Equine Infectious Anaemia (EIA) or "swamp fever” is a virus disease of horses, mules and donkeys causing intermittent fever, anaemia, emaciation and death.

If notifiable disease is suspected, contact the DAERA Helpline on 0300 200 7840 or your local DAERA Direct Regional Office. Failure to do so is an offence

What is swamp fever?

Although it is not necessarily fatal, recovered animals become carriers of the disease and can infect other horses.  It can be transmitted by mechanical transfer of blood by biting insects and occurs typically in low-lying swampy areas. The disease was first described in France in 1843. The term "swamp fever" was first used by Torrance in 1903 in Canada, where this disease had been known before 1882.

This is a notifiable disease and DAERA must be notified of any suspicion of EIA.

Clinical signs

EIA is a viral disease of horses transmitted by an insect vector, in this case by the mechanical transfer of infected blood from animal to animal by biting flies, usually involving clegs/horse flies (Tabanids).

The virus can also be spread by contaminated blood products and contaminated equipment. Transplacental spread also occurs.  The disease is found worldwide. In Europe, EIA has been reported in France, Ireland, Germany, Greece and appears to be endemic in Romania and increasingly common in Italy.

Disease may be acute, chronic or subclinical in horses infected with the same strain of the virus.

The incubation period is usually 1-3 weeks but can range from a few days to a few months.

In acute disease clinical signs include: pyrexia, depression, ataxia, rapid weight loss and occasionally haemorrhage. Disease may be fatal within days with no sign of anaemia.

In chronic cases recurring cycles of anaemia, oedema, weight loss and lethargy are seen. An enlarge spleen may be detected on rectal examination. Mares may abort. Persistent replication of virus within the host leads to the periodical emergence of new strains which vary antigenically and are frequently resistant to the neutralising antibodies circulating in the horse causing a recrudescence of clinical signs which usually decrease in severity over time.

Subclinical disease resulting in seropositive asymptomatic carriers.

The differential diagnosis includes other febrile illnesses including equine viral arteritis, purpura hemorrhagica, leptospirosis, babesiosis, severe strongyliosis.

Further information on EIA

How is it transmitted?

The virus is usually transmitted mechanically, most commonly through blood-sucking insects (Tababus or Stomoxys species) or through the use of contaminated blood or blood products, instruments or needles. Transmission of the disease may occur where there are large numbers of horseflies in proximity to acutely affected horses and occurs most often during periods of high insect activity, in low-lying swampy areas close to woodlands.

Contaminated needles and blood products have also been implicated in the infection of horses, and transmission via colostrums or semen is uncommon. Pregnant mares may pass the disease to their foals in the womb.

Is there any risk to humans?


How widespread is this disease?

Although the infectious nature of this disease had long been suspected, a viral case was not definitely established until 1904. The virus was classified as a retrovirus in 1976 and later grouped with the lentiviruses (that is, non-tumour forming exogenous retroviruses) of which the human AIDS virus is also a member.

EIA has a worldwide distribution. Early in the twentieth century serous outbreaks occurred in France, Japan and America. The disease has been reported in many parts of America, Asia (India, Malaysia, Myanmar, Philippines, Thailand), Europe (Austria, France, Greece, Italy, Romania, USSR, Yugoslavia) and Australia. The last case in Northern Ireland was in 2006.

How long does it take for the disease to become apparent?

The incubation period is variable, from a matter of days to a few months but generally 1 to 3 weeks. Antibodies usually, but not always, develop 7 to 14 days after infection and last for life. Horses have been known to take several months before clinical signs or antibodies are apparent.

Is it notifiable?

Yes. DAERA must be notified of any suspicion of this disease.

Are restrictions placed on suspected infected or exposed to infection horses?

Yes, suspect horses are tested and placed in quarantine for a minimum of 60 days post exposure. This quarantine requires the separation of suspect horses from other horses, use of vector control methods such as fly repellents, and insect proof barriers. Should this initial test prove negative, further testing is carried out over the course of this quarantine period. If further tests prove negative restrictions are removed. If further testing proves positive, the affected animal would be euthanased and further testing of at risk horses carried out to detect any disease spread.

What are the EIA health requirements for moving horses between Member States of the European Union?

In order to move between EU Member States, horses must originate from premises which are not subject to prohibition for animal health reasons, must not have had contact with equidae from premises subject to prohibition. In the case of EIA the prohibition period lasts until the date on which, after slaughter of the infected animals the remaining animals at the same premises have had two negative tests for EIA carried out three months apart

What are the symptoms?

These depend upon the stage of the disease. Haemorrhages, oedema and jaundice occur in the acute disease: enlargement of the liver, spleen and lymph nodes are common in the chronic stage.

More useful links

Back to top