FEEVA animation on African Horse Sickness (AHS)


What is AHS?

  • AHS is a frequently fatal disease of horses caused by infection with an Orbivirus with 9 serotypes (types 1-9), there is limited cross protection between most serotypes
  • AHS virus is related to another Orbivirus called Bluetongue virus (BTV)
  • The horse is the most susceptible equine species, followed by donkeys. Zebra are the least susceptible with the infection being largely subclinical
  • AHS is considered endemic in sub-Saharan Africa but occasionally breaks out in other countries, most commonly following movement of infected zebra


Transmission AHS virus

  • AHS is not directly contagious between horses and viral transmission is by biting Culicoides midges
  • AHS virus is maintained in a cycle of infection between equine hosts (involving intrinsic viral incubation) and midge vectors (extrinsic viral incubation)
  • Zebra are critical to the endemic maintenance of AHS in Africa
  • Maintenance of AHS in non-endemic areas would probably require mechanisms for over-wintering of AHS virus


Clinical syndromes of AHS

  • AHS virus is transferred to blood from infected biting midges, with viral replication in the regional draining lymph nodes
  • Dissemination to the lungs, spleen and other lymph nodes is through a marked secondary viraemia leading to severe vascular permeability with disseminated haemorrhage and oedema
  • There are four clinical syndromes associated with AHS
    • Pulmonary form (‘Dunkop’), which presents as acute disease, fever, dyspnoea, sweating, pulmonary oedema, terminal frothy discharge (90% fatal)
    • Cardiac form (‘Dikkop’), which presents as subacute disease, fever, oedema – head, neck, chest, supra-orbital fossa, conjunctivitis, +/- colic (50% fatal)
    • Mixed form, which is as the name suggests a mixture of signs of Dunkop & Dikkop (80% fatal)
    • Horse sickness fever, which is milder and non-fatal and occurs in partially immune animals, either by vaccine or following survival after natural infection



  • Virus detection in blood or tissues (spleen, lung or lymph nodes), traditionally using virus isolation but now based on detecting viral RNA using reverse-transcription (RT) PCR
  • Serological diagnoses can be made using virus neutralization (VN) test or ELISA detecting antibodies against viral protein 7 (VP7)
  • World Organisation for Animal Health (WOAH) recommended tests are RT-PCR based on segment 8 (encoding VP7) and VP7 ELISA


Aims of AHS investigation and control in non-endemic areas

  • Identify any suspect cases & determine if AHS positive as quickly as possible
  • Eliminate AHS virus infection as quickly as possible to avoid spread to endemic midge vectors
  • Determine the route of introduction of AHS virus (e.g. recent live animal import, transfer of infected vector)
  • Regain AHS free status as soon as possible based on use of disease control zoning principles


Use of zones for AHS control

  • Zones are declared on confirmation of disease, together comprising the Restricted Zone (RZ)
    • Control Zone (CZ): at least 20km from infected premises (IP)
      • Vaccination of all equidae permitted
    • All suspect holdings under surveillance within the zone
    • All movements prohibited
  • Protection Zone (PZ): at least 100km from IP
    • Vaccination of all Equidae may be permitted
  • Surveillance Zone (SZ): at least another 50km
    • No vaccination permitted


AHS vaccination

  • The AHS vaccine currently used in South Africa is based on 7 live attenuated vaccine strains (polyvalent) presented as two separate injections (bottles 1 and 2) given at least 3 weeks apart before highest risk periods
    • Bottle 1: serotypes 1, 3 and 4
    • Bottle 2: serotypes 2, 6, 7 and 8
    • Cross protection is conveyed between serotypes 6 and 9 and 8 and 5
  • Viral surveillance has demonstrated recombinations between field and live-attenuated vaccine strains in South Africa
  • So use of polyvalent live attenuated vaccine to control AHS in non-endemic areas may present issues in restoring AHS free status
  • Safer, monovalent AHS vaccines with DIVA (differentiate infected from vaccinated animals) capability are required to assist effective control and eradication of AHS in non-endemic areas


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