Bovine Rotavirus

rotavirus (22)

Genus: Rotavirus (1)

Family: Reoviridae (1)

Bovine rotavirus has a genome of 11 double stranded RNAs enclosed in a triple layered capsid (1). There are at least 7 antigenic groups of rotaviruses (A-G), groups A, B and C are known to occur in mammals (1). Group A bovine rotaviruses are among the enteropathogenic agents more commonly associated with neonatal diarrhea in calves up to 30 days old (1). Group A rotaviruses have a high prevalence of infection; overall it is approximately 30% in normal neonates and 40% in diarrheic animals (17). The most prevalent serotypes of group A rotaviruses in dairy and beef calves with diarrhea in the United States are G6 and G10 (17).

Factors influencing the severity of rotavirus infection include: age, immune status of dam and absorption of colostral antibody, ambient temperature, degree of viral exposure, the occurrence of weaning, and the presence of other enteropathogens (17).

Rotaviruses are the most common cause of neonatal calf diarrhea, they most often affect calves that are 4-14 days old but are also probably in younger and older calves (15). Calves are most susceptible between 1 and 3 weeks of age, this is related in part to the rapid decrease in specific colostral antibodies to rotavirus that occurs at this time (17). Rotavirus can be considered a primary pathogen in calves but field and laboratory investigations indicate that multiple mixed infections are probably more common than single pathogen infections (17). Rotavirus is most commonly associated with diarrhea outbreaks in beef calves raised in groups outdoors but it has also been recovered from dairy calves raised together in large groups (17).

Viral Transmission

The incubation period of rotavirus is 16-24 hours (14). The virus is able to persist through the infection of adult cattle; there are no clinical signs in adults but viral excretion is a potential source of infection for calves (15). Virus survival in air and on surfaces is influenced by the level of relative humidity; it survives well in an aerosol state and medium range of relative humidity thus air may be a vehicle for dissemination of the virus (17).

Virus is excreted in the feces of infected animals in high titer (1011 viral particles per gram) and maximum shedding occurs on the 3rd and 4th days post-infection (14). This amount of excretion occurs in both infected adults and calves and excretion may last for several weeks (17). Rotavirus can spread rapidly among calves that contact each other frequently, especially during calving season (17). Calves are infected after birth from the contact with the virus in the dam’s feces or from other infected diarrheic calves or from subclinically infected calves (17). The virus is able to survive in feces for several months (14). Pregnant cows excrete the virus intermittently throughout pregnancy, from 1 calving to the next and provide a direct source of infection for the newborn (17).

Pathologic Effects

The virus invades small intestinal villous epithelial cells; the attack is usually self-limiting because once susceptible cells are destroyed there are no further target cells (15). Infected cells are sloughed, leading to partial villous atrophy (17). The infection is short lived but it takes time for the villi to repair themselves (15). Shrunken villi are initially covered by squamous and cuboidal cells from the intestinal crypts; the villi gradually regenerate as these cells differentiate into absorptive columnar epithelium (15). The immature cuboidal epithelial cells covering the shortened villi secrete decreased levels of disaccharidases (such as lactase) and are less able to carry out glucose coupled sodium transport (14). The undigested lactose in milk promotes bacterial growth and also exerts a further osmotic effect on the intestinal contents (14). In vitro studies have suggested that lactase may be a receptor and uncoating enzyme for rotavirus; this may explain the high degree of susceptibility of the newborn animal which have high lactase levels (17). The net effect of these morphological and functional changes is malabsorption resulting in diarrhea, dehydration, loss of electrolytes and acidosis (17).

Villous epithelial cell regeneration occurs within 4-6 days after the onset of diarrhea (17). The villi usually return to near normal within approximately 7 days after recovery from diarrhea but some calves may require 10-21 days for the villi to fully recover to their normal growth rate following infection (17).

scouring calf (23)

The Disease

Affected animals are moderately depressed and often continue to suckle (14). They will have voluminous feces which are soft to liquid and often contain large amounts of mucus (14). Ingestion of a large volume of milk contributes to the severity of the diarrhea because decreased production of lactase exacerbates the osmotic dysregulation (14). Other factors contributing to the severity of disease include pathogenic E. Coli, poor hygiene, chilling and overcrowding (14). Young animals may die due to dehydration or secondary bacterial infection but most calves will recover within 3-4 days (14). Recovery can be helped by feeding affected calves water instead of milk for 30 hours at the onset of diarrhea (14). Mortality rate depends upon the level of colostral immunity, incidence of enteric collibacillosis, and the level of animal husbandry and clinical management provided in the herd (17). Mortality is highest in the youngest animals that have received insufficient colostrum and are subjected to severe weather conditions (17).