Subclinical Aujeszky's disease virus infection in a pig herd and the characterisation of the strain of virus isolated.

The spread of antibody to Aujeszky's disease virus through a susceptible pig herd was monitored after the probable introduction of infection by a recently purchased boar. The infection spread slowly through the herd but no clinical signs of Aujeszky's disease were seen. The strain of virus isolated was designated NIA-6. It has been characterised by a series of experimental infections and extends the known range of virulence of isolates of Aujeszky's disease virus made in Northern Ireland. The strain caused no disease in four-week-old piglets and is therefore less virulent than other isolates from Northern Ireland pigs. However, it killed rabbits and a proportion of experimentally infected two-week-old piglets, which differentiates it from the avirulent bovine isolate (NIA-4).