An analysis of a presumed major outbreak of pseudorabies virus in a vaccinated sow herd

We describe a major outbreak of pseudorabies virus (PRV) in a sow herd in w hich the sows were vaccinated simultaneously three times a year with a vacc ine containing Bartha strain. Also in the associated rearing herd in which the gilts were vaccinated twice an outbreak of PRV occurred. The outbreak w as analysed with mathematical models, statistical methods and Monte-Carlo s imulation. Under the assumption that the outbreak started with one introduc tion of virus the reproduction ratio R-ind - as a measure of transmission o f PRV between individuals-in the sow herd was estimated with a Generalized Linear Model to be 1.6. Also under the assumption of one introduction of vi rus Ri,, in the rearing herd was estimated with a martingale estimator to b e 1.7. Both estimates were significantly larger than 1. Mathematical analys is showed that heterogeneity in the sow herd, because of the presence of no t-optimally immunized replacement sows could not be the only cause of the o bserved outbreak in the sow herd. With Monte-Carlo simulations, the duratio n of an outbreak after a single introduction of virus and R-ind = 1.6 did n ot mimic the data and thus the hypothesis of a single introduction with R-i nd = 1.6 could also be rejected and R-ind is thus, not necessarily above 1. Moreover, with statistical analysis, endemicity in the combination of herd s as a cause for the observed outbreak could be rejected. Endemicity in the rearing herd alone could not be excluded. Therefore, multiple introduction s from outside and most probably from the rearing herd were possibly the ca use of the observed outbreak(s). The implications for eradication of pseudo rabies virus were discussed.