Clinical Presentation

The clinical picture depends on the stage of infection and the individual animal. Fever is often one of the first findings. Other signs may include weakness, reduced appetite, weight loss and anemia. Thrombocytopenia is also common during clinical episodes.

In more severe cases, horses may develop jaundice, petechial hemorrhages, edema and muscle wasting. Some animals experience repeated episodes of fever and illness, while others progress to an inapparent carrier state.

This variation can make diagnosis based on clinical observation alone difficult. An apparently healthy horse may still have a history of EIAV infection and remain a potential source of infection for other horses.

How EIAV Spreads

EIAV is transmitted mainly through infected blood. In the field, horse flies and deer flies are important mechanical vectors. When a blood-feeding insect moves between horses, infected blood can be transferred from one animal to another.

Transmission does not depend entirely on insects. Contaminated needles, syringes and veterinary equipment can also transmit the virus. Blood transfusion is another recognized route. These routes are especially relevant in places where horses are handled frequently or where equipment is shared between animals.

The risk is affected by local conditions. Areas with large horse populations, substantial insect activity and frequent movement of animals may present different epidemiological conditions from regions where horse populations are smaller or surveillance is more established.

Available studies illustrate this variation. A study conducted in Goiás, Brazil, tested 1,170 equids from 332 farms and identified 14 seropositive animals, giving an overall seroprevalence of approximately 2.0%. In the United States, the situation is quite different at the population level. USDA APHIS reported 1,292,655 EIA tests in 2024, with 147 positive horses from 63 premises. The estimated prevalence in the U.S. horse population was approximately 0.004%.

These numbers should not be interpreted as a direct comparison of disease risk between the two countries, since surveillance systems, study design and the populations tested are different. They do, however, demonstrate that EIA prevalence can vary substantially according to geographic area and horse population.

The Role of Antibody Testing

Antibody testing has an important place in EIA surveillance because infected horses can remain carriers even after clinical signs have disappeared.

Following EIAV infection, horses develop antibodies that can be detected by serological methods. The traditional diagnostic method is the agar gel immunodiffusion assay, commonly known as the Coggins test. ELISA-based assays are also used for antibody detection and screening.

Rapid antibody tests use the same basic principle of identifying antibodies generated in response to EIAV infection, but in a format designed for a shorter and simpler testing process. Depending on the product and its validated intended use, a rapid test can be useful when a veterinarian needs an initial assessment without waiting for a conventional laboratory workflow.

This can be relevant for horses with an unknown testing history, animals newly introduced to a farm, or horses being screened before entering a group. A rapid result can also make it easier to decide whether a sample should be submitted for further laboratory investigation.

Practical Uses in Equine Management

EIA testing is relevant wherever horses from different sources are brought together.

At breeding farms, testing newly introduced animals can form part of the farm's health-screening program. Quarantine facilities may also use EIA testing when evaluating horses with an unknown health status.

Horse movement is another important consideration. Horses transported between farms, competitions and other facilities may have different levels of exposure to infectious diseases. In some jurisdictions, EIA testing is required for particular types of movement or certification.

Veterinary clinics may consider EIA when investigating horses with unexplained fever, anemia, thrombocytopenia or recurring illness. Testing may also be relevant when assessing equine blood donors because EIAV can be transmitted through contaminated blood.

For routine screening, a rapid antibody test has a practical advantage in that testing can be carried out close to the animal and does not necessarily require laboratory equipment. Results obtained within a short period can support immediate decisions about isolation, further testing or management of the horse.

Why Surveillance Remains Necessary

The difficulty with EIA is not only the disease itself, but the possibility of infection remaining unnoticed. Clinical signs may disappear, while the infected horse continues to carry EIAV.

For horse owners, veterinarians and facility managers, regular testing is therefore more useful than relying on clinical appearance alone. The appropriate testing interval depends on the disease situation, animal movement, local regulations and the risk profile of the population.

Rapid EIAV antibody testing can be used as part of this process, particularly for convenient screening and preliminary assessment. When combined with confirmatory laboratory testing and appropriate biosecurity measures, it provides another practical tool for managing the risk associated with equine infectious anemia.

References

World Organisation for Animal Health (WOAH). Equine Infectious Anaemia.

Merck Veterinary Manual. Equine Infectious Anemia.

USDA APHIS. Equine Infectious Anemia.

USDA APHIS. 2024 Equine Infectious Anemia Cases in the United States. 2025.

Preventive Veterinary Medicine. Seroprevalence and risk factors associated with equine infectious anemia in the state of Goiás, Brazil. 2022.