The effects of bovine viral diarrhoea virus on cattle reproduction in relation to disease control.

Bovine viral diarrhoea virus (BVDV) is a major reproductive pathogen in cattle. Infection of the bull can lead to a fall in semen quality and the isolation of infectious virus in the ejaculate, while infection in the cow leads to poor conception rates, abortions and congenital defects. BVDV also reduces the animal's resistance to other respiratory and enteric pathogens. The prevalence of BVDV is primarily due to the efficiency with which the virus crosses the placenta of susceptible females. Calves that survive infection during the first trimester of pregnancy are born with a persistent and lifelong infection. These persistently infected (PI) animals represent between 1.0% and 2.0% of the cattle population and continuously shed infectious virus. The availability of reliable diagnostic ELISA and PCR techniques, which can test milk or serum samples for virus or antibodies, has simplified BVDV surveillance and improved the prospects for control. Although PI animals are the principal vectors within and between herds, they can be readily identified and removed. By contrast, cows carrying a PI foetus are particularly problematic. These animals have been compared to 'Trojan Horses' because they are virus-negative and antibody-positive but they deliver PI calves. In general, acutely infected cattle are much less efficient vectors but infections at the onset of puberty have resulted in a localised and persistent infection within the testes. Under these circumstances, virus shedding into the semen may remain undetected. Transmission of BVDV can be controlled through vaccination or eradication. BVDV vaccine technology has been developing over the past 30 years, but currently available vaccines are still of the conventional inactivated or attenuated sort. In general, vaccination has not been applied with sufficient rigor to make a significant impact on the level of circulating virus, unlike the national and regional eradication programmes established in areas such as Scandinavia, Austria, the Netherlands and Scotland. Eradication confers the added advantage of improved herd health; however, it also creates a susceptible cattle population that needs to be protected by stringent biosecurity. In this article, we discuss how BVDV influences reproductive function, the potential for viral transmission during breeding and the measures that must be taken to avoid the spread of infection to susceptible cattle populations via semen, embryos, culture fluids and infected cows.