Bovine Coronavirus

coronavirus  (22)

Genus: Coronavirus (19)

Family: Coronaviridae (19)

Bovine coronavirus is an enveloped, single stranded RNA virus (19). The genome is a positive sense RNA molecule of 27-32 kilobases (10). It is a pneumoenteric virus that causes mild or severe respiratory infections and severe lower intestinal infections with enteritis (19). Enteric disease is often seen in combination with respiratory clinical signs (10). In addition to causing diarrhea in young calves, it is associated with winter dysentery in cows (19).

Coronavirus is particularly common in 4-30 day old calves (15) and is seen most commonly in calves approximately 1 week of age which is the time when antibody levels in the dam’s milk starts to wane and calf exposure is maximal (13). Disease due to coronavirus is most common during the winter months which may reflect the enhanced survival of the virus in a cool, moist environment (17). One study shows that the incidence of coronavirus is 8-69% in diarrheic calves, and 0-24% in healthy calves (17).

Viral  Transmission

The virus can be transmitted nasally and via fecal-oral routes (19) and is often shed in both nasal secretions and feces (10). Aerosol transmission is also possible (15). Virus particles are excreted by adult cattle which are usually asymptomatic (15). The majority of adult cattle are seropositive and the virus can be shed by up to 70% of adults despite the presence of specific antibodies in the serum and feces (17). Calves born to carrier cows are at a higher risk for developing coronaviral diarrhea (17). Virus excretion tends to increase around the time of parturition (15) and peak shedding also tends to occur during the winter months in North America (17). Once infected, calves initially excrete high levels of virus and are potent sources of contamination of pens (15). The infection persists for weeks in apparently recovered calves and these animals excrete low levels of the virus for weeks (15). Subclinical persistence and recurrent infections are common in neonatal and older calves and excretion from these animals may maintain a reservoir of infection (17). Infection often results in high morbidity but usually has low mortality (10).

Pathologic Effects

It exerts an effect on both the small and large intestine (15). In the small intestine it destroys the absorptive cells of the villi and is able to cause more severe villous atrophy than rotavirus (15). In the large intestine it causes widespread destruction of cells of the colonic ridges (15). The crypt epithelium in the intestine is also affected, which prolongs regeneration of villous cells and results in persistent diarrhea for several days and potentially death from dehydration and malnutrition (17). Coronavirus is more likely to be associated with signs of colitis (straining, passage of mucus or occasionally blood in the feces) because it causes a more widespread pathology than rotavirus (15).

scouring calf (23)

The Disease

The diarrhea caused by coronavirus usually lasts 4-5 days (13). There is rapid loss of water and electrolytes (13). Glucose and lactose metabolism are also affected resulting in hypoglycemia, lactic acidosis and hypervolemia which can lead to acute shock, cardiac failure and death (13). Experimental infection of calves has resulted in depletion of lymphocytes in the mesenteric lymph nodes and Peyer’s patches, low levels of immunoglobulins and generalized immune suppression (17).