United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Veterinary Services 

Centers for Epidemiology and Animal Health September 2006 

_________________________________________________________________________________________________________________________ 

Equine Infectious 

Anemia (EIA) 

EIA is an infectious and potentially fatal viral disease of 

horses. No vaccine or treatment exists for the disease. 

Clinical signs of EIA include fever, weight loss, icterus 

(yellowing of body tissues), anemia, swelling in the 

limbs, and weakness. However, not all equids infected 

with the equine infectious anemia virus (EIAV) show 

signs of illness, and these animals serve as inapparent 

carriers. 

EIAV transmission 

EIAV is usually transmitted from horse to horse 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 

flies’ blood feeding. When interrupted, flies are motivated 

to complete feeding as soon as possible. They then 

attack the same or a second host and feed to complete 

their meal. In this manner, any infective material from the 

blood of the first host that is present on the mouthparts 

of the flies can be mechanically transmitted to the 

second host. 

 Needles and equipment contaminated with blood 

from an infected horse can also spread the virus when 

used on unexposed horses. Horses demonstrating 

clinical signs of EIA pose the greatest risk of spreading 

the virus because they have the greatest concentrations 

of circulating virus. However, even inapparent carriers 

pose a risk to other horses. 

Diagnosis 

Diagnosis of disease associated with EIA and 

inapparent EIAV infection was not readily available until 

an effective serologic test for antibodies specific to the 

virus was developed in 1970 by Dr. Leroy Coggins. This 

agar–gel immunodiffusion or Coggins test is able to 

identify carrier animals. Although other serologic tests 

have been developed and approved for diagnosing EIA, 

the Coggins test is internationally recognized as the 

standard to which all other tests are compared. 

Currently, the Coggins test or some of the newer tests 

for EIAV are commonly required when moving horses 

within and among States. 

Testing 

EIAV testing in the United States utilizing the 

Coggins test began in 1972. Data exist from 1972 to 

2005 on the number of tests performed by each State 

annually and the percentage of those tests that were 

positive for EIAV (figures 1 and 2). The percentage of 

test-positive samples in the United States decreased 

dramatically from nearly 4 percent in 1972 to less than 

0.01 percent in 2005 

<www.aphis.usda.gov/vs/nahps/equine/eia>. 

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

1975 1980 1985 1990 1995 2000 2005

Year

Number 

of tests

Figure 1. Number of EIAV Tests Performed in the United States, 1972-2005

*Data not available for 1981 and 1982

*

 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1975 1980 1985 1990 1995 2000 2005

Year

Percent tests

positive

Figure 2. Percentage of Samples that Tested Positive for EIAV in the United 

States, 1972-2005

*Data not available for 1981 and 1982

*

 
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

 Historically, until the mid-1990s, 92 percent of testpositive samples have originated from horses located in 

what is referred to as the “hot zone” (see map 1). Risk of 

EIAV infection is highest in this region in part because 

environmental conditions are ideal for the insect vectors 

that transmit the virus and, presumably, because a 

reservoir of untested horses infected with EIAV exist. 

However, in the past decade clusters of EIAV-infected 

horses on individual premises have been recognized in 

areas outside the historical hot zone. Interactive maps of 

EIA testing results are available at: 

<www.aphis.usda.gov/vs/nahps/equine/eia/webmapping.html>. 

Control measures 

Minimizing or eliminating contact between 

nonexposed horses and the secretions, excretions, and 

blood of EIAV-infected horses can help control the 

spread of EIAV. This is accomplished in most areas of 

the world by testing horses for EIAV and either 

euthanizing or segregating test–positive horses from 

test-negative horses. Once the reservoirs of EIAV are 

identified, separated, and maintained at a safe distance 

from nonexposed horses, the transmission of EIAV is 

broken. 

The major regulatory actions to control EIAV are 

carried out by individual States. However, States’ rules, 

while encompassing a broad scope of EIA concerns, are 

not consistent. In 2002, USDA created a Uniform 

Methods and Rules to facilitate the development of a 

uniform control program for EIA 

<www.aphis.usda.gov/lpa/pubs/eiaumr.pdf>. 

NAHMS Equine ’98 serum bank 

While there has been a decline in the percentage of 

positive tests among horses tested as part of the EIA 

programs, the EIA subcommittee of the Infectious 

Diseases of Horses Committee of the U.S. Animal 

Health Association suggested looking at the prevalence 

of EIAV in the entire U.S. equine population. To 

accomplish this task, the subcommittee requested that 

the existing serum bank generated during the NAHMS 

Equine ’98 study be tested for EIAV, thereby providing a 

more accurate representation of EIAV infection in the 

general equine population. The American Horse 

Council’s Health and Regulatory Committee along with 

State Veterinary Regulatory officials from the 28 States 

participating in the Equine ‘98 study (see map 2 next 

page) agreed that testing the serum bank was a priority 

for the industry and an appropriate use of the banked 

sera. 

The Equine ‘98 study estimated that overall 40.9 

percent of equine operations tested at least one resident 

horse for EIAV in 1997 and that just over 35 percent of 

horses were tested annually. For operations that tested 

for EIAV, the most common reason for testing was 

attending an event like a show within the State (41.4 

percent of operations that tested) followed by interstate 

transportation (19.2 percent of operations that tested). 

As part of the NAHMS Equine ’98 study, biologic 

samples including blood were collected. A serum bank 

was established—based on the collection of blood from 

a subset of horses enrolled in the study—and stored at 

the National Veterinary Services Laboratory (NVSL) in 

Ames, IA. 

To participate in the biologic sampling phase of the 

study, operations had to have three or more horses as of 

January 1, 1998, and complete questionnaires 

associated with the study. Sera contributed to the serum 

bank were collected from horses in 28 States during 

summer 1998 (June 15 to September 23) or winter 1999 

(October 2, 1998, to March 3, 1999). The number of 

horses sampled per operation was on a sliding scale up 

to 20, based on the number of horses on the operation; 

each horse was sampled only once. For this study 

horses were defined as full-size breeds, usually standing 

14 hands or taller when mature. No horses on race 

tracks were included in Equine ’98 biological sampling. 

Sera in the Equine ’98 serum bank were tested by 

NVSL using the Coggins test to detect antibodies to 

EIAV. There were certain limitations in using these sera 

to precisely predict EIAV test-positive horses in the 

general population—the main limitation being the 

relatively small number of operations and horses 

included in the study. If the study’s original goal had 

been to precisely estimate the prevalence of EIAV testpositive horses— which was expected to be low—then a 

larger number of operations and horses would have 

been included. 

FL

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 Map 1. Historical Equine Infectious Anemia (EIA) "Hot Zones" 

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Shaded States = Hot-Zone States
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

FL

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Map 2. Equine '98 Particpating States 

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Shaded States = Participating States

Based on testing of banked sera from Equine ‘98, 

the weighted estimate of EIAV-positive horses was 

0.003 percent with a 95-percent confidence interval for 

the estimate between 0.001 and 0.008 percent. This 

equates to being 95-percent confident that the true 

national prevalence of EIA-positive horses (as described 

above) was less than 8 in 100,000. 

Inferences cover the population of horses on 

operations with 3 or more horses in the 28 participating 

States. All respondent data were statistically weighted to 

reflect the population from which they were selected. 

Due to the low prevalence of test-positives in the 

serum bank no further analysis could be performed, 

such as regional estimates or animal-risk factors for 

positivity. It would appear that no clustering of positive 

horses occurred on a given premises based on this 

testing of sera from the approximately 900 equine 

operations included in the Equine ’98 study. 

The EIAV prevalence estimate based on regulatory 

testing activity from 2002 through 2005 (0.01 to 0.02 

percent) is consistent with the level estimated form 

testing the NAHMS Equine ’98 serum bank (0.001 to 

0.008 percent).

More information on the Equine ’98 study can be 

found at 

<http://nahms.aphis.usda.gov/equine/index.htm> 

_______________________________ 

For more information, contact: 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMS@aphis.usda.gov 

<http://nahms.aphis.usda.gov> 

#N461.0806 

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