For the Health of It
African Horse Sickness : Potential Threat for Disease-free Countries
Reprinted with permission from Volume 29 , Issue Number 4 of Equine Disease Quarterly
A major outbreak of African horse sickness ( AHS ) in Thailand earlier this year was a timely reminder for animal health officials , veterinarians , and members of the horse industry of the vulnerability of equine populations to this disease . Of all known equine infectious diseases , AHS is the single most important in terms of devastating losses in naïve horses and economic impact on international trade . It is a dreaded , non-contagious , vector-borne disease with the potential to kill 50 % to 95 % of affected horses .
The etiological agent of AHS is an RNA virus belonging to the family Reoviridae , genus Orbivirus , that is transmitted naturally by species of Culicoides or midges . There are nine antigenically distinct serotypes of the virus . While the primary host species are members of the family , Equidae , evidence of infection has also been found in African elephants , black and white rhinoceroses , camels and dogs , none of which are considered epidemiologically significant .
Historically , AHS was considered restricted to tropical and subtropical regions of sub-Saharan Africa , where it had been known to occur regularly for over 200 years . Although infrequent , the disease has spread from west and north Africa to various southern European or Middle Eastern countries . The most significant such event occurred between 1959 and 1963 when serotype 9 of AHS virus spread out of Africa into and throughout the Middle East , as far north as Turkey , and extended as far east as Afghanistan , Pakistan , and India .
Concern over the risk that AHS poses for horse industries in Europe has been keenly felt for some time . Southern European countries are faced with the potential threat of disease introduction from migration of the virus northwards from regions in Africa , where the disease is endemic . Spread of the virus could result from the movement of nomads and their animals , passive wind-borne carriage of infected Culicoides over long distances , and legal or illegal trade in zebra from countries where the disease is currently active .
10 • Walking On
A similar if not greater risk exists for the Middle East which experienced incursions of AHS more frequently than any other region or country .
As the most recent occurrence of AHS in Thailand has shown , distance is no guarantee of safety from the risk of introduction of this disease . Thailand is approximately 6,000 miles distant from where the virus responsible for this event probably originated , and the furthest east in Asia where AHS has ever been recorded . The most plausible explanation as to the source of virus lies in the fact that a shipment of zebra arrived in Thailand three to four weeks before the first AHS outbreak of the disease was discovered . Zebra are considered the natural reservoir of the virus , developing viremias lasting up to 40 days .
This would not be the first occasion that importation of zebra from a country in which AHS is a seasonal occurrence has been implicated in the introduction of disease into a disease-free country . A shipment of zebra was the confirmed source of this virus for a major disease event that occurred in Spain in 1987 and subsequently involved Portugal and Morocco .
International trade in wildlife , both legal and illegal ,