APHIS Factsheet

Veterinary Services January 2008

Equine Infectious 

Anemia

Equine infectious anemia (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 lentivirus: it contains genetic RNA 

material, which it uses to produce DNA. This DNA is 

then incorporated into the genetic makeup of infected 

cells. Identified in France in 1843 and first tentatively 

diagnosed in the United States in 1888, EIA has commanded a great deal of attention over the years. No 

vaccine or treatment exists for the disease. It is often 

difficult to differentiate from other fever-producing 

diseases, including anthrax, influenza, and equine 

encephalitis.

 EIA is significant historically because it is the first 

disease of horses proven to be caused by a “filterable virus”—one that can survive a special laboratory 

filtering procedure and remain infectious. EIAV is the 

first lentivirus-induced disease proven to be transmitted by insects. And EIA is the first persistent virus for 

which antigenic drift was defined. (Antigenic drift is 

the virus’ ability to change its form sufficiently so that it 

is no longer vulnerable to existing antibodies.) Finally, 

EIA is the first lentivirus-induced disease for which a 

diagnostic test was approved.

Degrees of Infectiousness

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 1 million horses.

 The clinical signs of the acute form of EIA are 

nonspecific; in mild cases, the initial fever may be 

short lived (often less than 24 hours). As a result, 

horse owners and veterinarians may not observe this 

initial sign when a horse is infected with EIAV. These 

infected 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 

well be a positive result on a routine annual test.

Chronic—If the horse survives this first acute bout, it 

may develop a recurring clinical disease with these 

signs:

• Fever—An infected horse’s temperature may rise 

 suddenly to about 105 F or, rarely, as high as 

 108F. Then it may 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—The horse appears more or less 

 dejected (head hangs low) and generally listless.

• Weight loss—The horse may refuse feed or may 

 eat an inordinate amount but still continues on an 

 obvious decline from normal weight.

• Dependent edema—The 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 experience 

 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.

 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 EIAV. 

One-fifth of a teaspoon of blood from a chronic case 

during a feverish episode contains enough virus to 

infect 10,000 horses.

Inapparent—By far the majority of horses are inapparent carriers: they show no overt clinical abnormalities 

as a result of infection. They 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 horsefly out of 6 million is likely to 

pick up and transmit EIAV from this horse. All horses 

infected with EIAV are thought to remain virus carriers 

for life. The inapparent form may become chronic or 

acute due to severe stress, hard work, or the presence 

of other diseases.

Transmission

EIA is considered a classic bloodborne infection. 

People have played an important role in EIAV transmission over the years by using blood-contaminated 

materials on different horses. The EIAV most 
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frequently is transmitted between horses in close 

proximity by large biting insects, such as horseflies 

and deerflies. The bites from these flies stimulate 

defensive movement by the horse, which 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 to repletion. In this manner, 

any infective material from the blood of the first host 

that is 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 habits 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 in the 

blood of the infected horse from which the initial blood 

meal was obtained. The rate of transmission cannot 

be predicted accurately because of these variables.

Preventing the Spread

Diagnosis of EIA and of infections with EIAV was not 

possible until an effective test for antibodies specific 

to EIAV was described in 1970 by Leroy Coggins. 

The agar-gel immunodiffusion (AGID) or Coggins 

test was shown to correlate with horse innoculation 

test results for EIAV and, therefore, could 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.

 Controlling the spread of EIAV involves minimizing or eliminating contact of horses with the secretions, excretions, and blood of EIAV-infected horses. 

This has been carried out in most areas of the world 

by testing and segregating test-positive horses from 

those with negative test results. 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, one 

must assume that each horse is a potential reservoir 

of EIAV and take precautions to see that his or her 

horses commingle only with horses with impeccable 

backgrounds (i.e., they came from farms with only 

test-negative horses and have never been exposed to 

test-positive horses).

What Equine Owners Can Do To Help

Horse owners can take a number of precautions to 

reduce the risk of infection:

• Use disposable syringes and needles. Follow the 

 rule of one horse-one needle.

• Clean and sterilize all instruments thoroughly after 

 each use.

• Keep stables and immediate facilities clean and 

 sanitary. Remove manure and debris promptly, 

 and ensure that the area is well drained.

• Implement insect controls. The local veterinarian 

 or animal health official can provide information 

 about approved insecticides and other insect- 

 control measures. Avoid habitats favorable to 

 insect survival.

• Do not intermingle infected and healthy animals. 

 Do not breed EIAV-positive horses.

• Isolate all new horses, mules, and asses brought 

 to the premises until they have been tested for 

 EIA.

• Obtain the required certification of negative EIA 

 test status for horse shows, county fairs, race– 

 tracks, and other places where many animals are 

 brought together.

• Abide by State laws that govern EIA.

 The major regulatory actions to control EIA are 

carried out by the States. States’ rules, while encompassing a much broader scope of EIA concerns, vary 

considerably and lack uniformity among individual 

State control programs. USDA created a Uniform 

Methods and Rules (UMR) to facilitate the development of a uniform control program for EIA and the 

interstate movement of horses.

Report Suspicious Cases

Veterinarians and horse owners who suspect an animal may have EIA should immediately contact State 

or Federal animal health authorities.

Additional Information

For more information, contact

USDA, APHIS, Veterinary Services

National Animal Health Program

4700 River Road, Unit 43 

Riverdale, MD 20737–1231

Telephone (301) 734–6954

Fax (301) 734–7974

or visit our Web site at

http://www.aphis.usda.gov/vs/ceah/index.htm