milk intake, or give antibiotics.

The estimated severity of dehydration can be combined with estimates of losses through diarrhea and for the maintenance of essential functions to give the total daily fluid requirement (see Table 20-6). The hydration status of the calf can be estimated from the degree of enophthalmus, the degree of skin tent on the neck, and evaluation of the mucous membranes (see Table 20-6). The volume (L) required to replace the deficit is determined as follows:

The ongoing losses through diarrhea should be estimated from the nature and volume of the diarrhea. Fecal losses can range from 1 to 6 L in diarrheic calves.⁴⁵⁸ Maintenance requirements have been estimated at 50 to 100 mL/kg/day.^6,458 The degree of hydration and the volume of feces passed should be reassessed daily, and the treatment adjusted accordingly. Only 60% to 80% of oral fluids are absorbed, and this needs to be accounted for in the calculation.⁴⁵⁹

Bicarbonate requirements can be calculated from base deficit values (based on blood gas measurements or estimated from physical findings) as follows:

A chart of bicarbonate requirements for various body weights and base deficit values is available (Table 20-7).

Table 20-7 Calculation of Bicarbonate Requirement from Calf Body Weight and Severity of Acidosis

Measurements of serum total carbon dioxide content or bicarbonate are also reliable estimates of bicarbonate requirements.⁴⁶⁰ Bicarbonate requirements are as follows:

For example a 40-kg calf has a serum bicarbonate or TCO₂ of 10 mmol/L. The calf has a bicarbonate deficit of 30 mmol/L − 10 mmol/L = 20 mmol/L, so 40 kg × 20 mmol/L × 0.5 = 400 mmol bicarbonate required to replace existing deficits. Ongoing diarrhea may require additional bicarbonate.

Calves that are unwilling to suck and that are severely depressed are best treated with intravenous fluids. Calves that are only moderately depressed may also be treated with intravenous fluids if the condition is worsening rapidly. Catheterization is easier if a No. 15 scalpel blade is used to nick the skin. If it proves very difficult to catheterize the calf, it can be suspended upside down so that blood will pool and distend the jugular veins. The calf’s neck should be clipped and prepared before inversion, and the calf laid flat as soon as the catheter is placed. It should be possible to place a catheter in less than a minute even in severely dehydrated calves using this technique. Once the catheter is placed, fluids can be administered. If the calf is hypothermic the fluids should be warmed before administration because cold fluids can decrease cardiac output and may kill a critically ill calf. Fluids can be warmed by a number of methods; one convenient technique is to run the fluids through a coil of tubing immersed in a bucket of hot water (check the temperature regularly) on the way to the calf.

Although saline-based fluids are suitable for rehydration (Table 20-8), most severely depressed calves are acidotic, and more consistent recovery is obtained if an alkalizing agent is also used. A wide variety of alkalizing agents is available (lactate, acetate, gluconate), but clinical trials show that only bicarbonate is consistently effective in severely acidotic calves (Fig. 20-6).^93,461 Many diarrheic calves require large amounts of bicarbonate to correct their acidosis.⁹¹ An isotonic solution (156 mmol/L) of bicarbonate can be readily made by dissolving 13 g of sodium bicarbonate (baking soda) in 1 L of water.⁴⁶² Sodium bicarbonate solutions can be mixed with saline; there is a possibility that precipitates may form if bicarbonate is mixed with calcium-containing solutions such as Ringer’s.

Table 20-8 Fluids Commonly Used in Intravenous Therapy

Fig. 20-6 Comparison of the alkalizing effect of various bases in diarrheic calves with severe dehydration and acidosis. At the start of the trial, all calves were at least 8% dehydrated and had a mean blood pH of 7.032 and a base deficit of 18 mmol/L. All calves received a total of 7.2 L of fluid containing 102 mmol/L of saline plus 50 mmol/L of the sodium salt of the respective alkalizing agent. Letters (a, b, c, d) are statistically different (p < .5) from one another at that point.

From Kasari TR, Naylor JM: Clinical evaluation of sodium bicarbonate, sodium L-lactate, and sodium acetate for the treatment of acidosis in diarrheic calves, J Am Vet Med Assoc 187:392, 1985.

Some clinicians may prefer to rehydrate the neonate first and then reconsider the need for bicarbonate if it is not up and sucking within 12 hours of therapy. However, this is time-consuming. It is not always necessary to completely correct acidosis; blood pH from 7.25 to 7.45 has little adverse affect (normal calves have a venous blood pH of 7.34, bicarbonate of 30 mmol/L, and a base excess of 5 mmol/L).

Ideally, dehydration and acidosis should be corrected over a 24-hour period. However, it is unusual to see problems when the fluid and acid-base deficits are corrected over 4 hours, although the calf may continue to improve after this period. Rapid resuscitation techniques include the administration of either hypertonic saline dextran or hypertonic sodium bicarbonate. Hypertonic saline dextran (7.2% saline containing 6% dextran 70) administered at 4 mL/kg of body weight during a 4-minute period concurrently with an isotonic alkalizing oral electrolyte solution is effective in resuscitating dehydrated calves with diarrhea.^463,464 Hyperosmotic sodium bicarbonate solutions may also be used to rapidly resuscitate acidotic dehydrated calves.⁴⁶⁵ The base deficit can be corrected by administering 8.4% sodium bicarbonate IV at a rate of 1 mL/min/kg over 15 minutes.^466,467 Hypertonic fluids should always be given concurrently with an isotonic oral electrolyte solution. After 24 hours of appropriate therapy, one would expect the calf to be standing and show a good suck reflex. Persistent depression is usually a sign of uncorrected acidosis or toxemia.

Most diarrheic calves are not markedly hypoglycemic, but glucose supplementation is needed to treat severe hypoglycemia (glucose concentrations <2 mmol/L or <36 mg/dL). Severe hypoglycemia is treated by adding glucose to the intravenous fluids to a final concentration of 2.5% to 5%. Severe hyperkalemia is seen in some dehydrated diarrheic calves, but this responds to rehydration (restores renal perfusion and dilutes out potassium) and correction of acidosis (redistributes potassium into the cells). Nutritional support is not needed if the calf is in good body condition but should be given if the calf is emaciated or if the calf has been deprived of milk for more than 3 days.

There are increased losses of potassium in diarrhea; the significance of this is uncertain, although potassium depletion can result in weakness. Usually there is no need to add potassium to intravenous fluids; the majority of calves respond to infusion of saline and 1.3% sodium bicarbonate. After 12 to 24 hours of intravenous fluid therapy most calves start on oral electrolyte solutions. These usually contain 10 to 30 mmol of potassium per liter. Clinical trials comparing the efficacy of low and high potassium solutions have not been reported.

Once the calf is able to nurse or drink, therapy is usually switched to oral electrolytes. This is also the route of choice for treatment of mildly affected calves on the farm. Calves with weak suck reflexes and calves that are unused to hand-feeding can be administered electrolytes using an esophageal feeder. Oral electrolyte solutions need to supply sufficient sodium to facilitate normalization of extracellular fluid deficits, nutrients to facilitate absorption of sodium from the intestine, alkalizing agents to treat metabolic acidosis, and supplemental energy.⁴⁶⁸ The first two requirements depend on the coupled active transport of glucose and sodium ions across the brush border membranes of enterocytes, which results in passive absorption of water and other electrolytes.⁴⁶⁹ This function remains largely intact in calves with ETEC diarrhea, but when there is endothelial damage it may be impaired. Certain amino acids (glycine, L-alanine, L-glutamine) enhance the absorption of sodium and water,⁴⁶⁹ as do acetate and propionate.⁴⁷⁰ In order to effectively combat acidosis, oral electrolyte solutions need to contain 50 to 80 mmol of alkalizing agent per liter. Acetate, lactate, citrate, gluconate, and bicarbonate are all used as alkalizing agents. Bicarbonate combines with hydrogen ions directly, whereas the other agents remove hydrogen ions during their metabolism within cells.⁴⁵⁸ Electrolyte solutions that contain >40 mmol of bicarbonate or citrate per liter have marked adverse effects on milk clotting.⁴⁷¹ Bicarbonate raises abomasal pH, whereas citrate binds calcium, and so the presence of either interferes with the normal clotting of milk in the abomasum. Breakdown of abomasal milk clots results in the gradual release of some nutrients into the small intestine. Bicarbonate also reduces milk digestibility. A reduced growth rate was recorded when electrolyte solutions with bicarbonate were fed to milk-fed calves.⁴⁷² Solutions containing bicarbonate may also alkalize the gastrointestinal tract of milk-fed calves and promote bacterial overgrowth in the small intestine as well as ETEC attachment and toxin production.⁴⁷³ Acetate is the best alkalizing agent to include in electrolytes that are to be fed to calves that are still receiving milk; it has excellent alkalizing ability and does not interfere with milk clotting in the abomasum. Any of the commonly used alkalizing agents are likely acceptable for calves held off milk.⁴⁷¹

A wide variety of oral electrolyte preparations are on the market, and different products are suited to different situations. Almost all the products contain water and electrolytes and are suitable to use for rehydration (Table 20-9). Beware of products that are designed for medicating hundreds of liters of water; the final solution is often very dilute (<10 g of electrolytes per liter) and will not rehydrate sick calves. These products are often marketed as “boosters” and “stress relievers.” Glucose and glycine are usually added to oral electrolyte solutions to facilitate sodium absorption. Research in people has shown that adding glycine to solutions containing 110 mmol/L of glucose aids rehydration. It is probable, however, that there is little additional benefit to supplementing solutions that contain more than 200 mmol/L of glucose with glycine. Solutions that contain large amounts of glucose are hyperosmolar and are absorbed more slowly than isotonic solutions, but the differences are too small to be clinically important.⁴⁷⁴ The ionic composition also affects absorption; mixtures containing sodium chloride and citrate, bicarbonate, or acetate have improved absorption over chloride salts alone. Oral electrolyte solutions are almost completely absorbed in healthy calves, but absorption can be as low as 60% in severe E. coli diarrhea.⁴⁷⁵

Table 20-9 As-Fed Composition of Some Available Oral Electrolyte Solutions for Calves

The ability to counteract acidosis varies greatly among oral electrolytes. Some have a net acidifying effect, whereas others alkalize blood (Fig. 20-7). These differences are therapeutically important and are responsible for differences in survival rates among products. Highly alkalizing solutions give the best results. One study showed that it was more important that an electrolyte solution contain bicarbonate than chloride.⁴⁷⁶ This is particularly important in older calves. Recently there has been interest in adding glutamine to oral electrolyte solutions because it is an important fuel for the gastrointestinal tract and can promote mucosal repair.^477-479 However, studies show that oral electrolytes containing glutamine as the sole amino acid are no more effective in diarrheic calves than other well-designed solutions that use glycine as the amino acid.⁴⁸⁰ Psyllium has been added to some oral electrolyte solutions for a number of perceived benefits, but controlled studies show no clinical advantages, although there may be some moderation of bacterial fermentation within the gastrointestinal tract.^481,482

Fig. 20-7 Comparison of the alkalizing abilities of various oral electrolyte solutions. The Y axis shows the mean alkalizing effect after administration of one treatment (1.9 to 2.25 L) of the fluid to healthy calves, and the X axis shows the net amount of alkalizing agents in the solution. Bicarbonate and acetate are experimental solutions. (Life-Guard/Enterolyte is manufactured by SmithKline Beecham Labs, Rogar is Rogar STB’s electrolyte powder, Hydra is manufactured by Vetrepharm, Electro-Plus A by Pitman-Moore, Ion Aid by Syntex, and Re-Sorb by Beecham.)

Modified fromProc 14th World Congr Dis Cattle 1:362, 1986.

The other problem to be considered in the chronically scouring calf is the need for nutritional support. Maintenance metabolizable energy requirements for a 50-kg calf are about 2000 kcal, and 3500 kcal are required to support a weight gain of 0.5 kg/day. These requirements can be met by 3.3 and 5.7 L of whole cow’s milk, respectively. Comparative studies indicate that weight loss in calves fed oral electrolytes are inversely proportional to the energy content of the solutions.⁴⁸³ Assuming a 4-L daily intake and 100% digestibility of oral electrolyte nutrients, regular electrolyte solutions supply between approximately 15% and 25% of energy needs. As a result, diarrheic calves that are held off milk for prolonged periods lose weight⁴⁸⁴ and can become emaciated. When maintaining body condition is a concern and little milk or solid food is being ingested, then a high-energy oral electrolyte should be fed. Products such as Enterolyte HE provide about 50% of energy requirements if fed twice a day (total intake 4 L) and about 75% if fed three times a day (total intake approximately 6 L). The energy content of various oral electrolyte solutions is presented in Figure 20-8.

Fig. 20-8 Comparison of the energy contents of various oral electrolyte solutions. Milk is also shown for comparative purposes. *ME estimated from water-soluble carbohydrate composition. ^†ME estimated from glucose and other water soluble carbohydrate composition of as-fed electrolyte solution assuming 1g (cry matter) of carbohydrate = 4kcal. This figure assumes electrolyte solutions are fed to neonatal calves prior to rumen development. (See text for manufacturers.)

Role of Antimicrobials

There is some controversy regarding the use of antimicrobials for the treatment of calf scours. Reports questioning the use of antimicrobial therapy cite lack of efficacy, potential for adverse effects, potential for violative residues, and selection for antimicrobial resistance. Conversely, there are reports describing attenuation of clinical disease, reduced pathogen shedding, and lower mortality after the use of antimicrobials to treat scouring calves.

Antiprotozoal Drugs

Drugs reported to have some efficacy against Cryptosporidium in calves include, halofuginone,^516-522 paromomycin,^523,524 decoquinate,^525,526 and β-cyclodextrin.⁵²⁷ Halofuginone is licensed for treatment of calves in Europe and appears to be the most efficacious. The efficacy of decoquinate is questionable, with the only controlled clinical study failing to demonstrate a beneficial therapeutic effect with daily treatment at 2 mg/kg/day.⁵²⁶ A trial of lasalocid for treatment of Cryptosporidium infection has been conducted. Using a toxic dose of 8 mg/kg was found to reduce the shedding of cryptosporidia; however, the calves suffered adverse side effects. At a dose of 0.8 mg/kg, lasalocid was not effective.⁵²⁸ The registered dose for preventing coccidiosis in calves is 1 mg/kg per head per day.

Coccidiosis is uncommon in calves less than 6 weeks of age. In hand-reared calves coccidiostats (lasalocid, amprolium, or decoquinate) may be added to milk replacer. Prophylactic options for beef calves are restricted to coccidiostat medicated pellets (monensin, lasalocid, amprolium, or decoquinate) or water (amprolium or sulfonamides). Therapeutic options include amprolium or sulfonamides such as sulfadimidine.

Both fenbendazole (5 mg/kg PO once daily for 3 days ) or albendazole (20 mg/kg PO once daily for 3 days) have been shown to be effective treatments for Giardia.^222,298,529 Because of the high level of subclinically affected animals, all cows and their dams need to be treated, and reinfection is likely to occur unless calves are removed from environmental sources of infection.

Antiinflammatory Drugs

A nonstatistical trend toward decreased morbidity has been reported in a study evaluating the benefits of a single or double injection of flunixin meglumine in scouring calves.⁵³⁰

Probiotics

Probiotics are foods or drugs containing live microbes that are expected to confer beneficial physiologic effects to the host animal through microbial actions. Bacterial and fungal species included in these products include Enterococcus faecium, Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus plantarum, Lactobacillus rhamnosus, Bifidobacterium bifidum, Streptococcus salivarius subsp. thermophilus, Aspergillus oryzae, and Candida pintolopesii. General mechanisms of action that have been ascribed to probiotics include competition for receptor sites on the intestinal surface, immune system stimulation, excretion of antimicrobial substances, and competition with pathogens for intraluminal nutrients.⁵³¹

The number of controlled clinical trials evaluating probiotic formulations in calves is limited. In one report, feeding antimicrobial resistant Streptococcus faecalis to calves reared on an antibiotic-containing diet reduced Salmonella intestinal colonization of calves.⁵³² An improvement in weight gain and a reduction in diarrhea have been reported when calves were fed either 3 × 10⁹ Bifidobacterium pseudolongum or L. acidophilus daily from 1 to 56 days of age or a cell mixture containing 10¹⁰ colony forming units (cfu) of Bacillus thermophilum, 10¹⁰ cfu of Enterococcus faecium, and 10⁹ cfu of Lactobacillus acidophilus for 28 days.⁵³³

Intestinal Protectants

Several products that include intestinal protectants are marketed for treatment of calves with scours. Intestinal protectants include bismuth subsalicylate, kaolin or pectin, and activated charcoal. There are no efficacy data available regarding the use of kaolin in scouring calves. Suggested advantages of bismuth subsalicylate are its neutralization of bacterial toxins and antisecretory effect through its local antiprostaglandin activity.^534,535

Prognosis

The prognosis for recovery decreases with the severity of depression. Severe hypothermia and the presence of intercurrent disease are grounds for a guarded prognosis.^536-538 An initial examination should be performed. Calves with a primary problem of septicemia are not usually worth treating because of the poor prognosis. The severity of dehydration, hypothermia, and acidosis should be estimated. Recumbent calves are usually treated IV with a saline-based rehydrating fluid (0.9% saline, Ringer’s, lactated Ringer’s, and so on) and isotonic sodium bicarbonate (especially for older and comatose calves). Calves that can suck are treated with oral electrolytes that contain 50 to 80 mmol/L of alkalizing agent. Products that use mainly acetate as the alkalizing agent are best for calves that are still drinking cow’s milk (small quantities, frequently). Any alkalizing agent is likely effective in calves held off milk.