African Horse Sickness
African horse sickness (AHS) is exotic to the U.S. and endemic in Africa. Its distribution in Africa is in the dry tropical zones on either side of the equator, corresponding to the distribution of the insect vector, Culicoides. African horse sickness periodically makes incursions outside of the African continent and has been responsible for major outbreaks in the Middle East and more recently, in Spain (1987-1991).
| Why would you even think of this foreign animal disease? |
Infectious agent: African horse sickness virus is a member of the Family Reoviridae, Genus Orbivirus. These are all dsRNA viruses. Other orbiviruses of veterinary importance include bluetongue and epizootic hemorrhagic diseases.
All three of these agents are transmitted by Culicoides
insects. Other terms for Culicoides include: midges, no-see-ums, and punks.
Clinical Signs: labored breathing with increased respiratory rate, fever, and depression; there may be paroxysmal coughing; edema of the supraorbital fossa is thought to be diagnostic for the disease but is only seen in about 10% of cases.
Froth from nares seen at the time of death
Swelling of the supraorbital fossa
What is the pathogenesis of AHS?
| This virus enters the animal through the bite of an infected Culicoides and then replicates preferentially in endothelium of the heart and lung (although any vessels can be affected). Vessels become leaky and result in heart failure and pulmonary edema. Postmortem lesions therefore include adventitial edema of blood vessels, especially in the lungs, alveolar edema, and rarely acute myocardial necrosis +/- multifocal myocardial hemorrhage. Strangely, vasculature of the muscles around the ligamentum nuchae can also become leaky and result in a “lemon jello” type appearance to the deep cervical muscles when examined on necropsy. If vessels of the gut become leaky, clincial signs of colic may result. |
Look at these two striking examples of pulmonary edema caused by African Horse Sickness!
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Diagnosis: Diagnosis of a foreign animal disease is never made in the field. Definitive diagnosis is through laboratory confirmation after you have alerted proper regulatory authorities of your suspicions. Virus isolation from whole blood from a live animal or spleen of recently dead animal can be used to make the diagnosis.
Treatment and Prevention: There is no treatment for this disease. Affected animals are destroyed. If isolated in the U.S. prevention would include Culicoides vector control and surveillance for the presence of the viral organism.
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