THE TRANSMISSION AND GEOGRAPHICAL SPREAD OF AFRICAN HORSE SICKNESS AND BLUETONGUE VIRUSES

African horse sickness virus (AHSV) and bluetongue virus (BTV) are dsR NA viruses within the genus Orbivirus. Both are able to cause non-cont agious, infectious, arthropod-borne diseases in their respective verte brate hosts. AHSV infects equines and occasionally dogs, whereas BTV r eplicates in ruminants. The disease caused by AHSV is usually at its m ost severe in horses, whereas certain breeds of sheep are particularly sensitive to BTV infection. AHSV is endemic in sub-Saharan Africa but periodically makes brief excursions beyond this area. BTV occurs much more widely and can be found in a band around the World, stretching f rom approximately 40 degrees N to 35 degrees S. In the wild, both viru ses are transmitted between their vertebrate hosts almost entirely via the bites of arthropod vectors, although dogs can occasionally acquir e AHSV by eating virus-contaminated meat and BTV may be infrequently t ransmitted via infected semen or transplacentally. Because of their re liance upon arthropod vectors, BTV and AHSV have a global distribution which is limited not only by the requirement for susceptible vertebra tes but also by the necessity for competent arthropod vectors. The maj or vectors of AHSV and BTV are certain species of Culicoides biting mi dge, which are true biological vectors but mosquitoes and/or ticks may also be involved to a greater or lesser extent. Until recently, AHSV has apparently been unable to survive beyond its traditional endemic z ones in sub-Saharan Africa for more than 2-3 years at most. This has b een interpreted as being due to a number of factors, including the abs ence of a long-term vertebrate, reservoir, a lower prevalence, shorter , seasonal incidence and decreased transmission efficiency of the loca l vectors and also possibly to the effect of control measures (vector abatement, vaccination). The recent outbreaks of African horse sicknes s (AHS) in Spain, Portugal and Morocco, which persisted for at least 5 years (1987-1991) therefore seem to have established a new pattern in AHSV survival in an epidemic zone. This extended persistence may be d ue to the 'all-year-round' presence in the area of adult Culicoides im icola, the-major AHSV vector. This is basically an Afro-Asiatic specie s and its continuous presence in parts of Iberia may be due to some re cent moderation in the climate. Further northerly extensions in the ra nge of Culicoides imicola, in response to 'climatic moderation', canno t be ruled out and could substantially increase the area of Europe 'at risk' to AHS. Furthermore, since Culicoides imicola is also the major Old World Vector of BTV, extensions in its range would also have seri ous implications in regard to areas 'at risk' to bluetongue (BT).