Epizootiology and vectors of African horse sickness virus.

African horse sickness (AHS) virus causes a non-contagious, infectious, arthropod-borne disease of equines and is enzootic in sub-Saharan Africa. The major vectors are species of Culicoides but mosquitoes and ticks may be involved. Periodically the virus makes excursions beyond its enzootic zones but until recently has not been able to maintain itself outside these areas for more than 2-3 consecutive years. This is probably due to a number of factors including the absence of a long term vertebrate reservoir, the prevalence and seasonal incidence of the vectors and the efficiency of control measures. The recent AHS epizootics in Iberia and North Africa seem to have established a new pattern in AHS virus persistence. This is probably linked to the continuous presence of adult C. imicola in the area. Culicoides imicola is basically an Afro-Asiatic insect and prefers warm climates. Therefore its continuous adult presence in parts of Iberia may be due to some recent moderation of the climate in these areas.