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Key takeaways

African horse sickness (AHS) is a highly fatal disease endemic to sub-Saharan Africa.

  • Biting midges are the primary route of transmission
  • Symptoms of AHS include difficulty breathing, swelling of the head and neck, frothy nasal discharge, and sudden death
  • Suspected cases of African horse sickness must be reported to authorities
  • African horse sickness has never been identified in North America to date
  • Current policies surrounding the import of equines require strict testing protocols and quarantine procedures for any horses arriving from AHS-endemic countries
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A closer look: African Horse Sickness

African horse sickness is endemic to sub-Saharan Africa, and is uncommon in other parts of the world. Outbreaks have occurred in the Middle East, Asia, and Europe as a result of horse transportation.

** Most cases of AHS have an extremely poor prognosis, with a high fatality rate, making disease control and prevention critical. For this reason, the World Organization for Animal Health has declared this disease internationally reportable, meaning all cases must be reported to authorities and managed under strict guidelines. **

Currently, African horse sickness has never been identified in North America due to rigorous importation testing protocols.

Risk factors

There are two major forms of African horse sickness: the respiratory form and the cardiac form. Both forms have a high fatality rate, with the respiratory form having a fatality rate of up to 95%.

Vaccinated animals tend to have a milder form of disease, showing only symptoms of lethargy and fever. These horses typically make a full recovery. Some vaccinated horses develop subclinical infections, where they show no symptoms whatsoever.

Possible causes

African horse sickness is a viral disease primarily transmitted by biting midges. Midges acquire the virus from feeding on an infected animal. Zebras are the primary source of the virus in African countries where the disease is endemic.

Main symptoms

African horse sickness mainly affects the lungs and heart.

Testing and diagnosis

Severe respiratory symptoms or cardiac symptoms, combined with a history of travel to areas where AHS is endemic is typically sufficient for diagnosis. Confirming a diagnosis involves blood testing to detect the virus directly. Suspected cases of AHS in North America must be reported to federal authorities and managed according to federal policies.

Steps to Recovery

There are no specific treatments for AHS, and most horses with the cardiac or respiratory forms die despite supportive care efforts. Many horses suspected of having AHS are euthanized for welfare reasons.

Vaccinated horses with milder forms of disease typically show symptoms for 4-8 days, however some cases last for up to 21 days. During this time, infected horses are a source of infection for biting midges, allowing for spread of the disease. After recovering from the illness, horses with mild disease show no long-lasting effects.


African horse sickness is not directly contagious between horses, as it requires a biting midge for transmission. Quarantine of horses testing positive for the virus away from all other horses is crucial to preventing spread of the virus. Insect-proof housing and insect control measures, such as insecticides, further reduce the risk of transmission. Many current recommendations require euthanasia of horses showing serious symptoms, both to reduce spread of disease and for welfare reasons.

In countries where AHS is common, vaccines are available as a preventative measure. Export of horses vaccinated with the AHS vaccine to other countries is typically impossible, as they test positive on blood tests. Distinguishing between vaccination and true infection is extremely difficult with currently available testing modalities.

All horses exported from AHS-endemic countries require viral testing. Typically, these horses live in an insect-proof housing unit for several days to weeks as a quarantine period prior to export. Federal regulations surrounding import quarantine and testing vary significantly.

Is African Horse Sickness common?

AHS is common in sub-Saharan Africa, and occasionally causes outbreaks in other parts of Africa. There has never been a case of AHS in North America to date.

Typical Treatment



Peter Timoney, MVB, MS, Ph.D., FRCVS - Writing for American Association of Equine Practitioners
Camilla Weyer , BVSC, MSc, PhD - Writing for Merck Veterinary Manual

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