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African Horse Sickness



  • A non-contagious infection caused by African Horse Sickness (AHS) virus (AHSV) usually causing acute and often fatal disease in horses.
  • Notifiable in the USA and the EU, OIE-listed disease.
  • Incidence: transmitted by night-flying insect vectors; primarily seen in sub-Saharan Africa. The disease has both seasonal (late summer/autumn) and cyclical incidence (warm-phase events).
  • Signs: there are four classical clinical forms of AHS;
    • Pulmonary: the pulmonary form is periacute and occurs in fully susceptible animals with death usually occurring in a few hours. Clinical signs include: respiratory distress, sweating, extension of head and neck, froth exudes from the nostrils.
    • Cardiac: the cardiac form is subacute with mortality around 50%. Clinical signs include: swelling of neck, suborbital fossae and conjunctiva. Paralysis of the esophagus can occur resulting in aspiration pneumonia.
    • Mixed: clinical signs of both the cardiac and pulmonary forms are present with mortality around 70%.
    • Horse sickness fever: clinical signs are mild, often only involving transient fever. Most often observed in zebra and donkeys
  • Diagnosis: virus isolation, serology, RT-PCR, RT-LAMP. AHS can be confused with encephalosis, equine infectious anemia, equine morbillivirus pneumonia, equine viral arteritis, babesiosis and purpura hemorrhagica so laboratory confirmation is essential.
  • Treatment: no specific therapy. There are attenuated (monovalent and polyvalent) vaccines commercially available.
  • Prognosis: poor to grave - mortality is related to the species of equidae affected and the serotype/strain of the virus. Horses are most susceptible (50-95% mortality), followed by mules/hinnys (50% mortality). Donkeys and zebra are very resistant in enzootic regions but can be susceptible (10% mortality) in European and Asian countries.



  • African Horse Sickness virus (AHSV) is a double-stranded RNA Orbivirus in the family Reoviridae, similar to Bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV).
  • There are 9 immunologically distinct serotypes:
    • Serotype 9 is widespread in endemic areas and is responsible for the majority of AHS outbreaks outside Africa.
    • Serotypes 1-8 are found in limited geographical areas.
    • Serotype 4 was involved in the outbreak in Spain and Portugal in 1987-1990.
  • Virus is transmitted by night-flying biting insects primarily by Culicoides spp. It is possible that mosquitoes and biting flies (Stomoxys and Tabanus) Biting and nuisance flies may also be able to transmit although the significance of this is probably low.
  • Transmission cannot occur directly between animals.

Predisposing factors

  • Geographical area - endemic to sub-Saharan, central and eastern Africa.
  • Seasonal incidence following rains - late summer and warm-events.
  • Low-lying areas.


  • Presence of suitable vector and environmental conditions to promote the vector.
  • Contaminated veterinary equipment, infected blood-containing products.


  • Infection of endothelial cells of vascular system → vasculitis → edema and hemorrhage.


  • Variable depending upon clinical form but generally 5-14 days for the horse sickness fever; 3-5 days in peracute (pulmonary) form; 5-7 days for mixed form.


  • Virus is transmitted by night-flying biting insects, especially Culicoides spp Culicoides spp.
  • There is evidence in enzootic areas of Africa of a reservoir between seasonal attacks, probably in zebras and donkeys.
  • Incidence decreases to zero after cold weather in temperate countries.
  • Epidemics occur in cyclic intervals due to droughts followed by heavy rains.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Fowler V L, Howson E L A, Flannery J et al (2016) Development of a novel Rt-Lamp Assay for the rapid detection of African Horse Sickness Virus. Transboundary and emerging diseases 64 (5), 1579-1588 PubMed.
  • Diouf N D, Etter E, Lo M M, Lo M & Akakpo A J (2013) Outbreaks of African Horse Sickness in Senegal and methods of control of the 2007 epidemic. Vet Rec 172 (6), 152 PubMed.
  • Aradaib I E (2009) Pcr detection of African Horse Sickness Virus serogroup based on genome segment three sequence analysis. J Virol Methods 159 (1), 1–5 PubMed.
  • Fernández J, Fernández-Pacheco P, Rodríguez B et al (2009) Rapid and sensitive detection of African Horse Sickness Virus by real-time Pcr. Res Vet Sci 86, 353–358 PubMed.
  • Agüero M, Gómez-Tejedor C, Angeles Cubillo M et al (2008) Real-time fluorogenic reverse transcription polymerase chain reaction assay for detection of African Horse Sickness Virus. J Vet Diagn Invest 20, 325–328 PubMed.
  • Herholz C, Fussel A E, Timoney P, Schwermer H, Bruckner L & Leadon D (2008) Equine travellers to the olympic games in Hong Kong 2008: a review of worldwide challenges to equine health, with particular reference to vector-borne diseases. Equine Vet J 40 (1), 87-95 PubMed.
  • Rodriguez-Sanchez B, Fernandez-Pinero J, Sailleau C et al (2008) Novel gel-based and real-time Pcr assays for the improved detection of African Horse Sickness. Virus J Virol Methods 151, 87–94 PubMed.
  • Coetzer J A W & Guthrie A J (2005) African Horsesickness. In: Infectious diseases of livestock. 2nd edn. Eds: Coetzer J A W & Tustin R C. Oxford University Press, Cape Town. pp 1231–1246.
  • Maree S & Paweska J T (2005) Preparation of recombinant African Horse Sickness Virus Vp7 antigen via a simple method and validation of a Vp7-based indirect elisa for the detection of group-specific Igg antibodies in horse sera. J Virol Methods 125 (1), 55–65 PubMed.
  • Bremer C W, Gerdes G H et al (2000) The prevalence of different African Horse Sickness Virus serotypes in the onderstepoort area near Pretoria, during an outbreak of African Horse Sickness in South Africa in 1995/1996. Onderstepoort J Vet Res 67 (1), 65-70 PubMed.
  • Meiswinkel R, Baylis M & Labuschagne K (2000) Stabling and the protection of horses from culicoides bolitinos (Diptera; Ceratopogonidae), a recently identified vector of African Horse Sickness. Bull Entomol Res 90 (6), 509-515 PubMed.
  • Baylis M, Mellor P S & Meiswinkel R (1999) Horse Sickness and enso In South Africa. Nature  397-574 PubMed.
  • Portas M et al (1999) African Horse Sickness in Portugal - a successful eradication program. Epidemiol Infect 123 (2), 337-346 PubMed.
  • Bremer C W, Dungu-Kimbenga B & Viljoen G J (1998) Detection of African Horsesickness Virus in Zebra by Rt-Pcr and the development of different methods for confirming AHSV specificity of Rt-Pcr products. In: Proceedings of the eighth international conference on equine infectious diseases. R & W Publications.
  • Laviada M D, Sanchez-Vizcaino J M, Roy P & Sobrino F (1997) Detection of African Horsesickness Virus by the polymerase chain reaction. Invest Agr Sa 12, 97–102 VetMedResource.
  • Sakamoto K, Punyahotra R, Mizukoshi N et al (1994) Rapid detection of African Horsesickness Virus by the reverse transcriptase polymerase chain reaction (Rt-Pcr) using the amplimer for segment 3 (Vp3 Gene). Arch Virol 36 (1–2), 87–97 PubMed.
  • Laviada M D , Roy P & Sanchez-Vizcaino J M (1992) Adaptation and evaluation of an indirect elisa and immunoblotting test for African Horse Sickness antibody detection. Bluetongue, African Horse Sickness and related orbiviruses: proceedings of the second international symposium, 646-650 VetMedResource.
  • Hamblin C, Graham S D, Anderson E C & Crowther J R (1990) A competitive elisa for the detection of group-specific antibodies to African Horse Sickness Virus. Epidemiol Infect 104 (2), 303–312 PubMed.
  • House C, Mikiciuk P E & Berninger M L (1990) Laboratory diagnosis of African Horse Sickness: comparison of serological techniques and evaluation of storage methods of samples for virus isolation. J Vet Diagn Invest 2, 44–50 PubMed.

Other sources of information

  • OIE (2016) Terrestrial Animal Health Code. Website:
  • Department for Environment, Food and Rural Affairs (2012) African Horse Sickness control strategy for Great Britain. Website:
  • Weyer et al (2010) Improved detection of African Horse Sickness Virus in naturally infected horses: A prospective study. In: Proc 9th annual congress of the Southern African society for veterinary epidemiology and preventative medicine. Pretoria, South Africa. pp 101-103 VetMedResource.
  • Iowa State University (2006) African Horse Sickness. In: Center for food security and public health. Iowa State University, Iowa. pp 4.

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