African horse sickness is a viral disease of equidae transmitted by mosquitoes with a high mortality rate. It is not dangerous for humans.

Depends on the distribution area of the vectors. Sub-Saharan regions such as Central Africa, South Africa and West Africa are particularly affected. Individual outbreaks occurred in Asia in 2020.

Horses, mules, donkeys and zebras, and rarely camelids, Angora goats and elephants. Dogs can be affected after eating meat containing the virus.

Transmission by mosquitoes of the genus Culicoides (gnats)

2-14 days

High fever (40-41 °C), disturbance of the general condition, muscle tremors, staggering, sweating, respiratory and cardiac problems up to heart failure, oedema formation. Depending on the form of progression, mortality varies between 50-95 %.

African horse sickness is a notifiable animal disease transmitted by mosquitoes that is associated with considerable mortality in horses. Infected animals must be killed and destroyed.

Preventive measures can be taken to reduce the contact of susceptible animals with mosquitoes, e.g. by stabling them during the night. Preventive vaccinations are available but prohibited in Austria.

Austria is officially free of African horse sickness. The disease has never been detected in Austria.

African horse sickness, also known as equine plague, pestis equorum, African horse sickness (AHS) or equine plague, is a peracute to acute, seasonal viral disease of equidae that is transmitted by blood-sucking insects. There is no transmission from animal to animal, but the disease can be transmitted indirectly through used syringes.

The pathogen, an orbivirus from the Reoviridae family, is ingested through the bite of mosquitoes (midges); the virus multiplies in mosquitoes of the genus Culicoides and is transmitted further with the next act of sucking. Other mosquito species can spread the virus mechanically. To date, nine serotypes of AHS viruses have been differentiated. Strains with different virulence occur under natural conditions.

The African horse sickness virus is unstable to low pH values and is rapidly destroyed by temperatures of 50-60 °C, sunlight and putrefaction.

Symptoms

African horse sickness is generally characterised by fever, oedematous swellings in the head area, pulmonary oedema, bleeding in internal organs and exudation into the body cavities. This is caused by virus replication in endothelial cells and the resulting increased permeability of the capillary and vascular walls.

There are four different forms of African horse sickness:

• Peracute pulmonary form: incubation period 2-5 days, high fever (40-41 °C), increased respiratory rate, distended nostrils, muscle tremors, swaying, dyspnoea, sweating and coughing. Mortality 95 %, death usually occurs within a few hours of the first clinical symptoms
• Subacute cardiac form: fever for 3-6 days and oedema in the head, neck, chest and limbs. Foamy discharge from the nostrils, petechiae on the conjunctiva, muscle weakness, colic symptoms. Lethality up to 50 %
• Acute mixed form: a mixture of the symptoms of the two forms mentioned, subclinical cardiac form followed by acute dyspnoea. Most common form, death from heart failure after approx. 1 week in 70-80 % of cases
• Abortive form ("horse sickness fever"): mild form, incubation period 5-14 days, intermittent mild fever over 5-8 days. Complete recovery, usually occurs in less susceptible animals (donkeys) or vaccinated animals.

The suspected diagnosis, especially in the mosquito season, is made clinically by frequent peracute deaths of equidae and the presence of typical oedema.

The diagnosis is confirmed by PCR diagnostics, virus isolation or serology, especially by means of the elisa technique. Commercially available elisa test kits (VP7 protein) allow reliable detection of antibodies from serum of horses from 8-12 days after infection. With competitive formats, testing of sera from other equids (donkey, zebra, etc.) is also possible.

Virus isolation as well as antigen detection by real-time PCR is possible from blood during the fever phase, after which spleen, lungs and lymph nodes are suitable as sample material.

Differential diagnosis: equine arteritis, trypanosomiasis, spirochetoses, piroplasmoses, babesioses, infectious anaemia, anthrax, various poisonings.