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Author: Dr Melvyn Quan
Adapted from: Coetzer, J.A.W and Guthrie, A.J. 2004. African horse sickness, in Infectious diseases of livestock, edited by J.A.W. Coetzer & R.C. Tustin. Oxford University Press, Cape Town, 2: 1231-1264

Introduction

African horse sickness (AHS) is a peracute, acute, subacute or mild infectious but non‑contagious disease of equids caused by African horse sickness virus (AHSV). The virus is classified in the genus Orbivirus of the family Reoviridae, of which there are nine serotypes, all transmitted by Culicoides midges. AHSV is similar in morphology and shares many properties with other orbiviruses, such as equine encephalosis virus, bluetongue virus and epizootic haemorrhagic disease virus. The virus is closely associated with erythrocytes. Of the nine serotypes of AHSV, there is cross-neutralization between serotypes 1 and 2, 3 and 7, 5 and 8 and 6 and 9.

AHS is a World Organisation for Animal Health (OIE) - listed disease and manifests pyrexia and clinical signs compatible with impaired respiratory and circulatory functions, characterized by oedema of subcutaneous and intermuscular tissues and of the lungs, transudation into the body cavities and haemorrhages on serosal surfaces. The mortality rate in horses, the most susceptible species, may be as high as 95 per cent, while mules are less susceptible and donkeys with rare exceptions only develop inapparent infections.

Although mortality in horses as a result of AHS occurs every year in South Africa, major epidemics prior to 1953 seem to have occurred at intervals of roughly 20 to 30 years. The timing of these epizootics has been associated with the warm (El Niño) phase of the El Niño/Southern Oscillation (ENSO) phenomenon. Severe losses were reported in 1780, 1801, 1839, 1855, 1862, 1891, 1914, 1918, 1923, 1940, 1946 and 1953.