Terminating Aujeszky vaccination in the Netherlands?

Since 1993, the Netherlands have been using obligatory vaccination against Aujeszky's disease to eradicate this virus (ADV) from the porcine population. The disease incidence has become rather low, but can we stop vaccinating? This leads to the following questions. When is the incidence low enough to try a non-vaccination policy? What is the expected size of an outbreak when the virus is introduced? How often do we expect outbreaks? What is the best way to stop an outbreak? To answer such questions, we developed a model describing the spread of the virus in Dutch pig husbandry. It contains a submodel describing the dynamics of the infection within one farm. The results of this submodel were used in an overall model that describes the transmission of the infection between farms. We distinguished several different types of farms and different transmission routes. The basic reproduction ratio of ADV between different herds $\rm (R_h)$ could be calculated from the model. When this reproduction ratio is below one, the epidemic will fade out to extinction, when above one, a major part of the dutch herds may become infected during a major outbreak. Data from a large survey for animal transport were used to quantify transmission by that route. Data from a recent outbreak of classical swine fever were used to quantify transmission by other routes than animal transport. Finally data from an Aujeszky outbreak in Denmark were used to calibrate the model for ADV in an unvaccinated population. The model shows that presently in the Netherlands $\rm R_h$ ranges from 0.6 in areas with low pig density to 0.8 in areas with high pig density. This was used to validate the model for the decreasing prevalence over the last 5 years. If vaccination is stopped only amongst finishing pigs, $\rm R_h$ will remain below 1 in areas with rather low pig density, but in very dense areas, $\rm R_h$ will be slightly above 1. Thus, in areas with low pig density, major outbreaks are not expected when vaccination of finishing pigs is discontinued. We found that eradication is difficult at this moment because (1) the prevalence of the infection is still rather high in some areas and (2) the risk of introducing the virus from abroad is substantial. However, in a northern part of the Netherlands, both pig density and Aujeszky's disease prevalence are very low. In this areas, the risk of new introduction of the virus is also rather low and eradication is feasible if re-introduction of the virus can be minimised. The vaccination program could be stopped after an intensive survey and final stamping out of the last sources of infection. We calculated the expected size of such outbreaks and methods to stop the outbreak. For the northern part of the Netherlands we found that the basic reproduction ratio between herds, $\rm R_h$ was 3.2, whereas $\rm R_h$ was 6.8 in the area with the highest pig density, so in all areas an outbreak may occur when the virus is reintroduced. In areas with high pig density, an outbreak cannot be stopped by a ban on animal transport, supported by an extra clinical survey $\rm (R_h = 1.1)$