MSD Manual

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Pet Owner Version

African Horse Sickness


Thomas St. C. McKenna

, DVM, PhD, USDA, APHIS, Veterinary Services

Reviewed/Revised May 2019 | Modified Oct 2022

African horse sickness is a short-term insect-borne viral disease of equids (horses, donkeys, mules, and zebras) that is widespread in Africa. Horses and mules are considered most susceptible. It is characterized by signs of lung and circulatory system impairment. During epidemics, the fatality rate can reach 90% in groups that have not been previously exposed to the virus.

African horse sickness virus is most frequently transmitted by midges, although other insects, including mosquitoes, may also transmit the disease.

The virus causes lung and heart disease in horses. Incubation time is up to 2 weeks, after which fever and respiratory signs develop. Signs include fever, coughing, difficulty breathing, and dilated nostrils. Fever lasts about 1 week, followed by swelling near the eyes. The swelling usually extends to the neck, shoulders, and chest. Death usually follows within a week and may be preceded by colic. The mortality rate is about 80%.

In areas where the disease is common, the signs and tissue changes may allow a veterinarian to make a provisional diagnosis. However, laboratory testing is needed to confirm the diagnosis and identify the strain of virus involved. Knowing the strain can be important for control measures.

The disease is not contagious from horse to horse but is spread by bites from carrier insects. In the US, horses, donkeys, mules and other equids from African countries are quarantined for 2 months and then tested for the virus.

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