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

Cho, Yong-il; Yoon, Kyoung-Jin

Bovine coronavirus is an enveloped virus with a positive-sense, single-stranded RNA genome (27∼32 kb). This pathogen is a member (Betacoronavirus 1) of the genus Betacoronavirus that was formerly classified as group 2a coronaviruses [24] . Virus infection can present as three distinct clinical syndromes in cattle: a) calf diarrhea in calves at 1 to 2 weeks of age; b) winter dysentery with hemorrhagic diarrhea in adult animals; and c) respiratory diseases including bovine respiratory disease complex in both young and adult cattle [17, 77] . The spike (S) protein of the virus plays an important role in virus entry and pathogenesis besides the ability to neutralize antibody [76] . The S protein consists of two subunits (S1 and S2) and is crucial for virus-host interaction. While the S1 subunit facilitates binding of the virus to host cell receptors, the S2 subunit functions in the fusion of the viral envelope to host cellular membranes [146] . Viral infection begins in the small intestine and usually spreads through the entire small intestine and colon. Microscopically, villi of the affected small intestine and colonic crypts become atrophic, and the lamina propria becomes necrotic. Initially, the S protein and hemagglutinin-esterase (HE) protein of the virus attach and fuse to the intestinal epithelial cells. [122] . The virus replicates in enterocytes and progeny viruses are released through a normal secretory mechanism and cell lysis. Mature villous epithelial cells are the primary target of the virus although crypt enterocytes are also affected. Clinical signs in affected animals often have a longer duration due to the damage done to crypt enterocytes by the virus.