African horse sickness

African horse sickness (AHS) is a highly infectious, and deadly disease. It commonly affects horses, mules, donkeys and zebras. It is caused by a virus of the genus Orbivirus belonging to the family Reoviridae. This disease can be caused by any of the nine serotypes of this virus. AHS is not directly contagious, but is known to be spread by insect vectors.

Epidemiology
AHS 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 Americas, 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. Larvae do not carry the virus and long, cold winters are sufficient to break epidemics in non endemic areas.

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 show no signs of the disease.

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.

Clinical signs
Horses are the most susceptible host with close to 90% 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, a mild (horse sickness fever) form, and a mixed form.

Pulmonary form

The peracute form of the disease is 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

This subacute form of the disease has an incubation period longer than that of the pulmonary form. 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. The survival rate is 100%

Mixed form

Diagnosis is made at post-mortem exams. Affected horses show signs of both the pulmonary and cardiac forms of AHS.

Diagnosis
Presumptive 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 immunofluorescence of infected tissues. Serological test are only useful for detecting recovered animals, as sick animals die before they are able to mount an effective immune respond.

Treatment and prevention
There is currently no treatment for AHS.

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.

History
African horse sickness was diagnosed in Spain in 1987-90 and in Portugal in 1989, but was eradicated using slaughter policies, movement restrictions, vector eradication and vaccination.

Related Diseases
AHS is related to bluetongue disease and is spread by the same midge (Culicoides species).