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Equine Infectious Anaemia

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Equine Infectious Anaemia or “swamp fever” is a viral disease of horses, mules and donkeys.

Background to the disease

Although not necessarily fatal, recovered animals become carriers of the disease and can infect other horses.  It is usually transmitted through blood-sucking insects but can be transmitted through the use of contaminated blood or blood products, instruments or needles.  

Pregnant mares can pass on this disease to their foals whilst in the womb. The disease may occur where there are large numbers of horseflies in proximity to acutely affected horses, usually during periods of high insect activity, in low-lying swampy areas which are close to woodlands.

The disease does not commonly occur in Great Britain but it is present in other parts of the world. The disease has been reported in many parts of America, Asia, Europe and Australia. Two cases of Equine Indectious Anaemia (EIA) were detected in England in 2012.

Signs of the disease

Following the first exposure to the virus, acute cases experience fever and bleeding 7-30 days post infection. Very few horses with this initial fever are detected by owners. The more classical case of EIA is that of a chronically infected horse. These animals experience episodes of fever, weight loss, depression, progressive weakness and swelling.

These signs occur every two weeks in recurring cycles. Other symptoms which may occur during the course of the disease include:

  • loss of appetite
  • frequent urination
  • diarrhoea
  • weakness
  • paralysis of the hindquarters
  • paleness of the mucous membranes
  • yellowish discoloration of the eyes
  • small pinpoint bleeding beneath the tongue
  • rapid breathing and accelerated pulse
  • pregnant mares may abort.

Most horses that recover clinically will still be carriers of the virus. Horses can relapse and show signs of the chronic form.

The onset of these signs is often associated with stresses such as hard work, hot weather, racing, pregnancy or use of steroid drugs. The most difficult animals to identify are the unapparent carriers. These horses show no clinical signs associated with the infection and go undetected unless subjected to a blood test.


There is no known treatment that can eliminate the virus from the body. To date there is no satisfactory vaccine for EIA. Research work continues on attempting to produce suitable vaccines. The key to prevention is the identification and restriction of infected horses.


This is a Notifiable Disease. If you suspect signs of Equine Indectious Anaemia (EIA), you must immediately notify your local Animal and Plant Health Agency office.

Related Links

The Department of Environment, Food and Rural Affairs (DEFRA) - Equine Infectious Anaemia (external link).