Estimation of the reproduction ratio (R(0)) of bluetongue based on serological field data and comparison with other BTV transmission models.

Bluetongue virus serotype 8 (BTV-8) emerged in north-western Europe in 2006. In 2007, one of the affected countries (the Netherlands) implemented a sentinel network in dairy cattle. This data offered the opportunity to estimate transmission parameters. From our field data, the number of secondary infected cows that became infected by one infectious cow in a completely susceptible herd through the bites of infectious Culicoides i.e. the basic reproduction ratio (R(0)) was calculated. With that information, the R(0) of BTV-8 was estimated using an formulae of a general SIR model. In 2007, the BTV-8 epidemic started in the south and spread northwards in the following months. R(0) could be estimated for 197 herds in which transmission occurred. The median R(0) was 2.3 and the mean R(0) was 3.7 (5th percentile=1.8; 95th percentile=11.0). In the northern region where BTV-8 transmission occurred later in the season with less favorable conditions for transmission, R(0) remained significantly lower than in the south. Our model differed from earlier published more theoretical models on BTV-8 transmission because we estimated transmission from serological field data while other models used literature based assumptions for the majority of the parameters included in their models. Although there were many differences between our model and the previously developed more theoretical models, the results showed similar ranges of R(0) for BTV-8. The reasons for the similarity between the results may be that, although the part of the vector was not included with parameters in our model, the transmission based on serological field data in cows represented both BTV-8 transmission influenced by cows and by its vector, Culicoides. Furthermore, in the earlier models the assumptions made on the vector part, although derived from literature, probably gave a good representation of the true behavior of the Culicoides species that were associated with BTV-8 transmission in north-western Europe.