African horse sickness

African horse sickness (AHS) is a highly fatal insect born viral disease of equidae caused by an Orbivirus (family Reoviridae) and characterized by severe pyrexia, respiratory distress and high mortality (up to 95%). with extensive oedema and serous effusions. The disease occurs during summer, immediately after rainfall and is endemic in South Africa. Horses are the most susceptible to this disease, followed by mules, donkeys and zebras in decreasing order of susceptibility.AHS virus is an Orbivirus belonging to family Reoviridae, of which nine antigenic serotypes of different virulence are known. AHS is an insect-borne disease and cannot be transmitted between horses by contact. Biting midges of the family Culicoides are recognised as the most important vectors.AHS was reported in India during 1960-61 in several states in northern, western and central India resulting in over 22,000 deaths among equines. It was effectively controlled by vaccination and zoo-sanitary measures. No case has been reported after 1964.

Clinical signs: The incubation period is 7 days. Three clinical form of the disease occur, an acute or pulmonary form, a cardiac or sub acute form and a mild form known as 'horse sickness fever'.In the acute form, fever, dyspnoea, increased breathing rate (60-75 per minute), paroxysmal cough and nasal discharge are commonly seen. Profuse sweating and spasmodic cough with frothy fluid exudating from nostril is observed in terminal stage. Mortality up to 95%. In the subacute cardiac form, fever and extensive swelling of the supra-orbital fossa are noticed. Marked subcutaneous oedema and hydropericardium are the lesions observed in the cardiac form.In Mild or 'horse sickness fever' form of infection, the  clinical signs consist of influenza-like symptoms and a transient fever up to 40 C can occur for 2-3 days. It is the most common form of the disease manifested in mules, donkeys, elephants and zebras. The conjunctiva may be slightly congested and pulse rate may be increased.

Diagnosis: It is diagnosed by clinical symptoms and lesions, followed by the isolation and identification of the virus in the laboratory. Virus isolation is done from infected whole blood and spleen tissues collected after PM examination using tissue culture (BHK 21, Vero), eggs and newborn mice (intracerebrally). The serological tests include virus neutralization test (VNT), CFT, sandwich ELISA for group-specific antibody in serum samples of infected equines. VNT detects serotype specific antibodies. Polymerase chain reaction is done with the whole blood samples and spleen samples submitted for diagnosis in the laboratory. Indirect sandwich ELISA detects viral antigen from spleen tissues of AHS positive horses.

Prevention and control:  In endemic countries, AHS control may be achieved by vaccinating equines with available vaccine and maintaining a high level of population immunity. Three types of vaccines; inactivated ('killed'), attenuated ('live') and recombinant virus vaccines are available.   The vaccine must be made from the serotype(s) responsible for the outbreak of clinical disease. Vector-control programme should be enforced in disease free countries.Quarantine and movement controls to stop the entry of infection should be implemented.