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

Cho, Yong-il; Yoon, Kyoung-Jin

Bovine torovirus is an enveloped, positive-stranded, RNA virus (25∼30 kb) belonging to the genus Torovirus in the family of Coronaviridae, order Nidovirales [68] along with equine torovirus, porcine torovirus, and human torovirus. Toroviruses are infectious gastrointestinal agents in cattle, and a predominant cause of acute enteric infection in piglets and children [69, 78] . Fecal shedding of BToVs from diarrheic calves has been reported around the world including the USA (2003, 2002, 1983, and 1982 [29, 53, 56, 67, 108, 110, 139] . Morphological similarities and antigenic cross-reactivity between human and bovine toroviruses has raised a concern about the potential zoonotic nature of BToV [57] . Bovine toroviruses can produce mild to moderate diarrhea in young calves less than 3 weeks of ages [57] . After oral or nasal inoculation with the virus, epithelial cells in the middle and lower parts of intestinal villi extending into the crypt epithelium are infected, leading to cell death and epithelial desquamation in the small intestine together with necrosis in the large intestine [32, 112] . Damage to the villous and cryptic enterocytes thus induces malabsorptive/maldigestive diarrhea. Thirty to 50% of lesions caused by the virus are present in the upper small intestine, which may account for the mild to moderate diarrhea in affected animals [144] . Similar to BCoV, BToV antigen and viral RNA have been detected in nasal secretions, but the role of these factors in respiratory disease remains to be clarified [55] .