Spatial analysis of Aujeszky's disease eradication in Catalonia, Spain

Abstract

Aujeszky's disease (AD) eradication programme started in Spain in 1995, but it was not until 2003, due to the additional guarantees in intra-community trade relating to Aujeszky's, that AD eradication programme was adapted and ensured. The aim of this study is to conduct a spatial analysis of the Aujeszky's disease (AD) eradication programme in Catalonia, Spain, from 2003 to 2007. The study has been divided in four periods, based on the phases designed in the AD eradication programme in Catalonia. In the first part of the study, we explore for high risk areas (clusters) in order to test whether the spatial distribution of AD in the region during the consecutive eradication periods was homogeneously distributed over the territory or clustered in space. Different purely spatial analyses, based on the Bernoulli model, were run with SaTScan® v6.1 in each period. Clusters of positive sow farms (farrow to weaning and farrow to finish) and/or fattening farms were identified in the four study periods in the western, central and north eastern part of the region. The prevalence ratio values of these clusters increased throughout the study period due to the fact that the risk of disease decreased faster outside the clusters than inside the clusters. In order to study the evolution of the disease, we explored for areas where more negative sow farms became infected and areas where more sow farms eliminated the infection. These analyses demonstrated areas with significantly higher proportions of sow farms that became negative, which indicates that the eradication of the disease has a spatial component. Clusters of negative sow farms that were infected again (reinfections) were also detected in the four study periods. The relative risk values of these clusters were much higher compared to the other cluster analyses. There was a geographical association between the clusters of positive sow farms, positive fattening farms and re-infected sow farms. This association could be attributable to the local spread of Aujeszky´s disease virus. Pig farm density could be a factor influencing the local spread of infection and was therefore evaluated for clusters of re-infected sow farms and clusters of sow farms that eliminated the infection. The mean density of pig farms was 0.40 farms per square Km (median of 0.28 and standard deviation of 0.33) in clusters of sow farms that became negative and 1.51 (median of 0.70 and standard deviation of 1.61) in clusters where more sow farms became positive (p-value Based on these results, in the second part of the study, we tested the role of geographical factors that could be implicated in local spread and persistence of AD in certain areas. Several geographic variables describing the possible risk factors associated to neighbourhood transmission: Distance to the nearest slaughterhouse, distance to conventional roads, mean number of AD serological positive sows and serological positive fattening pigs in the neighbourhood (750 meters radius) of each sow farm were included in a hierarchical Bayesian binomial model. A non geographic variable; type of farm (farrow to weaning versus farrow to finish) was also included. The use of Bayesian models allowed us to take into account the spatial dependence (autocorrelation) among the data; included in the model as a random effect. Spatial dependence was parameterised with a conditional autoregressive distribution (CAR) based on a set of neighbours. The set of neighbours was defined as those farms located in a 500 meters buffer radius around each sow farm. From the four geographical variables included in the model, only positive fattening animals in the neighbourhood of sow farms increased the probability of being AD positive. In the first period, 1,000 positive fattening pigs in the neighbourhood (750 meters buffer radius) increase the odds of each sow farm being AD positive by a factor between 1.005 and 1.36. In period 2.2, having positive fattening animals in the neighbourhood increased the likelihood of each sow farm to be AD positive between 1.84 and 3.22. In period 2.1 and period 3, none of the variables had a positive relation with the probability of being positive. The type of farm (farrow to weaning or farrow to finish) also did not influence the probability of being AD positive in any period. The geographical pattern of the residuals of the hierarchical bayesian binomial model (observed versus predicted) was very similar to the observed infection in sow farms in all the eradication periods, showing that neighbourhood transmission might not be the main factor related to the eradication of Aujeszky-s disease in sow farms. Other herd¬specific risk factors might be much more related to the probability of AD infection than the geographical variables included in this study.

Date of Award	28 Jul 2008
Original language	Undefined/Unknown
Supervisor	Marc Sáez Zafra (Director) & Jordi Casal Fàbrega (Director)