Other Names
- Swamp Fever
- EIAV (Equine Infectious Anaemia Virus) infection
- Coggins disease (named after the diagnostic test)
What is Equine Infectious Anaemia (EIA)?
Equine infectious anaemia (EIA) is a serious viral disease affecting horses, ponies, donkeys, mules, and zebras. It is caused by the equine infectious anaemia virus (EIAV), a retrovirus closely related to HIV in humans. EIA is a notifiable disease in the UK — any suspected or confirmed case must be reported to the Animal and Plant Health Agency (APHA) immediately.
Once a horse is infected with EIAV, the virus persists for life. There is no cure. The horse becomes a permanent source of infection, and current UK legislation requires that confirmed positive horses either be permanently isolated (a minimum of 200 metres from all other equids) or euthanised. This makes EIA not just a health issue but a significant legal and management concern for horse owners.
The disease is relatively rare in the UK compared to parts of Europe, North America, and South America, but sporadic cases occur — often associated with imported horses — and awareness is important.
What Causes EIA and How Does it Spread?
EIAV is a lentivirus (slow-acting retrovirus) that integrates into host immune cells and establishes a permanent infection. Transmission occurs through transfer of blood or blood-containing secretions:
- Biting insects (mechanical vectors): Horseflies (Tabanidae) and deerflies are the most important transmission route. Unlike biological vectors, these insects transmit the virus mechanically — they take a blood meal from an infected horse and, if interrupted, may move to an uninfected horse and immediately resume feeding, transferring virus-laden blood. The virus does not replicate in the insect. Stable flies are less efficient vectors. Transmission risk is directly related to insect population density and proximity of infected and uninfected horses.
- Contaminated equipment: Needles, syringes, blood transfusion equipment, and surgical instruments contaminated with infected blood are efficient transmission routes. Reuse of needles between horses — even briefly — is sufficient to transmit the virus. This was a historically significant route in working horse populations and in poorly-resourced veterinary settings.
- In-utero and perinatal transmission: Infected mares can transmit EIAV to foals in utero or, less commonly, through colostrum. Infected foals may be born apparently healthy but become carriers.
- Semen: Transmission through semen from infected stallions has been documented, though is less common than the above routes.
Clinical Signs and Phases of EIA
EIA is highly variable in clinical expression. Many infected horses, particularly those with inapparent infections, show no obvious signs for extended periods — this is why testing is so important and why the disease can spread silently within a population.
Acute Phase
Occurs 1–4 weeks after infection:
- High fever (often the first sign)
- Depression and weakness
- Loss of appetite
- Petechial haemorrhages on mucous membranes
- Rapid weight loss
- Oedema (swelling) of the lower abdomen and limbs
- Anaemia — due to destruction of red blood cells by the immune response
- Thrombocytopaenia (low platelet count) causing bleeding tendency
Some horses die in the acute phase. Others recover apparently, entering a chronic phase.
Chronic Phase
Recurrent episodes of fever, anaemia, weight loss, and depression interspersed with apparent normality. Episodes tend to be triggered by physiological stress (heavy exercise, intercurrent illness, pregnancy) or may occur spontaneously as the virus replicates in periodic bursts.
Inapparent (Carrier) Phase
Many horses stabilise into an apparently healthy state where they show no clinical signs. These are the most epidemiologically dangerous animals, as they are indistinguishable from uninfected horses without laboratory testing, yet carry virus and can transmit it indefinitely.
Diagnosis — The Coggins Test
The standard diagnostic test for EIA is the agar gel immunodiffusion (AGID) test, universally known as the “Coggins test” (named after Dr Leroy Coggins, who developed it in the 1970s). It detects antibodies against EIAV’s p26 protein in the horse’s blood.
The competitive ELISA (cELISA) is a more sensitive alternative, widely used in surveillance programmes. In the UK, both tests are used depending on laboratory and circumstance.
Key points about EIA testing:
- A negative test result is generally reliable for horses more than 45 days post-exposure
- Foals of positive mares will have maternally-derived antibodies for up to 6 months and may test positive without being truly infected — confirmatory testing must be delayed until maternal antibodies wane
- In the UK, a positive test result on an approved test in an APHA-approved laboratory is the trigger for legal action (notification, isolation, or euthanasia)
The Coggins test is routinely required for international movement of horses, entry to competitions in many countries, and is advisable when purchasing any horse with an unknown history.
Treatment
There is no treatment for EIA. No antiviral drugs are effective against EIAV in horses, and there is no licensed vaccine. Supportive care (fluids, anti-inflammatories, rest) can help manage acute episodes and improve the horse’s comfort, but the infection cannot be cleared.
The management of EIA is therefore entirely focused on:
- Early detection through testing
- Permanent isolation (minimum 200 metres from all other equids in the UK) or euthanasia of confirmed positive horses
- Preventing further transmission through insect control and biosecurity
Prevention
- Test before purchase: Any horse being purchased — particularly if imported or with an unknown history — should have a current negative Coggins test. This is standard practice in the USA and increasingly expected in the UK.
- Insect control: Reduce horsefly populations and exposure. Stabling horses during peak fly activity periods (particularly dawn and dusk in summer), using fly sheets and masks, applying repellents, and locating stables away from woodland and water where flies breed all help reduce transmission risk.
- Strict needle hygiene: Never reuse needles or syringes between horses. This is basic infection control that eliminates the iatrogenic transmission route entirely.
- Blood product safety: Only use EIA-tested blood donors. This is standard practice for any equine blood bank or transfusion service.
- Annual testing: Recommended for horses regularly travelling, competing abroad, or mixing with horses of unknown status.
Legal Requirements in the UK
EIA is a notifiable disease under the Animal Health Act 1981. Any suspicion of EIA must be reported to the APHA. Confirmed positive horses must be isolated or euthanised under official supervision. Horses in contact with a confirmed case may be subject to movement restrictions and testing. Failure to report is a legal offence.
When to Call the Vet
Contact your vet immediately if:
- A horse develops unexplained recurring fever, anaemia, weight loss, or limb oedema — particularly if recently imported or in contact with horses of unknown origin
- You have purchased a horse with no clear Coggins test history
- A horse that has travelled internationally shows signs consistent with EIA
- You suspect any route of blood transfer between a horse of unknown status and your horses
Frequently Asked Questions About EIA
How common is EIA in the UK?
EIA is relatively rare in the UK compared to other parts of the world, but it is not absent. Sporadic cases occur, frequently associated with importation of horses from continental Europe (particularly Eastern Europe and parts of the Mediterranean where prevalence is higher) or directly from endemic regions. The UK government’s APHA publishes annual reports on notifiable disease cases, which include EIA findings.
If a horse tests positive for EIA, does it have to be put down?
Not necessarily. In the UK, confirmed EIA-positive horses must be permanently isolated at a minimum of 200 metres from all other equids, or euthanised. The option to keep a positive horse alive under strict isolation conditions exists but is demanding and expensive long-term. Many owners choose humane euthanasia, particularly for horses that are symptomatic or where long-term isolation cannot be guaranteed. Your vet and the APHA will advise on options following a positive result.
Can EIA affect humans?
No. EIAV is a horse-specific retrovirus. Despite its relatedness to HIV, it does not infect humans. Horse handlers are not at risk of contracting EIA.
What is a Coggins test and how long is it valid?
A Coggins test is the standard blood test for EIA using the AGID method. In the USA, a Coggins test is typically required annually for interstate travel. For international movement to certain countries, a test within 30–90 days is commonly required. In the UK, there is no mandatory annual testing requirement for most horses, but a test is strongly recommended when purchasing horses with unknown backgrounds, particularly if imported.