African Horse Sickness
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About African Horse Sickness

What is African horse sickness?
African horse sickness (AHS) is a highly infectious non-contagious, vector born viral disease affecting all species of Equidae. It is classified as an Orbivirus of the Reoviridae family of which there are 9 serotypes. All serotypes (1-9) are distributed throughout South Africa, although there is a variation in their temporal distribution. It is endemic to (occurs naturally on) the African continent, and is characterised by respiratory and circulatory damage, accompanied by fever and loss of appetite.

Host and Vector
Animals affected are, all breeds of horses (mortality rate of 70-90%), mules and donkeys. Wild life Equine species (Zebras) are resistant to the disease. The vector host, Culicoides midge, spreads AHS virus.

How do horses contract the disease?
AHS does not spread directly from one horse to another, but is transmitted by the Culicoides midge, which becomes infected when feeding on other infected equidae. It occurs mostly in the warm, rainy season when midges are plentiful, and disappears after frost, when the midges die. Most animals become infected in the period associated with sunset and sunrise, when the midges are most active.

The disease manifests in three ways, namely the lung form, the heart form and the mixed form. The lung (dunkop) form is characterised in the following manner:

  • very high fever (up to 41 degrees).
  • difficulty in breathing, with mouth open and head hanging down.
  • frothy discharge may pour from the nose.
  • sudden onset of death.
  • very high death rate (90%).

The heart (dikkop) form is characterised in the following manner:

  • fever, followed by swelling of the head and eyes.
  • in severe cases, the entire head swells (“dikkop”).
  • loss of ability to swallow and possible colic symptoms may occur.
  • terminal signs include bleeding (of pinpoint size) in the membranes of the mouth and eyes.
  • Slower onset of death, occurring 4 to 8 days after the fever has started.
  • Lower death rate (50%).

The mixed form is characterised by symptoms of both the dunkop and dikkop forms of the disease.

Diagnosis and Notification
The symptoms described above may assist with an initial diagnosis of AHS. This diagnosis can only be confirmed by identifying the virus in a laboratory. It is, therefore, essential that blood samples be taken from the horse during the fever stage of the disease for analysis. As AHS is a controlled disease, horse owners are obliged by law to notify the local State Veterinarian of suspected cases.

Note: Excerpts taken from Onderstepoort Biological Products leaflet.

Control of African Horse Sickness

African Horse Sickness (AHS) is one of a number of diseases known to be potentially damaging to the livestock economy. By way of the Animal Diseases Act (Act No. 35 of 1984), AHS has been declared a state controlled disease, thereby empowering the state to implement measures to control the disease. Horse owners are also required by this law to notify their local state veterinarian of any cases of AHS. The Act also requires that all equines (horses, donkeys and mules) must be vaccinated at least once a year with an approved AHS vaccine.

Movement controls into the AHS Control Area in the Western Cape were introduced when the export protocol (1997/10/EC) was ratified by the European Commission in 1997. Based on guidelines given by the World Animal Health Organisation (OIE), and South African and European legislation, the South African Veterinary Authority established a policy for the movement of all equines into this area.

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