The objective of this study was to investigate the dynamics of PRRSV infection and to
quantify transmission within a breeding herd, and its impact on herd performance. For this
purpose a longitudinal study was performed in a closed breeding herd of 115 sows. Statistical
methods and Monte Carlo simulations based on stochastic SIR models were used to analyse
the observational data. Moreover, a case-control study was performed to determine whether
seroconversion of sows during gestation was associated with aberrant litters. The transmission
parameter R was estimated to be 3·0 (95% confidence interval 1·5–6·0) for the model version
based on the most plausible assumptions that the infectious period lasts 56 days and no
lifelong immunity exists after infection. Based on simulations using a breeding herd of equal
size the average time-to-extinction was estimated to be 6 years; using a herd of twice the size,
it was 80 years. Furthermore, in contrast to the epidemic phase of the disease, the endemic
phase was not detrimental to herd performance.