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 genusOrbivirus 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 Morocco, the Middle East, India and Pakistan. More recently, outbreaks have been reported in the Iberian Peninsula. 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 biting 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%^[1][2] 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 characterized by high fever, depression and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing frothy fluid from nostril and mouth, and shows signs of pulmonary edema 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 symptom. The disease also manifests as conjunctivitis, with abdominal pain and progressive dyspnea. Additionally, edema is presented under the skin of the head and neck, most notably in 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 animals may have low grade fever and congested mucous membrane. The survival rate is 100%.

Mixed form

Diagnosis is made at necropsy. Affected horses show signs of both the pulmonary and cardiac forms of AHS.

Diagnosis

Presumptive diagnosis is made by characteristic clinical signs, post mortem lesions and presence of competent vectors. Laboratory confirmation is by viral isolation, with such techniques as Real Time PCR for detecting viral RNA, antigen capture ELISA and immunofluorescence of infected tissues. Serological tests are only useful for detecting recovered animals, as sick animals die before they are able to mount effective immune responses.

Treatment and prevention

Control of an 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.