An overview of calf diarrhea - infectious etiology, diagnosis, and intervention

Cho, Yong-il; Yoon, Kyoung-Jin

Bovine rotavirus is a primary etiological agent of calf diarrhea. The virus belongs to the genus Rotavirus within the family Reoviridae. Rotavirus is a non-enveloped virion possessing 11 double-stranded RNA segments (16∼21 kb) and is very stable over a wide pH range with heat lability [38] . There are seven serogroups (A through G) of rotaviruses based on antigenic and genetic similarities of the intermediate capsid protein (VP6) [129] . Group A rotaviruses are the major cause of rotaviral infection in domestic animals [129] . Most BRVs (95%) belong to group A, although groups B and C rotaviruses have also been identified in field cases [45, 133] . Group A rotaviruses can be further classified into P or G types based on genetic and antigenic similarities of VP4 (protease sensitive protein) and VP7 (glycoprotein) which constitute the outer capsid of the virion and induce anti-viral neutralizing antibody production [25] . Sixteen G types and 27 P types have been reported in domestic animals [25] . Bovine rotaviruses are G1, G6, G8, or G10 types [49, 82] . G6 and G10 type are reported to be the most prevalent in cattle [82] . While VP4, VP6, and VP7 play a major role in maintaining viral structure, virus attachment, and antigenicity, nonstructural glycoprotein 4 (NSP4) holds a special role as a viral enterotoxin. This protein also interferes with cellular homeostasis by elevating calcium ion influx into the cytoplasm [4] . These alterations account for drastic changes in the movement of nutrients and water across the intestinal epithelium and are more important for viral pathogenesis than histopathological lesions. Bovine rotavirus usually causes diarrhea in calves at 1 to 2 weeks of age. The milk uptaken by calves can provide a good environment for rotavirus survival under a wide range of gastrointestinal pH levels and infection of the intestinal epithelial cells [26] . This may explain why weaning calves are more susceptible to calf diarrhea. The virus has a very short incubation period (12∼24 h) [129] and induces peracute diarrhea in affected calves. Once infected, the calves shed a large amount of virus via feces for 5∼7 days, thus contaminating the environment and allowing the virus to be transmitted to pen mates. The virus replicates in the cytoplasm of epithelial cells of small intestinal villi. Destruction of mature enterocytes in the villi, activation of the enteric nervous system by vasoactive components from the damaged cells, and secretion of a viral enterotoxin (e.g., NSP4) account for maldigestive/ malabsorptive diarrhea promoted by rotavirus infection. Viral infection causes villus atrophy and usually affects the caudal part of the small intestine. Evidence for interspecies transmission along with genetic reassortment between human and animal rotaviruses (e.g., swine, bovine, feline, and canine) has raised concerns about zoonotic rotaviruses [81] .