A novel SYBR Green based real-time RT-PCR assay for detection of genogroup III bovine noroviruses (BoNoV) was developed and the assay applied to 419 faecal samples from calves with and without diarrhoea. The samples were obtained from 190 Norwegian dairy and beef herds. BoNoV was detected in 49.6% of the samples from 61.1% of the herds indicating that BoNoV is ubiquitous in Norway. The overall prevalence was not significantly different in diarrhoea and non-diarrhoea samples. Analyses of polymerase gene sequences revealed both genotype III/1 and III/2 with genotype III/2 (Newbury2-like) being the most prevalent. Detected capsid sequences were restricted to Newbury2-like and the chimeric Bo/Thirsk10/00/UK strain. The RNA polymerase genotypes of the circulating BoNoVs in Norway were predicted by melting temperature analysis. Additional data from a challenge experiment suggest that a high proportion of young calves are shedding low levels of BoNoV for a prolonged time after recovering from the associated diarrhoea. The findings may explain some of the discrepancies in detection rates from previous studies and explain why some studies have failed to detect significant prevalence differences between calves with and without diarrhoea. It may also shed new light on some epidemiological aspects of norovirus infections.
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