Bovine toroviruses (Breda virus): Development of diagnostic tests and epidemiology.

摘要

Bovine Torovirus (BoTV) is an uncultivable, enveloped, single-stranded RNA virus that causes diarrhea in cattle. The main objectives of this dissertation were to develop diagnostic assays to identify BoTV antigen and viral nucleic acids, and to use these assays in epidemiological surveys of BoTV in various cattle populations, focusing on an understanding of its effects on cattle health and performance. In the feedlot study, fecal and nasal shedding respectively, of BoTV was detected by ELISA (37% and 26%) and RT-PCR (95% and 100%), in a high number of steers with peak shedding on day 4 after arrival. Differences were not detected between calves shedding BoTV and nonshedders in relation to disease and treatment. BoTV was also detected by ELISA and/or RT-PCR in 9.7% of 259 bovine fecal samples from cases of gastroenteritis submitted to the Ohio Animal Disease Diagnostic Laboratory during a 19-month period. Of the BoTV-positive samples, 56% were from calves, which represented 26.4% of the total calves tested; and of the BoTV-positive calves, 71% were under 3 weeks of age. In the last study, the overall rate of fecal shedding of BoTV in veal calves was 24% by ELISA and/or RT-PCR, with peak shedding on day 4 after arrival at the veal farm. A significant independent association between BoTV shedding and diarrhea was observed (Odds ratio, 6.95). Calves seronegative or with low antibody titers to BoTV at arrival seroconverted to BoTV, and were more likely (OR, 7.4) to shed virus than those already seropositive at arrival. In conclusion, BoTV was prevalent in cattle of different ages in Ohio and near-by states, especially in calves ≤1 month old. The presence of BoTV infections was determined by seroconversion measured by HI and BoTV shedding in feces and nasally detected by ELISA and RT-PCR assays developed in our lab. The BoTV shedding was strongly associated with diarrhea in neonatal veal calves during the first week after arrival at the farm, which provides evidence that BoTV is an important pathogen of neonatal calves. Future studies of infectious diarrhea in cattle should also include assays for this etiologic agent.