Equine Infectious Anemia in Horses

Equine infectious anemia, commonly known as swamp fever, is a viral disease that attacks the horse's immune system. There are no cure and no vaccine for this viral infection, which is caused by a retrovirus closely related to the HIV virus in humans--the cause of AIDS. It's recommended by the American Association of Equine Practitioners that horses testing positive for EIA antibodies be euthanized humanely. Most horses exposed to the equine lentivirus, the virus that causes EIA, progress without outward symptoms of the disease.

EIA was first identified in France in 1843, and initially was diagnosed in the United States in 1888. The virus is found worldwide, and steps have been taken by authorities around the globe to control the spread of this dreaded disease. According to Chuck Issel, DVM, PhD, of the Gluck Equine Research Center in Lexington, Ky., when EIA was first able to be diagnosed, it was discovered that some farms and ranches in the United States had a 100% infection rate, while other farms and ranches had a zero percent infection rate. Even today, Issel said, "There are populations of horses found each year with an EIA infection rate approaching 100%."

In 1970 Leroy Coggins, DVM, developed the first reliable test to diagnose EIA. In an effort to control the spread of EIA, and initially in hopes of totally wiping out the disease in the United States, the USDA, along with state regulatory bodies, recommended that all horses which come in close proximity to other horses be tested for EIA. Blood samples taken from the horse must be sent to a state-approved laboratory for a Coggins test to be performed. The test now is mandatory to register your horse for a show or to cross state boundaries in all 50 states. Some states even go so far as to require a negative Coggins test to sell your horse.

"Testing for EIA virus antibodies should be conducted on any horse which the past history is not known," said Issel.

The reason it's so important to test these questionable horses is that if the horse comes into contact, or even is within close proximity to other horses, the disease can be transmitted if the horse is carrying the EIA virus, even if no outward symptoms are apparent. Since EIA affects all breeds and ages of equines equally, it's advised by the USDA to have even a weanling tested if his contact history with other equines is not known. If the mare is EIA positive, the foal must be quarantined for at least 60 days and have a negative test at the end of the quarantine period before being released with other horses. If the mare is positive for EIA, the foal will get passive EIA antibodies from the mare's colostrum. This foal might test positive for more than six months due to the colostrum he has ingested from his dam.


Some horses appear to recover from the disease and exhibit no outward signs of infection, but are carriers of the virus. For this reason it's important to test for EIA antibodies so that EIA positive horses can be isolated from the general population in order to prevent the spread of the EIA virus. In most cases, a positive EIA test is the first time a horse is recognized as being infected by the virus. The familiar Coggins test is the common name for the agar gel immunodiffusion test that determines the presence of EIA antibodies in the horse's blood. If present, it means the horse is EIA positive, is a carrier of the EIA virus, and is able to infect nearby stablemates and horses at shows. The Coggins test is very reliable, with a proven statistical accuracy of at least 95%. The USDA estimates about 1,600 EIA positive horses are detected each year through testing.

While the Coggins test is by far the most familiar test for EIA antibodies, there are other ways to diagnose the disease. Literature from Animal Plant Health Inspection Services of USDA states that three ELISA tests are also used in diagnosing EIA, two detecting antibodies against the p26 antigen and one detecting antibodies against the gp45 antigen. One of the ELISA tests is a competitive enzyme-linked immunosorbent assay (cELISA). This test has fewer false negatives than the Coggins, but false positives do occur on rare occasions. Positive tests must be confirmed using the Coggins test.

Synthetic antigen enzyme-linked immunosorbent assay (saELISA) is another test used in EIA diagnosis. This test often is used when cELISA and Coggins tests give conflicting data. According to literature from the USDA, "The test detects antibodies against the gp45 (viral transmembrane protein) antigen and thus provides additional power to diagnosis through detection of an antibody to a second antigen." And as with the cELISA, any positive must be confirmed by conducting a Coggins test.


EIA is considered a classic blood-borne infection. Transmission of the virus results through inoculation of the blood from an infected animal into an uninfected animal. Humans have played an important role in the EIA virus transmission by using blood-contaminated materials, such as reusing hypodermic needles between horses, blood transfusions, or other methods where the blood of an infected animal is inoculated into an uninfected animal. The rule, according to Issel, should be "one horse, one needle, one syringe."

Blood-feeding insects such as the deer fly, horse fly, stable fly, and mosquito also have the potential to carry the virus from one horse to another. If the bites from these flies stimulate defensive movements by the horse, it often results in an interruption of the blood-feeding. 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 until it is satisfied. Because of this, any infective material from the blood of the first host and present on the mouth parts of the insect can be mechanically transmitted to the second host.

Transmission of EIA virus by insects depends on the number and habits of the insects present, 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 in the blood of the infected horse from which the initial blood was obtained. Because of all these variables, the rate of transmission cannot be predicted accurately, according to Issel.

Stages Of Infection

After an incubation period of two to four weeks, horses which have been infected with the EIA virus may begin to exhibit symptoms of the disease. Symptoms of EIA infection include fever, weakness, weight loss, incoordination, anemia, minute blood-colored spots on the mucous membranes, and dependent edema--a swelling, and evidence of fluid collecting under the skin in the legs, under the chest, and under other underbody surfaces.

In severe cases of EIA, acute signs of disease emerge and the horse can die within two to three weeks. This acute response to the virus is not typical of horses infected by blood-feeding insects such as deer flies, stable flies, mosquitoes, and horse flies--the most common way the virus is transmitted. Infections of EIA caused by insects vectors rarely transmit enough of the virus to initiate the onset of an acute case of EIA.

When smaller amounts of the virus are deposited in the uninfected horse's blood stream, they will infect the horse, but not on such a detrimental scale. USDA literature states, "In the early stages of infection, the horse usually tests negative for antibodies to EIA virus, and blood samples must be collected at a subsequent date (generally 10 to 14 days later) to confirm or exclude EIA as a diagnosis. During this period, it is prudent to quarantine the horse (or the farm) if EIA is strongly suspected on the basis of history or signs."

If the horse survives the initial onset of EIA, it will progress to either a chronic case or inapparent carrier of the virus. "The horse with chronic EIA is the classic 'swamper' who has lost condition, is lethargic and anorexic, has a low hematocrit, and demonstrates a persistent decrease in the number of blood platelets, especially coincident with fever induced by the EIA virus," according to the USDA. Appearing to be normal and healthy horses, the inapparent carriers are the horses most dangerous to other horses because they show no overt symptoms of illness and move throughout the horse population freely. Horses infected with the EIA virus might have repeated relapses when they become stressed.

Degrees Of Infectiousness

How concentrated the virus is in the blood of a horse will depend on whether the horse is an acute, chronic, or inapparent carrier of the EIA virus. According to the USDA, "In the acute horse, one-fifth of a teaspoon of blood from a horse with acute EIA contains enough virus to infect one million horses. This can pose a problem because the clinical signs of the acute form of EIA are nonspecific; and in mild cases, the initial fever may be short-lived (often less than 24 hours). Oftentimes the owner or veterinarian will not observe this initial response when a horse is infected with the EIA virus."

The problem arises when these horses recover and roam freely in the population. "The first indication that a horse was exposed and infected with the EIA virus may very well be a positive on a routine Coggins test," noted Issel.

"The chronic EIA horse is one that experienced a mild acute bout of EIA and survived. These horses exhibit recurring clinical disease. One fifth of a teaspoon of blood from a chronic case during a feverish episode contains enough virus to infect 10,000 horses," states the USDA.

In the inapparent carrier horse, which is the majority of horses infected with EIA virus, "only one horse fly out of six million is likely to pick up and transmit EIA virus," according to the USDA. All horses infected with EIA virus are thought to remain virus carriers for life. Inapparent carriers have a much lower concentration of EIA virus in their blood than horses with active clinical signs of the disease. This inapparent form may turn into chronic or acute disease due to stress, hard work, or the presence of other diseases.

The USDA reports that, "An acutely ill horse harbors a substantial concentration of EIA virus in its bloodstream, as does a chronic case experiencing a fever. But an inapparent carrier may have only an infinitesimal amount of virus in its blood."


Controlling and minimizing the spread of EIA virus involve eliminating contact of non-infected horses with the secretions, excretions, and blood of EIA infected horses. This is done by testing and separating positive horses from the general population that tests negative. Once the positive horse, or horses, are identified, they should be kept at least 200 yards away from the general equine population or be euthanized. Once this is done, the transmission of the EIA virus is broken. It's wise to consider all horses positive for the EIA virus--and a potential carrier--until a test is performed on that animal proving it's negative for the antibodies to the virus.

All breeds and all ages of horses are equally susceptible to EIA. However, there are seasonal and regional differences in cases of the disease. Most EIA cases show up in what is termed the "hot zone," which includes Alabama, Arkansas, Florida, Georgia, Illinois, Iowa, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, and Wisconsin. EIA occurs seasonally in the summer and autumn months because of insect populations, said Issel. He added that, in southern states, environmental stress on the horse due to the heat may induce a bout of EIA, while in northern states, environmental stress due to the severe cold may bring on a bout of EIA.

An added precaution to the spread of EIA in your horse population is an insect control program, especially if you're located in the "hot zone."


  • Obtain negative Coggins test results on all horses on your premises.
  • Isolate for 45-60 days all new horses at least 200 yards from the general population until a negative Coggins test is obtained.
  • Use disposable syringes and needles and use them only once.
  • Implement insect control programs on your farm.
  • Clean and sterilize all instruments after each use.
  • Keep stables and facilities clean, which will reduce the fly population.
  • Follow state laws that govern EIA.
  • Report horses suspected of having EIA to state or federal animal health authorities.


The week of July 1 saw the completion of the third phase of the gathering of BLM horses in the Agency Draw area where all 50 animals tested negative for EIA. Previously, in the East Bonanza Horse Management area only one horse of 97 was found to be positive for EIA. However, 51% of the horses tested on BLM ground west of the management unit, in the area known as Natural Buttes, and on the adjacent White River area of the Ute Reservation, were found to be positive for EIA. A total of 84 animals out of 163 tested were positive for EIA in that area. The remaining negative animals in that herd are considered exposed and will be quarantined for 45 days and retested before being released. Further gathering and testing will be done on the herd next year as part of an ongoing surveillance effort. Initially, 172 animals in the Tabayago Management unit tested negative for the disease.

The gathering of Ute Tribal horses in the Hill Creek area continues as well, where 260 animals have been gathered and only two have tested positive for EIA. The tribe is concerned about the risk to privately owned animals, and has focused attempts to have all the threatened animals in the White River risk area tested. Another three privately owned animals near Touwave Reservoir tested positive this week. State Veterinarian, Dr. Michael R. Marshall, is recommending that all horse owners in the area have their horses tested.

All the positive animals have been eliminated except 11 foals. The foals are the subject of a Temporary Restraining Order (TRO) filed by the International Society for the Protection of Mustangs and Burros against the BLM. It is possible that the foals have tested positive because of antibodies transferred in the colostrum from their mothers, and that, if given enough time, the foals may eventually test negative. The problem presented is where and how to house the foals without placing other animals at risk during the vector season.

The Utah State Veterinarian, BLM administrators, researchers, and animal rights advocates are working to seek a solution. The animals may not be transported interstate under federal regulations except to an approved research facility or to slaughter. In the meantime, the foals remain confined under quarantine. It is the desire of all involved to have healthy herds, free of disease.

--State of Utah Department of Agriculture & Food

About the Author

Tim Brockhoff

Tim Brockhoff was Staff Writer of The Horse:Your Guide to Equine Health Care from 1995 to 1999. His degree is in Agricultural Communications from the University of Kentucky, and his equine experience is with American Saddlebreds.

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