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

Cho, Yong-il; Yoon, Kyoung-Jin

Bovine norovirus is a non-enveloped, single-stranded positive-sense RNA virus (7.4∼8.3 kb) belonging to the genus Norovirus in the family Caliciviridae [20] . Five genogroups (GI through GV) have been identified based on sequence similarities of open reading frames (ORFs) 2 (VP1: major capsid protein) and 3 (VP2: minor capsid protein) due to high genetic diversity among noroviruses (NoVs) [147] . BNoVs belong to GIII that includes two prototype strains, Jena (genotype 1; GIII-1) and Newbury 2 (genotype 2; GIII-2) viruses, and are phylogenetically distinct from human (GI, GII, and GIV), porcine (GII-11, GII-18, and GII-19) and murine (GV) NoVs [84, 101, 123] . The possibility of interspecies transmission of NoV was demonstrated by a study in which gnotobiotic pigs were infected with a human NoV strain, raising a concern for the zoonotic potential of this virus worldwide [16] . Numerous studies have been conducted to survey BNoV infection in cattle and molecularly characterize the viruses compared to human NoVs [19, 27, 64, 66, 85, 102, 106, 116, 125, 136, 145] . The reported frequency of BNoV detection using molecular methods widely varied among different countries, ranging from 7.5% to 49.6%. All identified BNoVs have been phylogenetically distinct from human NoVs, suggesting that the zoonotic potential of BNoVs is very low. Noroviruses are a major cause of acute and sporadic non-bacterial gastroenteritis in humans (both adults and children). These pathogens have also been reported to cause gastroenteric disease in animals such as cattle, pigs, dogs, and mink [123] . Recently, an experimental challenge study with the Jena strain of BNoV was conducted on newborn calves infected via an oral route [104] . The investigators demonstrated that the virus infected epithelial cells of the small intestine and caused villous atrophy (in the jejunum and ileum) leading to diarrhea with virus shedding but not seroconversion. Detection of BNoV in feces from clinically healthy cattle has also been reported [64, 91, 116] , raising questions about the clinical significance of BNoV.