The disease may take an acute, chronic or sub-clinical form and clinical signs are extremely variable.
Outward signs of the acute form include fever, depression, increased heart and respiratory rate, haemorrhaging, bloody diarrhoea, loss of co-ordination, poor performance, ataxia, rapid weight loss, skin swelling and jaundice. Acutely infected horses carry high levels of virus in the blood and are potentially infectious to other horses and donkeys.
The chronic form may be characterised by recurring bouts of fever, depression, anaemia, weakness or weight loss, interspersed with periods of normality.
Any horse displaying severe, unexplained anaemia should be isolated and tested for EIA as soon as possible.
Sub-clinically infected horses may not show any clinical signs of disease.