The transmission and geographical spread of African horse sickness and bluetongue viruses

African horse sickness virus (AHSV) and bluetongue virus (BTV) are dsRNA viruses within the genus Orbivirus. Both are able to cause non-contagious, infectious arthropod-borne diseases in their respective vertebrate hosts. AHSV infects equines and occasionally dogs, whereas BTV replicates in ruminants. The disease caused by AHSV is usually at its most 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 from approximately 40 degrees N to 35 degrees S. In the wild, both viruses are transmitted between their vertebrate hosts almost entirely via the bites of arthropod vectors, although dogs can occasionally acquire AHSV by eating virus-contaminated meat and BTV may be infrequently transmitted via infected semen or transplacentally. Because of their reliance upon arthropod vectors, BTV and AHSV have a global distribution which is limited not only by the requirement for susceptible vertebrates but also by the necessity for competent arthropod vectors. The major vectors of AHSV and BTV are certain species of Culicoides biting midge, 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 zones in sub-Saharan Africa for more than 2-3 years at most. This has been interpreted as being due to a number of factors, including the absence of a long-term vertebrate reservoir, a lower prevalence, shorter, seasonal incidence and decreased transmission efficiency of the local vectors and also possibly to the effect of control measures (vector abatement, vaccination). The recent outbreaks of African horse sickness (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 due to the 'all-year-round' presence in the area of adult Culicoides imicola, the major AHSV vector. This is basically an Afro-Asiatic species and its continuous presence in parts of Iberia and may be due to some recent moderation in the climate. Further northerly extensions in the range of Culicoides imicola, in response to 'climatic moderation', cannot be ruled out and could substantially increase the area of Europe 'at risk' to AHS.(ABSTRACT TRUNCATED AT 400 WORDS)