We describe a major outbreak of pseudorabies virus (PRV) in a sow herd in which the sows
were vaccinated simultaneously three times a year with a vaccine containing Bartha strain. Also
in the associated rearing herd in which the gilts were vaccinated twice an outbreak of PRV
occurred. The outbreak was analysed with mathematical models, statistical methods and
Monte-Carlo simulation. Under the assumption that the outbreak started with one introduction
of virus the reproduction ratio Rind – as a measure of transmission of 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 virus Rind in the rearing herd was estimated with
a martingale estimator to be 1·7. Both estimates were significantly larger than 1. Mathematical
analysis showed that heterogeneity in the sow herd, because of the presence of not-optimally
immunized replacement sows could not be the only cause of the observed outbreak in the sow
herd. With Monte-Carlo simulations, the duration of an outbreak after a single introduction of
virus and Rind = 1·6 did not mimic the data and thus the hypothesis of a single introduction
with Rind = 1·6 could also be rejected and Rind is thus, not necessarily above 1. Moreover, with
statistical analysis, endemicity in the combination of herds as a cause for the observed
outbreak could be rejected. Endemicity in the rearing herd alone could not be excluded.
Therefore, multiple introductions from outside and most probably from the rearing herd were
possibly the cause of the observed outbreak(s). The implications for eradication of
pseudorabies virus were discussed.
Monte Carlo simulation and statistical analysis of genetic information coding
