It is caused by an orbivirus (a type of virus which also causes Bluetongue), and is transmitted by the same type of midge. The spread of disease can be influenced by climatic conditions that favour the proliferation of the vector midges, including warm, moist weather and high rainfall, as well as spread by wind dispersal.
The clinical signs seen are dependent upon which form of the disease is present:
Acute - high fever, severely laboured
breathing, coughing and profuse discharge from the nostrils. The mortality rate is very high with up to 95% of severely affected horses dying within a week.
Cardiac - swellings are present over the head and eyelids, lips,cheeks and under the jaw. The mortality rate is around 60% and death results from heart failure.
Mildest form - fever with low temperatures in the morning rising to a high peak in the afternoon.
The mixed form of the disease shows itself by mild respiratory signs followed by the typical swellings of the cardiac form.
Protection
No vaccine for AHS is currently licensed in the EU.
Protection from vectors could reduce the risk of infection.