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Nature of the disease
Equine infectious anaemia (EIA or Swamp Fever) is a debilitating viral disease of horses. It is caused by a virus of the Lentivirinae subfamily (Retroviridae family) which is closed to the human immunodeficiency virus (HIV).
OIE List B disease
Susceptible species
Horses and other equids. Donkeys are less severely affected than horses.
EIA is present in many countries, but is often confined to specific regions and has a variable prevalence and severity. In Australia, the only area where EIA can be regarded as endemic is along the inland river systems of central and western Queensland. In new Zealand it has not been recorded since 1999.
Clinical signs 
Whilst the disease may manifest initially in an acute form with the possibility of some early deaths, EIA is generally a chronic disease characterised by:
  • Progressive loss of condition
  • Muscle weakness and poor stamina
  • Rough coat
  • Depression
  • Anaemia.

There may be intermittent acute episodes in which affected animals show fever, severe depression, anorexia, ataxia, conjunctival congestion, oculo-nasal discharges, jaundice, diarrhoea, and dependent oedema of the head, trunk and limbs. Death may occur during an acute episode.

Sub-clinical and mild infections are common.

Post-mortem findings 
In horses dying of the disease there is:
  • Haemorrhages in the liver, spleen, kidneys, on serous membranes and in the mucosa of the intestines
  • Liver is enlarged and often yellowish-brown in colour
  • Spleen is enlarged with soft pulp
  • Heart muscle is pale, flabby and friable
Differential diagnosis 
Equine infectious anaemia can be difficult to diagnose. The following possibilities may need to be considered:
  • Purpura haemorrhagica
  • Babesiosis
  • Leptospirosis
  • Severe strongyliasis or fascioliasis
  • Phenothiazine toxicity
  • Auto-immune haemolytic anaemia
Specimens required for diagnosis 
In typical cases of acute and sub-acute disease, EIA can be suspected on clinical grounds.
Blood samples should be collected to confirm the diagnosis using the Coggins (agar gel diffusion) test or the more recent ELISA test.
EIA virus is transmitted by Tabanidae other biting flies and some mosquitoes, it is a mechanical transmission and the virus survives less than 4 hours in the insect.

In endemic countries the prevalence of EIA is often highest in areas that are low-lying, swampy and humid — hence the alternative name, swamp fever.

Iatraogenic transmission is also possible during any manipulation that may involve the transfer of infected blood or saliva, for example through the use of unsterilised instruments or equipment.

Vertical transmission (in utero or through the milk) is also possible.

Risk of introduction   
As recovered horses continue to carry the virus, the most likely source of introduction of EIA is through importation of horses. OIE guidelines suggest that, when importing horses, as well as showing no clinical signs, they should be tested negative during the 30 days prior to shipment.
Control / vaccines  
No specific treatment is available. Supportive treatment, including blood transfusions can be considered. However, because recovered horses can become carriers, if the disease occurs in a previously free area it is generally better to destroy affected horses.

In endemic areas, risk of infection can be reduced by protecting horses from insect vectors where practicable. For example:

  • Keeping horses away from low lying areas
  • Draining swamps
  • Use of insecticides
  • Insect-proof stabling
  • Equine Infectious Anemia, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 499-500
  • Equine Infectious Anemia, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 940-943
  • Office International des Epizooties, 2002