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Equine Infectious Anemia (EIA)

EIA is an infectious and potentially fatal viral disease of members of the horse family. The equine infectious anemia virus (EIAv) is categorized as a retrovirus: it contains genetic RNA material, which it uses to produce DNA. The DNA is then incorporated into the genetic makeup of infected cells. There is no vaccine or treatment for the disease. It is often difficult to differentiate from other fever-producing diseases, including anthrax, influenza, and equine encephalitis.

Clinical Forms

Acute—When horses are exposed to EIAv, they may develop severe, acute signs of disease and die within 2 to 3 weeks. This form of the disease is the most damaging and the most difficult to diagnose because the signs appear rapidly, and often only an elevated body temperature is noted. One-fifth of a teaspoon of blood from a horse with acute EIA contains enough virus to infect a million horses. Clinical signs of acute infectiousness are rather nonspecific; in mild cases the initial fever may be short lived (often less than 24 hours). Horse owners and veterinarians may not observe this initial response when a horse is infected with EIAv. Horses often recover and continue to move freely in the population. The first indication that a horse was exposed to, and infected with, EIAv may be a positive result on a routine annual test.
Chronic—If the horse survives the first acute bout, it may develop a recurring clinical disease with most or all of the following signs:

  • Fever: An infected horse’s temperature may rise suddenly to 40.5 °C (105 °F) or, rarely, as high as 42.2 °C (108 °F) but may then drop back to normal for an indeterminate period until the onset of another episode.
  • Petechial hemorrhages: Minute blood-colored spots appear on the mucous membranes.
  • Depression: A horse appears more or less dejected (head hangs low) and generally listless.
  • Weight loss: A horse may refuse feed or may eat an inordinate amount but still continue an obvious decline from normal weight.
  • Dependent edema: A horse may develop swelling, which is evidence of fluid collecting under the skin in the legs and under the chest and other underbody surfaces.
  • Anemia: The horse’s blood may manifest a marked drop in its red corpuscle count and appear thin and watery. The animal may also have an irregular heartbeat, and a jugular pulse may become evident. 

Inapparent—The majority of horses are inapparent carriers: they show no overt clinical abnormalities as a result of infection. These animals survive as reservoirs of the infection for extended periods. Inapparent carriers have dramatically lower concentrations of EIAv in their blood than horses with active clinical signs of the disease. Only 1 horse fly out of 6 million is likely to pick up and transmit EIAv from such horses. All horses infected with EIAv are thought to remain virus carriers for life. The inapparent form may become chronic or acute owing to severe stress, hard work, or the presence of other diseases.


EIA is a classic blood-borne infection. People have played an important role in EIAv transmission over the years by using blood-contaminated materials on different horses. But the EIAv is most often transmitted between horses in close proximity by large biting insects such as horse flies or deer flies. The bites from these flies stimulate defensive movement by the horse, which often results in an interruption of the bloodfeeding. When interrupted, the fly is motivated to complete the feeding as soon as possible. It then attacks the same or a second host and feeds to repletion. Any infective material from the blood of the first host present on the mouthparts of the insect can be mechanically transmitted to the second host. Insect transmission of EIAv is dependent on the number and habit of the insects, the density of the horse population, the number of times the insect bites the same and other horses, the amount of blood transferred between horses, and the level of virus obtained in the blood meal.


The AGID, or Coggins, test has been shown to correlate with horse inoculation test results for EIAv and therefore can be used to identify EIAv carriers. Although other serologic tests have been defined and approved for the diagnosis of EIA, the AGID test is recognized internationally as the “gold standard” serologic test. The use of AGID and additional tests has assisted in the control of EIA. Presently, USDA recognizes the AGID and a number of ELISA formats for conducting official tests.

Controlling the spread of EIAv involves minimizing or eliminating contact of healthy horses with the secretions, excretions, and blood of EIAv-infected horses. Once the reservoirs of EIAv are identified, separated, and maintained a safe distance from the other horses, the transmission of EIAv is broken. Until all horses are tested, precautions should be taken to prevent commingling with horses that do not originate from test-negative farms or that have been exposed to test-positive horses. All diagnostic laboratories are required to report positive test results to Federal and State authorities for appropriate action.

When an equine has a positive result on an official test for EIA, the animal must be placed under quarantine within 24 hours after positive test results are known in order to permit confirmation testing and to prevent further exposure of other equines. The equine must remain in quarantine until final classification and disposition are made.

All exposed equines (either individual or within a herd) within 200 yards of the location where a reactor equine is or was maintained must also be placed under quarantine. The quarantine area must provide no less than 200 yards of separation from all other equines. The quarantine area and the quarantined equines therein must be monitored periodically by regulatory personnel to ensure that provisions of the quarantine are not being violated. Additional information regarding the control program may be obtained by contacting your local APHIS –VS District Office.

Additional information may be found at the EIA web site

The most current version of the EIA Uniform Methods and Rules may be found at: EIA Uniform Methods and Rules

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