This study reanalyses data using a new analytical approach, taking into account, that some risk-factors will increase the risk for a herd to be positive, whereas other risk factors tend to increase the risk of a high prevalence in positive herds. The zero-inflated model showed that use of pelleted feed increased the risk of being positive for the herd. In positive herds, use of pelleted feed, dry feed, increasing herd size and conventional (non-SPF) health status increased the prevalence.