Scours in calves: which are the main pathogens behind?

Scours also known as neonatal diarrhoea (ND) is a gastrointestinal disease affecting calves during the first weeks of their lives and the most common cause of illness in both dairy and beef calves. The disease can be economically devastating to producers due to high mortality, impaired growth of affected animals and, later on, reduced milk production in the first lactation^1–3. ND is also a significant welfare issue with affected calves displaying sickness behaviour such as increased lying time⁴ and decreased milk consumption⁵.

Scours can be caused by parasites, bacteria or viruses alone or in combination

Various enteric pathogens including viruses, bacteria and parasites can cause scours (Table 1) with rotavirus group A, coronavirus, Cryptosporidium parvum and bacterial pathogens (enterotoxigenic Escherichia coli F5 and Salmonella spp.) being the ones the most frequently incriminated in ill calves^6–8. Co-infection, that is the detection within the same calf of more than one pathogen, is also frequently observed.  

Table 1. Pathogens associated with neonatal diarrhoea (ND) in calves. The most frequently isolated enteric pathogens are listed in blue.

Scours in calves: when does it occur?

The occurrence of the various agents responsible for calf scours varies with the age of the calf. This can be useful in establishing the likelihood that a particular agent is involved (figure 1). For example, F5 E. coli typically causes diarrhoea in calves less than 6-days old, whereas Cryptosporidium is not detectable in the faeces of calves before 3-5 days old.

Figure 1. Timeline showing the average age of occurrence for scours in calves according to the causative agent.

Scours in calves: what are the “pathophysiological” consequences?

Whatever the pathogen(s) behind the clinical illness, affected calves excrete watery faeces with increased frequency. Without any treatment, the loss of liquids leads to a progressive dehydration, the loss of electrolytes, metabolic acidosis and possibly death within 2 days (figure 2)⁹.

Figure 2. Pathophysiological consequences of scour. Adapted from Carter et al., 2021⁹.

What are the characteristics of the major scours pathogens?

Rotavirus

Rotavirus (RVA) causes mild to severe diarrhoea in calves younger than 3 weeks old. The transmission occurs by the faecal-oral route. Infected animals shed around 10¹¹ viral particles/gram of faeces. The virions are highly resistant to chemical and physical inactivation, remaining infectious in humid environments up to 4 months. Rotaviruses replicate in the mature enterocytes of the small intestine. The tips of the villi undergo cell lysis, resulting in villous blunting and ischemia, causing maldigested, malabsorptive and secretory diarrhoea^10–12.

Coronavirus

Bovine coronavirus (CoV) causes pneumonia and diarrhoea in calves and adult cattle (winter dysentery). Calves become infected early after birth from faecal contamination of carrier dams and are susceptible between 1 day and 1 month of life. Infected calves shed high ~1 billion virions/g of faeces, remaining infectious only for up to 3 days in the presence of organic material unless faeces remain frozen. Its stability in frozen faeces and the aerosol transmission explains its prevalence in the winter months. CoV diarrhoea is prevalent in artificially reared dairy calves. Scour due to bovine coronavirus is usually more severe than that of rotavirus, resulting in muco-hemorrhagic enterocolitis (figure 3). Virus replication in the intestinal crypts leads to destruction of the absorptive intestinal villus epithelial cells in the entire small intestine, and eventually spreads throughout the large intestine, causing atrophy of colonic ridges and malabsorptive diarrhoea. Undigested nutrients are fermented in the large intestine, promoting bacterial overgrowth and production of organic acids, especially D-lactate, contributing to electrolyte losses and leading to dehydration and metabolic acidosis. The severity of the disease is increased and the incubation period is shortened in younger calves, especially those with inadequate passive transfer of immunity. Most calves recover, but some may develop pyrexia, recumbency, cardiovascular collapse, coma and death. Some CoV strains behave as a pneumoenteric pathogen, also replicating in the upper respiratory tract. Infected calves may develop both diarrhoea and respiratory disease^15–17.

Figure 3. Calf with bovine coronavirus scour¹⁴.

Escherichia coli

E.coli is a gram-negative bacterium. Diarrheagenic E. coli are now broadly classified according to virulence mechanisms. Enterotoxigenic strains of E. coli (ETEC) bearing different pili (F5, F17 and CS31A) bind to the intestinal epithelium without causing apparent damage to enterocytes and produce a thermostable toxin (ST) that causes secretory diarrhoea mostly in the first week of life. Other pathotypes include enterohemorrhagic (EHEC) producing hemolisin, shiga-toxin (STEC) and attaching-effacing (AEEC) E. coli that are usually associated with dysentery in calves between 8 to 21 days of age. AEEC allows the attachment of the bacteria to the apical cell membrane of the enterocyte causing a lesion characterised by degeneration and exfoliation of the microvilli in the small and large intestine of calves between 1 and 5 weeks of age. The infection causes enterocolitis, diarrhoea and dysentery. Other E.coli strains invade the bloodstream causing septicemia ^18-22.

 Salmonella spp.

Salmonella is a genus comprising gram-negative bacteria belonging to the family Enterobacteriaceae. Salmonella enterica serovar Dublin is host-specific for cattle while Salmonella enterica serovar Typhimurium is pathogenic for cattle and other species. Calves from two weeks up to 3 months old may become clinically infected. During an outbreak, calves may show diarrhoea with or without blood and mucus, fever, depression, respiratory signs, septicemia, and death. Macroscopic observation shows jaundice, pseudomembranes on the small intestinal mucosa and enlargement of the mesenteric lymph nodes. After an outbreak is controlled, several animals may remain as carriers. Cows become carriers of S. Dublin, with intermittent shedding, at higher levels in the peripartum period. In dairy farms, especially those with large numbers of cattle, Salmonellosis may become endemic. Salmonella infections usually occur via the fecal-oral route²³.

Cryptosporidium parvum

Cryptosporidium parvum is a protozoan frequently isolated in calves with diarrhoea and that infects the distal small intestine causing a mild to severe diarrhoea. In severe cases faeces can contain mucus, blood, undigested milk, or bile. C. parvum is transmitted via fecal-oral ingestion of oocysts that are resistant to most disinfectants and can survive for several months in cool and moist conditions. Susceptibility windows in calves go from one to four weeks of age. Affected animals frequently need oral or parenteral fluids to restore their hydration and metabolic disturbances. Halofuginone and paromomycin are reported to be effective treatments.^1,24–27