Rev. sci. tech. Off. int. Epiz., 2006, 25 (3), 1097-1109 Investigations on outbreaks of African horse sickness in the surveillance zone in South Africa G.J. Venter (1) , J.J.O. Koekemoer (1) (2) & J.T. Paweska (3) (1) Agricultural Research Council-Onderstepoort Veterinary Institute, Private Bag X5, Onderstepoort, 0110 South Africa (2) Department of Veterinary Tropical Diseases, University of Pretoria, Pretoria, 0001 South Africa (3) Special Pathogens Unit, National Institute for Communicable Diseases, Sandringham, 2131 South Africa Submitted for publication: 2 September 2005 Accepted for publication: 20 March 2006 Summary Confirmed outbreaks of African horse sickness (AHS) occurred in the surveillance zone of the Western Cape in 1999 and 2004, both of which led to a two-year suspension on the export of horses. Light trap surveys in the outbreak areas showed that known vector competent Culicoides species, notably C. imicola, were abundant and present in numbers equal to those in the traditional AHS endemic areas. Isolations of AHS virus serotypes 1 and 7, equine encephalosis virus, and bluetongue virus from field-collected C. imicola in the surveillance zone demonstrated that this species was highly competent and could transmit viruses belonging to different serogroups of the Orbivirus genus. Molecular identification of recovered virus isolates indicated that at least two incursions of AHS into the surveillance zone had taken place in 2004. The designation of an AHS-free zone in the Western Cape remains controversial since it can be easily compromised, as evidenced by the two recent outbreaks. In light of the results reported in the present study, the policy of maintaining a large population of unvaccinated horses in the surveillance zone should be reconsidered, as it leaves them vulnerable to infection with AHS virus, which is the most pathogenic of all equine viruses. Keywords African horse sickness – Bluetongue – Culicoides imicola – Equine encephalosis – Field infection prevalence – Light trap survey. Introduction African horse sickness (AHS) is an arthropod-borne viral disease that affects all equids. The causal agent, AHS virus (AHSV), is endemic in sub-Saharan Africa and possibly Yemen; however, outbreaks do occur periodically in countries far beyond these areas (23). To date, nine distinct serotypes of AHSV have been identified (13, 17). African horse sickness is the most lethal viral disease of horses (8). It was the only equine disease included in the former List A of diseases notifiable to the World Organisation for Animal Health (OIE) (48). African horse sickness virus has been present in Africa for many centuries. In South Africa, where no indigenous horses existed, it was first noticed in 1652 after the introduction of horses from Europe and the Far East into the Cape of Good Hope (12). The disease was frequently mentioned in the records of the Dutch East India Company, and in 1719 nearly 1,700 horses in the Cape of Good Hope succumbed to the dreaded horsesickness, also known as ‘perreziekte’ or ‘pardeziekte’ (35). The decline in the number of AHS outbreaks over the last few decades of the 20th Century, particularly in the southern areas of South Africa, is partly due to the elimination of large free- ranging populations of zebra (Equus burchellii), which are considered to be the natural cycling host for the virus (2, 23). Of equal importance was the introduction of a polyvalent AHS vaccine in 1974, which created a barrier of immune horses that probably impeded the southerly