2020 Equine Infectious Anemia Cases 

in the United States

January 1– December 31, 2020

This document summarizes equine infectious anemia (EIA) cases and testing reported in the United 

States during 2020. Following recent guidance updates, the over 400 EIA-approved laboratories sent

monthly summary testing data directly to APHIS Veterinary Services (VS). All positive cases were 

confirmed by the National Veterinary Services Laboratories (NVSL) and case information was compiled 

by the VS Equine Health Team in collaboration with State animal health officials.

Reporting of EIA testing is summarized on a calendar-year basis. During 2020, a total of 1,337,829 EIA 

tests were conducted, resulting in detection of 29 positive horses (Table 1.) These results compared to 

1,151,584 tests and 89 positives in 2019. Figure 1 displays a map depicting reported numbers of horses 

and premises testing positive for EIA in 2020. Figure 2 presents a summary of EIA testing from 2001-

2020. Historical data and additional information on EIA are available online at

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine/eia

Table 1. Test results reported for equine infectious anemia in the United States, 2020

State Tests Performed Positive Horses Positive Premises

Alabama 21,830 0 0

Alaska 838 0 0

Arizona 14,821 0 0

Arkansas 35,373 0 0

California 26,930 1 1

Colorado 22,707 1 1

Connecticut 7,501 0 0

Delaware 3,120 0 0

Florida 109,146 0 0

Georgia 39,837 3 2

Hawaii 376 0 0

Idaho 11,820 0 0

Illinois 26,902 0 0

Indiana 20,601 0 0

Iowa 18,949 6 1

Kansas 14,502 0 0

Kentucky 63,885 0 0

Louisiana 29,140 0 0

Maine 4,361 0 0

Maryland 22,564 0 0

Massachusetts 12,163 0 0

Michigan 31,515 0 0

Animal and Plant 

Health Inspection 

Service

Veterinary 

Services

September 2021
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 2

Minnesota 28,070 0 0

Mississippi 19,072 0 0

Missouri 53,261 0 0

Montana 19,774 0 0

Nebraska 8,901 0 0

Nevada 13,199 0 0

New Hampshire 6,065 0 0

New Jersey 16,494 0 0

New Mexico 13,745 1 1

New York 36,689 0 0

North Carolina 52,371 0 0

North Dakota 6,865 0 0

Ohio 35,302 0 0

Oklahoma 54,079 0 0

Oregon 9,043 0 0

Pennsylvania 43,546 0 0

Rhode Island 1,506 0 0

South Carolina 41,946 0 0

South Dakota 9,495 0 0

Tennessee 50,636 0 0

Texas 163,544 17 13

Utah 8,598 0 0

Vermont 5,547 0 0

Virginia 41,359 0 0

Washington 8,717 0 0

West Virginia 9,325 0 0

Wisconsin 28,253 0 0

Wyoming 13,476 0 0

Puerto Rico 56 0 0

U.S. Virgin Islands 14 0 0

U.S. Totals 1,337,829 29 19
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 3

Figure 1. Reported numbers of horses and premises testing positive for EIA, 2020
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 4

Figure 2. Reported numbers of EIA tests and positive cases in the United States, 2001-2020

Although the current prevalence of EIA in the U.S. equine population remains very low, at an estimated 

national prevalence of around 0.004 percent, the epidemiology of EIA-positive cases has shifted in 

recent years. Most EIA cases were previously found to occur from natural transmission by biting fly 

vectors in untested and under-tested populations. Now, increasing cases of iatrogenic transmission

(disease resulting from medical activity) are being identified. 

In 2020, 23 of the 29 confirmed cases of EIA were found in Quarter Horse racehorses with iatrogenic 

transmission either suspected or confirmed as the route of infection. Iatrogenic transmission in this 

population is occurring through unhygienic practices by horse trainers and owners. Practices include reuse of needles, syringes, and IV sets; administration of blood transfusions from untested donor horses;

use of illegal blood products from other countries; and infectious blood contamination of multi-dose 

drug vials. 

Some of the iatrogenic transmission cases in the Quarter Horse racehorse population are found in 

horses participating in unsanctioned racing. However, there are also recognized crossover cases 

between unsanctioned and sanctioned racing in some parts of the United States. Cases in sanctioned 

Quarter Horse racehorses with no obvious connection to unsanctioned racing have been identified as 

well. Frequently, EIA-positive cases in this emerging high-risk population are found in clusters, indicating

a horse trainer or owner repeatedly using unhygienic practices that caused disease spread to multiple 

horses. These cases are preventable by good hygienic practices and basic biosecurity measures. Thus, 

increased education and outreach to trainers and owners of Quarter Horse racehorses is needed to 

mitigate the continued spread of EIA. 

0

500

1000

1500

2000

2500

Number of Tests x 1000 Number of Test-positive Horses
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 5

Following recent changes in APHIS guidance for EIA-approved laboratories, more comprehensive EIA 

testing data is now available. Figure 3 captures the number of EIA tests performed in 2020 by month. 

March, May, and June, respectively, were the busiest months when EIA testing occurred in 2020.

Figure 3. Reported numbers of EIA tests performed in the United States in 2020 by month

Two EIA test types are authorized for use by approved laboratories in the United States: 1. agar gel 

immunodiffusion (AGID) tests, commonly called Coggins tests, and 2. Enzyme-linked immunosorbent 

assay (ELISA) tests. Figure 4 depicts the number of AGID tests performed throughout the U.S. in 2020,

compared to the number of ELISA tests performed. Both test assays were used in 2020 with nearly 

equal frequency. 

Figure 4. Reported numbers of AGID and ELISA EIA tests performed in the United States in 2020

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

EIA Tests Performed in 2020 by Month

AGID and ELISA EIA Tests Performed in 2020

AGID Tests

ELISA Tests
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 6

Figure 5. Reported numbers of AGID and ELISA EIA tests performed by State of horse residence in 2020

0 20000 40000 60000 80000 100000 120000 140000 160000

Alabama

Alaska

Arizona

Arkansas

California

Colorado

Connecticut

Delaware

Florida

Georgia

Hawaii

Idaho

Illinois

Indiana

Iowa

Kansas

Kentucky

Louisiana

Maine

Maryland

Massachusetts

Michigan

Minnesota

Mississippi

Missouri

Montana

Nebraska

Nevada

New Hampshire

New Jersey

New Mexico

New York

North Carolina

North Dakota

Ohio

Oklahoma

Oregon

Pennsylvania

Puerto Rico

Rhode Island

South Carolina

South Dakota

Tennessee

Texas

Utah

Vermont

Virgin Islands

Virginia

Washington

West Virginia

Wisconsin

Wyoming

AGID and ELISA EIA Tests Performed in 2020 by State of Horse Residence

AGID Tests ELISA Tests
2020 Summary of Equine Infectious Anemia Cases in the United States

USDA-APHIS-VS 7

Figure 5 shows the number and type of EIA assays used to test resident horses in each State. Twenty-six

States, along with Puerto Rico and the Virgin Islands, reported fewer than 20,000 EIA tests performed on 

their horse populations in 2020. Twenty-one States reported between 20,000 and 60,000 EIA tests 

performed on their horse populations in 2020. And, three States—Texas, Florida, and Kentucky—

reported over 60,000 EIA tests performed on their horse populations in 2020. Horse populations in 23 

States were tested more frequently using an ELISA assay than the AGID assay, while horse populations in 

27 States as well as Puerto Rico and the Virgin Islands were tested more frequently using the AGID assay 

compared to an ELISA assay.