Trotting and Harness Racing
The Harness Racing Association of South Africa

African Horse Sickness Virus

African horse sickness (AHS) is a non-contagious, infectious, insect-borne disease of equids caused by a virus of the same name (AHSV). The virus is transmitted by a biting midge, the Culicoides Imicola, which is thought to be most active in the first four hours after sunset. Although zebras and donkeys rarely exhibit clinical signs, the effects of the disease, particularly in susceptible populations of horses can be devastating and mortality rates for this species may exceed 90%.

As a consequence of its severity and because it is able to expand rapidly and without apparent warning out of its endemic areas AHS has been allocated OIE List A status.

In horses AHS is characterized by clinical signs, which develop as a result of damage to the circulatory and respiratory systems giving rise to serious effusion and hemorrhage in various organs and tissues. The extent and severity of the clinico-pathological findings have been used to classify the disease into four (4) forms. In ascending order of severity these are horse sickness fever (which usually affects only mules, donkeys and partially immune horses), the subacute or cardiac form, the cardio-pulmonary or mixed form and the peracute or pulmonary form. All forms of disease can occur in any one outbreak but in susceptible populations of horses the mixed and pulmonary forms tend to predominate so mortality rates in these animals will be very high.

AHS is an ancient disease and was first noted in Yemen apparently in 1327. However the virus almost certainly originated in Africa. The frequency, extent and severity of outbreaks in southern Africa has declined significantly over the last century, coinciding with major decrease in horse and zebra populations and the introduction of AHS vaccines.

Pretoria University’s veterinary science faculty at Onderstepoort has developed and distributes the world’s only effective vaccine.

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