A110 - African Horse Sickness

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A110 - African Horse Sickness

Nature of the disease
African horse sickness (AHS) is an acute or subacute non-contagious viral disease transmitted by arthropods that is often fatal and is characterised by fever and oedema in the lungs and subcutaneous tissues.
Classification
OIE List A disease
Susceptible species
Horses, donkeys and mules are the most susceptible species, however the disease has been isolated in dogs, camels and a number of African wild species.
Distribution
AHS occurs in the tropical area of central Africa where it is endemic. The disease regulaly spreads from there to the Southern Africa and occasionally to Northern Africa. Outbreak occurred in Middle East, Spain and Portugal where it has not occured since 1991.
AHS has never occurred in the Pacific region.
Clinical signs
The severity of the disease depends on the susceptibility of the host and several different forms exist depending on the organs involved.

Subclinical form: High fever (40-40.5°C) and general malaise for 1-2 days, usually the fever is more prevalent during the afternoon.

Subacute or cardiac form: Moderate fever (39-41°C) followed by a characteristic swelling of the supraorbital fossa, followed by swelling of eyelids, facial tissues, neck, thorax, brisket and shoulders. Death usually within 1 week and is preceded by colics.

Acute respiratory form: fever (40-41°C) for one or days, dyspnoea, spasmodic coughing, dilated nostrils with frothy fluid oozing out, congestion of conjunctivae, death from anoxia within 1 week

A mixed form (cardiac and pulmonary) occurs frequently: pulmonary signs of a mild nature that do not progress, oedematous swellings and effusions, death from cardiac failure, usually within 1 week. A nervous form may occur, though it is rare.

Post-mortem findings
Respiratory form: pulmonary oedema, with distended and heavy lungs, thoracic lymph nodes are oedematous, there is congestion of the gastric fundus

Cardiac form: petechiae and ecchymoses on the epicardum and endocardum, often there is hydropericardium. Lungs are oedamtous and flaccid.

There are some yellow and gelatinous infiltrations of the subcutaneous and intramuscular tissues. Sometimes there is haemorrhagic gastritis and petechiae on the peritoneum and the tongue.
Differential diagnosis
  • Anthrax
  • Equine infectious anaemia
  • Equine viral arteritis
  • Trypanosomosis
  • Equine encephalosis
  • Piroplasmosis
  • Purpura haemorrhagica
Specimens required for diagnosis
For serology
Serum: preferably paired samples should be taken 21-days apart and kept frozen at -20°C
Serological techniques are ELISA and Complement Fixation, which are recommended by OIE. Immunobloting is also possible.

For virology
Blood specimens obtained at the peak of the fever are preserved in OPG (50% glycerol, 0.5% potassium oxalate, 0.5% phenol) or heparin 10 IU/ml and transported at 4°C to the laboratory
Spleen, lungs and lymph nodes have the highest viral titres and should be collected from freshly dead animals. These are preserved in 10% buffered glycerine and transported at 4°C to the laboratory.
Techniques for virus isolation are innoculation of Suckling mice or cell culture (BHK, MS, VERO), and for virus identification: ELISA, virus neutralisation (Serotyping) and polymerase chain reaction (PCR).

Transmission
The disease is not directly contagious and must be borne by a biological vector, usually a mosquito Culicoides spp. Other mosquitoes such as Culex, Anopheles and Aedes spp, can be vectors and sometimes ticks (Hyalomma, Rhipicephalus).
As it is transmitted by arthropods, moist mild conditions and warm temperatures favour the disease and usually outbreaks are preceded by periods of heavy rain, that alternates with hot and dry conditions.
Virus movement over long distances via windborne-infected vectors has been suggested but the role of the arthropod vector is not fully understood.
Risk of introduction
AHS could be introduced by:
  • illegally imported horsemeat,
  • introduction of contaminated vectors through flights from infected areas,
  • illegally imported horses, (or other Equidae) or dogs.
Quarantine is the main protection against the disease. In some countries (e.g. USA), imported horses stay for 2 months in quarantine and then are tested for the virus. Evidence of antibodies in the serum should be considered in regards to the origin of the animal (disease may be endemic) and vaccination history, as non recombinant vaccines may introduced reactive antibodies.
Control / vaccines
There is no treatment for AHS.
Vaccination of non-infected horses is possible. There are 9 different serotypes and vaccines have been developed for all of them.
Inactivated vaccines require two injections and give a shorter immunity than recombinant vaccines.
When the disease is introduced into a region, the population should be surveyed for infection and affected Equidae should be slaughtered and the carcasses destroyed. Non affected Equidae should be vaccinated with polyvalent vaccines prior to the virus identification. Once the virus has been identified, susceptible animals should be revaccinated with the homologous vaccine.
The control of vectors is advised, using insecticides, repellents and the destruction of mosquito breeding areas.
References

 

  • African Horse Sickness, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 496-498
  • African Horse Sickness, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 946-948
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002