Evaluation of the spatial and temporal distribution of and risk factors for Bluetongue serotype 1 epidemics in sheep Extremadura (Spain), 2007-2011

Abstract

Extremadura has been one of the regions in Spain most severely affected by Bluetongue (BT) epidemics. The first incursion of BTV, which was successfully eradicated one year later, occurred in 2004, involving the BTV-serotype 4. However, a second incursion occurred in September 2007, this time involving serotype 1. Since then, the implementation of intensive vaccination programs have significantly reduced BTV-1 occurrence, but the disease has not been completely eradicated yet. This study aimed to provide, for the first time, a complete description of the spatial and temporal patterns of BTV-1 occurrence in sheep in Extremadura from 2007 to 2011 and to identify the risk factors that contributed to the seasonal occurrence of BTV-1 in this region. The results showed that risk factors contributing to BTV-1 occurrence in sheep changed between 2007 and 2011. Initially, when the population was still immunologically naïve, the main risk factors for BTV-1 occurrence were extensive management practices, large sheep farms and Culicoides abundance on farms. However, after the implementation of vaccination, other factors became more relevant for BTV-1 occurrence, mostly related to BTV reservoirs, such as the proximity of cattle farms or the introduction of cattle into farms. The Talaverana sheep breed also seemed to be associated with a significantly higher risk of BTV-1 occurrence, although it may be due to confounding factors, such as the geographical concentration of where this breed is kept and/or management practises used for this breed. The results of this study suggest that preventive and control strategies, including vaccination and active surveillance strategies, should be primarily focused on cattle farms kept in close vicinity to sheep flocks as well as in high-risk sheep farms (i.e. farms with a large farm size keeping both cattle and sheep and with a high number of animal introductions). Methods and results presented here may be used to guide decisions for the annual update of the control and eradication program in Spain.

Keywords: Bluetongue; Logistic multilevel model; Risk factors; Scan statistics; Serotype 1; Spain.