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 cat tle. Infection of the bull can lead to a fall in semen quality and the isol ation of infectious virus in the ejaculate, while infection in the cow lead s to poor conception rates, abortions and congenital defects. BVDV also red uces the animal's resistance to other respiratory and enteric pathogens. Th e prevalence of BVDV is primarily due to the efficiency with which the viru s crosses the placenta of susceptible females. Calves that survive infectio n during the first trimester of pregnancy are born with a persistent and li felong infection. These persistently infected (PI) animals represent betwee n 1.0% and 2.0% of the cattle population and continuously shed infectious v irus. The availability of reliable diagnostic ELISA and PCR techniques, which can test milk or serum samples for virus or antibodies, has simplified BVDV su rveillance and improved the prospects for control. Although PI animals are the principal vectors within and between herds, they can be readily identif ied and removed. By contrast, cows carrying a PI foetus are particularly pr oblematic. These animals have been compared to 'Trojan Horses' because they are virus-negative and antibody-positive but they deliver PI calves. In ge neral, acutely infected cattle are much less efficient vectors but infectio ns at the onset of puberty have resulted in a localised and persistent infe ction 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 cur rently available vaccines are still of the conventional inactivated or atte nuated sort. In general, vaccination has not been applied with sufficient r igor 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 su sceptible cattle population that needs to he protected by stringent biosecu rity. 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 population s via semen, embryos, culture fluids and infected cows. (C) 2000 Elsevier S cience B.V. All rights reserved.