African Horse sickness
From Wikipedia, the free encyclopedia
African horse sickness (AHS) is a highly infectious, and deadly disease. It commonly affects horses, mules, donkeys and zebras. It is caused by Viscerotropic virus of the genus Orbivirus belonging to the family Reoviridae. This disease can be caused by any of the nine serotypes of this virus. This disease is not directly contagious, but is known to be spread by insect vectors.
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[edit] Epidemiology
This disease is known to be endemic in Sub Saharan Africa, and has spread to Morroco, the Middle East, India and Pakistan. More recently, outbreaks have been reported in the Iberian Peninsula and Morocco. AHS has never been reported in the Western Hemisphere, Eastern Asia or Australasia. Epidemiology is dependent on host-vector interaction where cyclic disease outbreaks coincide with high numbers of competent vectors. The most important vector for AHS in endemic areas is the midge Culicoides imicola which prefers warm, humid conditions. Larvaes do not carry the virus and long, cold winters are sufficient to break epidermics in non endemic areas.
[edit] Host
The common hosts of this disease are horses, mules, donkeys and zebra. However, elephants, camels and dogs (after eating infected horsemeat) can be infected as well, but often showing no signs of the disease.
[edit] Transmission
This disease is spread by insect vectors. The biological vector of the virus is the Culcoides (midges) species. However, this disease can also be transmitted by species of mosquitoes including Culex, Anopheles and Aedes, and species of ticks such as, Hyalomma and Rhipicephalus.
[edit] Clinical Signs
Horses are the most susceptible host with close to 100% mortality of those affected followed by mules (50%) and donkeys (10%). African donkeys and zebras very rarely display clinical symptoms, despite high virus titres in blood, and are thought to be the natural reservoir of the virus. AHS manifests itself in four different forms: the Pulmonary form, the Cardiac form, Mild (Horse Sickness Fever) Form and Mixed form.
Pulmonary form
Preacute form of the disease characterised by high fever, depression and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing, and shows signs of pulmonary oedema within four days. Serious lung congestion causes respiratory failure and results in death in under 24 hours. This form of the disease has the highest mortality rate.
Cardiac form
Subacute form of the disease. The incubation period is longer than the Pulmonary form where signs of disease start at day 7-12 after infection. High fever is a common characteristic. The disease also manifests as conjunctivitis, with abdominal pain and progressive dyspnoea. Additionally, oedema, presented under the skin of the head and neck: most notably swelling of the supra-orbital fossae, palpebral conjunctiva and intermandibular space. Mortality rate is between 50-70% and survivors recover in 7 days.
Mild or Horse Sickness Fever form
Mild to subclinical disease is seen in zebras and African donkeys. Infected animal may have low grade fever and congested mucous membrane. Survival rate is 100%
Mixed form
Diagnosis at post-mortem exams. Affected horses show signs of both the Pulmonary and Cardiac forms of AHS.
[edit] Diagnosis
Persumptive diagnosis is made by characteristical clinical signs, post-mortem lesions and presence of competent vectors. Lab confirmation is by viral isolation with such technique as Real Time PCR for detecting viral RNA, antigen capture ELISA and immunoflorescence of infected tissues. Serological test are only useful for detecting recovered animal as sick animals die before they are able to mount an effective immune respond.
[edit] Treatment and Prevention
There is currently no treatment efficient enough to treat this disease.
Control of outbreak in an endemic region involves quarantine, vector control and vaccination. To prevent this disease the affected horses are usually slaughtered and the uninfected horses are vaccinated against the virus. Three vaccines currently exist, which include a polyvalent vaccine, a monovalent vaccine and a monovalent inactivated vaccine. This disease can also be prevented by destroying the insect vector habitats using insecticides.
