Merck 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. African horse sickness is caused by a virus of the family Reoviridae. The appearance of African horse sickness follows seasons of heavy rain that alternate with hot and dry climatic conditions. Outbreaks in central and east Africa have extended to Egypt, the Middle East, and southern Arabia. Epidemics have also occurred in Spain after importation of an infected zebra. 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. Stray dogs, dog ticks, and camel ticks have also been shown to carry the disease and may also play a role in disease transmission, especially in areas where the disease is widespread.

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 outlook for recovery in infected horses depends on the viral strain involved and the susceptibility of the infected horse. Infected horses that survive develop immunity to the viral type with which they were infected. However, they remain susceptible to other types of the virus. There are vaccines that provide long-lasting protection. When the disease first appears in an area, affected horses should be eliminated immediately, and uninfected animals should be vaccinated and rested for 2 weeks. At the same time, attention should be paid to control of the insects that most frequently transmit this disease. The disease is not contagious from horse to horse but is spread by bites from carrier insects. Your veterinarian can recommend the most appropriate pest control program for your horses. Note that vaccinated horses should be kept in insect-proof housing because vaccine failure may occur. In the US, horses, donkeys, mules and other equids from African countries are quarantined for 2 months and then tested for the virus. The presence of antibodies does not interfere with importation of horses into countries free of the disease.

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