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Bovine viral diarrhoea virus in beef heifers in commercial herds in Australia: mob-level seroprevalences and incidences of seroconversion, and vaccine efficacy.

OBJECTIVES
To describe management group (mob)-level seroprevalences and incidences of seroconversion to bovine viral diarrhoea virus (BVDV), and to determine the efficacy of a vaccine against BVDV, in beef heifers in commercial herds in Australia.


METHODS
Seroprevalences were assessed in 38 mobs of beef heifers. Of them, 15 mobs that were considered to be at higher risk of BVDV transmission during the upcoming mating period underwent further serological monitoring, and were included in a double-blind controlled trial to assess vaccine efficacy.


RESULTS
In 66% of mobs, less than half the heifers were seropositive some months before mating start date. However, in only 2 mobs was the incidence of seroconversion during the mating period greater than 10%, with a very high incidence of seroconversion observed in only 1 mob. The pregnancy proportion in placebo-treated heifers in this mob was acceptable (89%), but a high proportion of placebo-treated heifers (26%) had persistently infected calves. The efficacy of the Pestigard® vaccine in preventing the birth of infected calves was estimated as 80%.


CONCLUSIONS
Outbreaks of serious BVDV-related disease are relatively uncommon in mobs of beef heifers, but when they occur, the impact can be large. This highlights the need to approach BVDV control from a risk-assessment perspective, where the likelihood and consequences of widespread BVDV infection in a mob are jointly assessed. Pestigard® vaccination of naïve heifers prior to mating reduces the risk of transplacental infection with BVDV if heifers are exposed to BVDV during early pregnancy.