Equine Infectious Anemia (EIA): Symptoms, Transmission, Diagnosis, and Legal Implications

What is Equine Infectious Anemia (EIA)?

Equine Infectious Anemia (EIA) is one of the most feared viral diseases in horses—not because of dramatic symptoms, but because it often spreads unnoticed. The disease is typically silent, highly infectious under certain conditions, lifelong, and currently incurable. A positive test result almost always leads to the same outcome: the affected horse must be permanently isolated or euthanised. Understandably, the fear of an outbreak in any equine facility is considerable.

Definition and differentiation from other types of anaemia

Anaemia in horses is a broad medical term referring to a decreased number of red blood cells or a lack of haemoglobin. It can result from harmless causes such as iron deficiency or parasites—but in the case of EIA, it is caused by a highly contagious retrovirus. While most anaemia types are treatable, infectious anaemia is a chronic viral condition that weakens the immune system and turns the horse into a lifelong carrier.

The virus: A retrovirus with serious consequences

EIA is caused by a retrovirus from the lentivirus family, similar to HIV in humans. It integrates itself into the horse’s DNA, making elimination impossible. The horse’s immune system attempts to fight the virus, resulting in cyclical flare-ups involving fever, weakness, and anaemia. During symptom-free periods, the horse may appear perfectly healthy—but remains highly infectious.

Transmission: How EIA spreads

The virus spreads exclusively via blood contact. Key transmission routes include:

• Bloodsucking insects such as horseflies and stable flies, which mechanically transmit the virus if they bite several horses in quick succession.
• Contaminated instruments, including needles, syringes, dental tools, and surgical equipment.
• Blood transfusions using untested donor animals.
• Vertical transmission during birth or via colostrum (rare).

Casual contact, such as mutual grooming, shared water troughs, or pasture time, does not spread the virus—significant blood exposure is required.

Is EIA notifiable or reportable?

Yes. Equine Infectious Anemia is a notifiable disease in the UK, and a reportable disease in the US. Any suspected or confirmed case must be reported immediately:

• UK: To the Animal and Plant Health Agency (APHA) – gov.uk
• US: To the USDA APHIS and the relevant State Veterinarian – aphis.usda.gov

A positive result typically triggers quarantine, movement restrictions, testing of all contact horses, and often euthanasia.

Classification among equine infectious diseases

EIA is classified alongside other serious equine diseases such as glanders, African horse sickness, and contagious equine metritis. Horses must test negative via Coggins test to be legally exported, moved interstate (in the US), or entered into events and sales.

How can a horse become infected with EIA?

Equine Infectious Anemia (EIA) is not highly contagious through casual contact, but under the right circumstances, it can spread extremely efficiently. The virus is transmitted exclusively through blood, not via respiratory droplets, saliva, or skin contact.

Common routes of infection

• Bloodsucking insects
  Horseflies, stable flies, and mosquitoes are the primary vectors. They feed on the blood of an infected horse and can mechanically transmit the virus if they bite another horse shortly afterward.
• Contaminated veterinary or dental instruments
  Even microscopic traces of blood on reused needles, syringes, dental floats, or surgical tools can transmit the virus.
• Blood transfusions
  If donor animals are not tested for EIA beforehand, transfusions carry a high risk of infection.
• From dam to foal (rare)
  Transmission can occur during birth or through infected colostrum if the mare is a carrier.

Direct contact between horses—such as mutual grooming or turnout in shared paddocks—does not pose a transmission risk, since blood exposure is required.

Symptoms: How does EIA manifest in horses?

The symptoms of Equine Infectious Anemia (EIA) are as diverse as they are deceptive. Some horses exhibit clear signs of illness, while others appear completely healthy for extended periods. These inapparent carriers are particularly dangerous because they can spread the virus without any outward symptoms.

Acute phase: When the virus becomes visible

During an acute episode, the following symptoms are commonly observed:

• High fever (over 40°C / 104°F)
• Loss of appetite
• Fatigue and general weakness
• Swelling (edema), particularly around the belly and limbs
• Bleeding from mucous membranes
• Yellowing of the eyes or gums (jaundice)
• Rapid heart rate (tachycardia)
• Anaemia, visible through pale mucous membranes

This stage may last from several days to a few weeks. Without targeted testing, these signs are often mistaken for colic, viral infections, or metabolic imbalances.

Chronic phase: Persistent but subtle

After the acute phase, EIA often enters a chronic or recurrent stage, with symptoms such as:

• Recurrent fever
• Lethargy
• Progressive weight loss
• Recurring anaemia

In this phase, horses may appear to recover intermittently, but the virus remains active and the animal stays infectious.

Inapparent form: The hidden carriers

Roughly one-third of infected horses show no clinical signs at all. These inapparent carriers are the biggest threat to herds, especially in situations involving horse sales, events, or transport. Without routine testing, these horses can silently spread the virus across stables and borders.

Distinguishing EIA from other causes of anaemia

Mild anaemia in horses can be caused by iron deficiency, chronic inflammation, parasites, or blood loss. Unlike EIA, these conditions are typically treatable. This makes thorough differential diagnosis essential—especially when signs like poor performance or pale mucous membranes occur.

Diagnosis: How is EIA detected?

Because Equine Infectious Anemia often presents with vague or no clinical signs, accurate diagnosis is essential—not only for the infected horse, but to protect the wider equine population. Only laboratory testing can confirm infection.

Blood test results: Helpful but not definitive

A standard blood profile might reveal clues that suggest EIA, such as:

• Low red blood cell count (anaemia)
• Abnormal white blood cell counts
• Low platelet count (thrombocytopenia)
• Signs of systemic inflammation

However, these findings are not specific to EIA. They merely indicate that something is wrong—further testing is needed for confirmation.

The Coggins Test: The gold standard for EIA

Which method is most commonly used to diagnose equine infectious anemia? The most reliable test for diagnosing EIA is the Coggins test, an Agar Gel Immunodiffusion (AGID) test. It detects antibodies to the EIA virus and is recognised internationally—legally required in both the UK and US for:

• Participation in shows or events
• Interstate or international transport
• Pre-sale health checks
• Routine surveillance in some regions

When is testing required?

Testing is mandatory or highly recommended in the following cases:

• Importation of horses from outside the EU/UK/US
• Entry into shows, events, or breeding programs
• Purchase of a new horse, especially from unknown origins
• Investigation of unexplained illness or suspected outbreaks

Some owners also request voluntary screening for all new arrivals at a yard or clinic to prevent potential spread.

Other diagnostic methods

In addition to the Coggins test, ELISA tests and PCR assays may be used to support diagnosis or for faster screening. However, only the Coggins test is typically accepted for official purposes, such as regulatory compliance and cross-border movement.

Treatment: What happens after a positive EIA result?

A positive test for Equine Infectious Anemia is devastating news for any horse owner—not only because there is no cure, but also due to the legal and ethical implications. EIA cannot be treated or eliminated, and infected horses remain a lifelong risk to others.

Can EIA be treated?

In short: no. There is no antiviral medication or vaccine that cures EIA. While supportive care may temporarily ease symptoms like fever or swelling, the virus persists in the horse's body.

How is equine infectious anemia treated?
With supportive care only—but in most countries, euthanasia or permanent isolation is required by law. Permanent Isolation for horses is not a long term solution, which is why euthanasia is the always the chosen measure.

What happens to infected horses?

Because infected horses are permanent virus carriers, the response to a positive result typically includes:

• Immediate quarantine of the affected animal
• Restriction of movement for the entire stable
• Testing of all in-contact horses
• Euthanasia, in most cases

In very rare exceptions, infected horses may be kept in strict lifelong isolation, subject to government approval and strict biosecurity.

Can a horse survive EIA?

Yes—clinically, some horses live for years with the virus, especially if asymptomatic. But from a disease control perspective, they are considered a permanent threat to other horses and the industry at large. That’s why euthanasia is the most common outcome.

What should owners do after a positive test?

• Notify authorities immediately (APHA in the UK, USDA in the US)
• Isolate the infected horse with no contact to others
• Cooperate with tracing and testing of all contact animals
• Cease participation in all equine events or sales involving the affected horse or its stablemates

Prevention: How can horses be protected from EIA?

Since Equine Infectious Anemia is incurable, the only effective defence is prevention. This means rigorous biosecurity, vector control, and consistent testing protocols, especially when introducing new horses or travelling.

Is there a vaccine for EIA?

No. There is currently no licensed vaccine for EIA in either the UK or the United States. The virus mutates frequently and integrates into the horse's DNA, making vaccine development extremely challenging. Additionally, vaccinated horses would test positive in the Coggins test, making disease surveillance and regulation impossible.

Hygiene: Everyday protection in the stable

Strong hygiene protocols significantly reduce the risk of transmission—not just for EIA, but for many other diseases. Key practices include:

• Always use sterile, single-use needles and syringes
• Never share dental tools, thermometers, or grooming kits between horses
• Ensure thorough disinfection of any surgical or medical instruments
• Use EIA-tested donor horses for transfusions only

Testing and travel protocols

To prevent disease spread across regions or borders, regular testing is essential:

• Coggins tests are required for horse shows, sales, or interstate/international transport (especially in the US)
• In the UK, testing is recommended before introducing a new horse to a stable or yard
• Any horse with unknown history should be tested before integration
• Remember: horses may appear healthy while still carrying the virus

Recent EIA Cases in the UK, Europe, and USA

Though Equine Infectious Anemia is considered rare in developed countries, recent outbreaks serve as reminders that the disease is still very much a threat—especially in the context of international movement, clinic visits, and insufficient vector control.

Germany (2025): Therapy pony diagnosed in Baden-Württemberg

In August 2025, a therapy pony at a riding centre in Neustetten-Wolfenhausen (Baden-Württemberg) was diagnosed with EIA after being taken to a veterinary clinic for colic-like symptoms. Blood tests confirmed the infection. The pony was euthanised in accordance with German animal health laws, and the entire facility was placed under a 90-day quarantine.

Local veterinary authorities began extensive testing of all horses on the premises, as well as those at potentially connected locations. The owner reported emotional and financial hardship, as riding lessons and therapy services had to be suspended, while costs for care, feed, and veterinary monitoring continued.

Netherlands (2025): Asymptomatic case

In March 2025, a horse in the Netherlands tested positive for EIA without showing any symptoms. The result was confirmed through both ELISA and Coggins testing. The animal was euthanised, and its contacts were placed under observation and testing. Authorities suspect insect transmission or a prior import as the source.

United States (2025): 21 confirmed cases across multiple states

The US recorded 21 EIA cases in early 2025, notably in Texas, California, Colorado, and Oklahoma. Several of these were traced back to a veterinary clinic, where improperly sterilised equipment may have facilitated the spread. This cluster demonstrates that iatrogenic (human-caused) infections can still occur, even in regulated environments.

State animal health officials responded with quarantine orders, contact tracing, and public awareness campaigns to prevent further spread.

Conclusion: EIA – rare, but a serious threat

Equine Infectious Anemia (EIA) is a chronic, incurable viral disease that—though rare—can devastate individual horses, entire facilities, and the broader equine community. Unlike treatable forms of anaemia, EIA turns horses into lifelong virus carriers. Because many infected animals show no symptoms, the disease often spreads silently—especially via insect vectors or contaminated instruments.

Recent outbreaks in Germany, the Netherlands, and the USA highlight the ongoing risk, particularly in connection with international transport, clinic visits, and inadequate biosecurity.

Preventing EIA comes down to three key strategies:

1. Routine testing (especially before sales, imports, or events)
2. Strict hygiene and use of sterile equipment
3. Effective insect control

Ultimately, protecting your horse means protecting the entire herd. Vigilance isn’t optional—it’s essential.

FAQ: Equine Infectious Anemia (EIA)

What is Equine Infectious Anemia?

EIA is a viral disease of horses, donkeys, and mules caused by a lentivirus. It spreads via blood and leads to chronic infection. There is no cure, and it is a notifiable disease in the UK and a reportable disease in the US.

Is EIA the same as anaemia in horses?

No. EIA is a contagious viral form of anaemia, whereas most other types (such as those caused by iron deficiency or parasites) are non-contagious and treatable.

What are the symptoms of EIA?

Typical signs include fever, weakness, weight loss, swelling (edema), pale mucous membranes, jaundice, and sometimes bleeding. However, many horses remain asymptomatic carriers.

How is EIA transmitted?

EIA is transmitted only through blood, primarily via biting insects (like horseflies), contaminated veterinary instruments, or blood transfusions. It is not spread through casual contact.

How is it diagnosed?

The Coggins test (AGID) is the international gold standard. ELISA and PCR tests may be used in some cases. Testing is mandatory for import/export, competition entry, and some sales.

What happens if a horse tests positive?

In most cases, the horse must be euthanised. In rare instances, lifelong isolation under strict biosecurity may be allowed—but this depends on local regulations.

Can EIA be cured?

No. There is no known treatment to eliminate the virus from an infected horse.

Is there a vaccine for EIA?

Currently, no vaccine is available. The virus mutates rapidly and integrates into the host’s DNA, making vaccination both difficult and problematic for diagnostic testing.

What can I do to protect my horse?

• Test all new arrivals to your yard or stable
• Use only sterile, single-use equipment
• Maintain strict fly control measures
• Avoid exposure to untested horses at shows or clinics