Analysis of the Association of Climate, Weather and Herd Immunity with the Spread of Equine Encephalosis Virus in Horses in Israel.

It is claimed that the distribution of Culicoides-borne viruses is highly influenced by climate. Equine encephalosis virus (EEV) is a Culicoides-borne orbivirus which affects horses and was recently found to be endemic in Israel. To test whether climate is a crucial factor in the geographical distribution of EEV, we collected blood samples from horses in Israel during the years 2002, 2007 and 2010 and tested them for the abundance of antibodies to EEV. Samples were also collected in 2011 from horses that were seronegative to the virus in 2010, to determine the rate of infection with EEV. It was found that seroprevalence fluctuated between the years and that in each year it was highest in a different climatic region. Interestingly, analysis of infection rate at the different farms showed a negative association with seroprevalence at prior observations. In addition, analysis of precipitation preceding the outbreak of EEV which occurred during 2008 revealed that an extremely dry period existed several months prior to the febrile outbreak with the average precipitation of spring 2008 being significantly lower than the average spring precipitation of the years 1997-2009. It is therefore conjectured that exposure to EEV is not climate specific. Rather, it is highly influenced by herd immunity and weather fluctuations which might change annually. This finding may have important implications for the prediction of the abundance of Culicoides-borne viruses in endemic regions.