Bovine parainfluenza virus type 3.

Bovine parainfluenza virus type 3 (bPIV-3) is a member of the family Paramyxoviridae, subfamily Paramyxovirinae, genus Respirovirus. Together with other viral and bacterial agents, bPIV-3 is associated with the clinical condition known as 'shipping fever', in which the animal develops clinical signs after transport over long distances, and bovine respiratory disease complex in cattle. Enveloped bPIV-3 virions have seven proteins encoded by a single-stranded RNA and have a negative-polarity genome, consisting of approximately 15,000 nt in length. Distribution of bPIV-3 infections is worldwide and the prevalence of specific antibodies is high in the cattle population. In North and South America, Asia and Europe, bPIV-3 infections are endemic, with high seropositivity rates in beef and dairy herds. The bPIV-3 isolates are antigenically similar, although viral diversity may be observed at the nucleotide level; three distinct genotypes have been described, and subgenotypes may occur. Prevention of clinically evident illness caused by bPIV-3 is likely to be achieved by general good husbandry practices, including hygiene measures, reduction of stocking density, control of animal transit, quarantine of new individuals arriving to the herd and vaccination. There is no consensus on the efficacy of vaccines presently on the market. The presence of evident clinical signs, levels of serum antibodies, shedding of the virus through the nostrils and lung lesions are commonly used to evaluate the efficacy of bPIV-3 vaccines under field conditions. These studies are highly controversial, since commercial vaccines carry other viral and bacterial agents related to respiratory disease, and factors such as environmental conditions and maternal immunity may also influence the results.