Equine infectious anaemia

What it is

Equine infectious anaemia (EIA, also called swamp fever) is caused by a lentivirus of the family Retroviridae, related to human immunodeficiency virus. The virus multiplies in tissue macrophages and, like all retroviruses, integrates into the genome of infected cells: infection lasts for life.

It is transmitted by blood. Horse flies and, to a lesser extent, stable flies (Stomoxys) carry blood on their mouthparts when their feeding is interrupted and they move on to bite another nearby horse; the insect is not infected, it only carries the virus (mechanical transmission). Shared needles and surgical equipment and transfusions of contaminated blood or plasma also transmit it, and less often the transplacental route.

Infection has three forms that can follow one another in the same horse:

• Acute: fever and thrombocytopenia in the weeks after infection; it often goes unnoticed.
• Chronic: repeated episodes of fever, anaemia, oedema and weight loss, each with a new viral variant that escapes antibodies.
• Inapparent carrier: most infected horses control the virus and have no signs, but remain infectious, especially if stress or corticosteroids lower their immunity.

The anaemia is mainly immune-mediated: red cells coated with virus, antibody and complement are removed in the spleen and liver, and bone marrow production is also reduced. It is a WOAH-listed, notifiable disease.

Who it affects

Horses, donkeys and mules. It is endemic in parts of the Americas, especially in wet areas of South America, and occurs in some European countries. Risk is highest in areas with many horse flies, near wetlands, in working horses kept in groups and where needles are reused or blood products shared.

Anatomy involved

• Spleen: rich in macrophages, it is one of the main sites of virus replication and where antibody-coated red cells are destroyed. Acute and chronic episodes cause splenomegaly, sometimes palpable per rectum, with lymphoid hyperplasia.
• Liver: the liver macrophages (Kupffer cells) also harbour the virus, phagocytose red cells and fill with haemosiderin. There is hepatomegaly with lymphocytic infiltrates in the portal areas.

Clinical signs

• Intermittent fever, depression and inappetence.
• Anaemia, with pale mucous membranes, in the chronic form.
• Petechiae on the mucous membranes, especially under the tongue, from thrombocytopenia.
• Ventral and limb oedema, progressive weight loss.
• Many horses have no signs at all and are only detected by testing.

Diagnosis

• Agar gel immunodiffusion (Coggins test): the WOAH reference test, highly specific.
• ELISA, more sensitive, for screening; positives are confirmed by immunodiffusion and, if the results disagree, by immunoblot.
• Antibodies take weeks to appear, so a negative result shortly after exposure must be repeated.
• PCR in particular cases: very recent infections or foals with maternal antibodies.
• Bloodwork: thrombocytopenia, anaemia and increased globulins.

Treatment and prevention

• There is no treatment. Depending on each country's rules, positive horses are culled or kept isolated for life, at a distance from other equids and protected from insects.
• There is no vaccine in general use.
• Mandatory serological testing before movements, sales, competitions and exports, and active surveillance of the population in risk areas.
• Prevention of iatrogenic transmission: single-use needles and syringes, disinfection of equipment and testing of blood and plasma donors.
• Insect control: repellents, stabling during peak horse fly activity and keeping horses of unknown status apart.

Key studies

The evidence on EIA comes from surveillance and observational studies: there are no trials of treatments or licensed vaccines to assess.

• Carvelli et al. (2024), analysis of mandatory active surveillance of Italy's entire equid population (about 406,000 animals) from 2007 to 2012: 1,337,899 samples, of which 0.18% were positive. Seroprevalence fell in all species over the period; mules had a much higher infection rate than horses (IRR 48.90) and donkeys a lower one (IRR 0.56), and holding-level prevalence was 1.15%. The authors conclude that passive surveillance alone is not enough to eradicate it. It is an observational study without a comparison group.
• Scicluna et al. (2013), evaluation of Italy's three-tier diagnostic system (ELISA, immunodiffusion and immunoblot) on 74,880 samples. 44 were negative by immunodiffusion but positive by ELISA and immunoblot, most of them from mules. In a follow-up study of 10 mules, viral loads in animals with negative or weak immunodiffusion results were equivalent to those of clearly positive animals. Using immunodiffusion alone can miss infected equids. The follow-up included few animals.
• de Pádua et al. (2022), cross-sectional study of 1,170 equids on 332 randomly selected farms in Goiás (Brazil). 2% of equids and 3.1% of farms were positive by immunodiffusion. Using a shared vaccination gun and having bodies of water on the farm were risk factors. Only immunodiffusion was used, which may underestimate prevalence.
• Borges et al. (2013), cross-sectional study of 547 equids in the Brazilian Pantanal. Adjusted seroprevalence was 31.5% (95% CI 17.4–48.8%) and 52.0% of farms had at least one positive animal. Animals in regularly flooded areas were 146 times more likely to be positive, and those with clinical signs 3.74 times more likely. It is a very particular area, with few farms (25).