African horse sickness virus

African horse sickness virus (AHSV) infects all equine species, including horses, donkeys, mules and zebras as well as camels.

It is one of the most deadly equine viruses and can be fatal in up to 90% of infected susceptible animals.

AHSV is spread by Culicoides biting midges that have been infected by biting an affected animal. 

It is prevalent in central and sub-Saharan Africa, but major outbreaks have also been reported in the Middle East, India, Pakistan, Spain, Portugal and Morocco.

  • African horse sickness is a notifiable disease and should be reported.
    Please see the Defra website for advice on how to spot and report the disease.

Clinical signs

AHSV can cause different forms of the disease:

Respiratory form:

  • Fever
  • Breathing difficulties, coughing
  • Sweating 
  • Frothy discharge from the nostrils with death occurring within a few hours

Cardiac form:

  • Fever
  • Swelling around the eyes, lips, cheeks, tongue and neck
  • In some cases colic may also been seen

Mixed form:

  • A combination of signs from the cardiac and respiratory forms can be seen

Horse sickness fever:

  • Fever for a few days
  • Depression and reduced appetite
  • These animals often recover from the disease


There are nine types (serotypes) of AHSV. It is from the family Reoviridae of the genus Orbivirus and there are nine types (serotypes). AHSV is unenveloped and made up of a two layered capsid. The genome comprises 10 double stranded RNA segments. It can be spread through the blood and infects namely the lungs, spleen and other lymphoid tissues.


AHSV-free countries such as the UK and the rest of the EU heavily control the movement of horses. Although AHSV is currently restricted to sub-Saharan Africa, it has a history of emergence into southern Europe. Strict international travel regulations are therefore in place to prevent infected animals being moved from regions where the virus is prevalent.

Vaccines are available using the live attenuated virus (a virus that has been made less virulent), in some countries such as South Africa where AHSV persists. These vaccines are not considered safe enough (as the pathogen is still live), for licensed use in countries where the virus is not present, including the EU.


There is no specific treatment available for AHS, other than supportive treatment. Measures to control the exposure of horses to biting insects, together with movement restrictions and efficient detection systems (rapid diagnosis), are essential to prevent an outbreak from spreading.

AHS is the only equine infectious disease for which the World Organisation for Animal Health (WOAH), issues an official declaration of disease freedom to its member countries.

Pirbright's research on African horse sickness

Scientists at The Pirbright Institute are leading the way in helping find a safe and effective vaccine for AHSV.

Existing vaccines using the live virus make it difficult to determine from blood samples if a horse has been vaccinated or infected with AHSV.

The vaccine strategy developed at Pirbright would enable horses to be differentiated and help ensure the safer movement of animals locally and internationally. This uses modified vaccinia Ankara virus (which is harmless to horses), to carry and deliver the gene of AHSV that creates the protection against the virus. This technique could potentially be used in a ‘polyvalent’ approach; meaning that it could be capable of protecting against each of the nine different AHSV serotypes.

With the support of the WOAH, the Institute is currently working with other research institutes around the world to evaluate potential new AHSV vaccines candidates, including the one being developed at Pirbright. We are also collecting the evidence needed to support a business case for the manufacture of these new vaccines.

Diagnostics and surveillance

Pirbright is one of four WOAH reference laboratories for AHSV and provides early warning, rapid diagnosis, emergency response and expert advice to the UK and international governments.

We coordinate international projects to standardise and harmonise the use of AHSV diagnostic tests. The Institute also advises on and contributes to comprehensive codes of practice for the safe international trade of horses. Our scientists also monitor global patterns of disease distribution and identify the correct vaccine to be used in the event of an outbreak.

Research papers

Ashby M, Moore R, King S, Newbrook K, Flannery J, Batten C (2024)

microorganisms 12 (5)
Nelson E, Thurston W, Pearce-Kelly P, Jenkins H, Cameron M, Carpenter S, Guthrie A, England M

viruses 14 (3)
Publisher’s version:
Jones LM, Hawes PC, Salguero FJ, Castillo-Olivares J (2023)

Frontiers in Veterinary Science 10 , 1114240
Tugwell L A, England M E, Gubbins S, Sanders C J, Stokes J E, Stoner J, Graham S P, Blackwell A, Darpel K E, Carpenter S (2021)

Parasites and Vectors 14 (1) , 55
King S, Rajko-Nenow P, Ashby M, Frost L, Carpenter S, Batten C (2020)

Transboundary and Emerging Diseases
Publisher’s version:


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