NEWS AND VIEWS
of gestation, the fetus will recognise the virus as part of itself, and be born as a ‘BVD carrier’ or a persistently infected (PI) calf. Unlike TI cattle, PI cattle never develop immunity to the virus. They go on to excrete enormous amounts of BVD virus during their lifetimes, which makes them the most important sources of new infections for naïve animals on dairy and beef farms. The early detection and immediate removal of PI animals is therefore an essential step in any BVD control programme.
BVD: effective testing for control or eradication Andrew MacPherson, Medical Affairs Veterinarian at IDEXX Laboratories, reports on research into using earnotch samples to test calves for bovine viral diarrhoea. INTRODUCTION Bovine viral diarrhoea (BVD) is a disease affecting cattle that is caused by the BVD virus. The virus is endemic in most cattlefarming countries and causes significant economic losses worldwide (Yarnall and Thrusfield, 2017). BVD manifests on dairy and beef farms. As it is immunosuppressive it can present in many ways, including as ill-thrift, reproductive losses and/or decreased milk production, with a predisposition to concurrent infectious disease. These variations in manifestation often impede management efforts and rapid diagnosis. In addition, and despite its name, the absence of diarrhoea does not rule out
50 – VetScript September 2020
the presence of the BVD virus on farm, as acute infection can be subclinical and cause mild or undetected disease. When a cow first becomes infected with the virus, no matter what their age, an acute transient infection (TI) occurs. These animals do not have a significant role in the ongoing transmission of BVD on farm, as they excrete only small amounts of the virus in a period of a few days (Evans et al., 2019). When a pregnant cow who has never been infected with BVD (and therefore hasn’t developed antibodies against the virus) is infected during gestation, the fetus will also become infected. If the dam is infected between day 30 and 125
THE BVD ANTIGEN-ELISA EARNOTCH TRIAL Maternal antibodies against BVD have been reported to have a half-life of 21 days and to persist for up to 30 weeks after birth. In New Zealand this has been thought to result in a ‘diagnostic gap’, where PI calves test negative via serum soon after birth. As a result, testing using antigen-enzyme-linked immunosorbent assay (ELISA) has not been recommended before calves are 35 days old. However, other studies have found that the use of tissue samples (eg, by ear notching) reduces the amount of maternal antibody interference (Kuhne et al., 2005; Hill et al., 2007). In response to these studies, Cognosco conducted a trial in spring 2019 to assess the sensitivity and specificity of calves’ earnotch samples from day 38 postpartum back to as close to their birth as possible. TRIAL METHODS The study enrolled 1,030 calves from a total 11 dairy herds where the BVD virus had been detected in bulk tank milk and/ or where individual animals had been confirmed as BVD-virus antigen-positive before the 2019 spring calving season.
IMAGERY: SUPPLIED
