By C.J. Issel, DVM, PhD, Wright-Markey Chair of Equine Infectious Diseases, University of Kentucky
These days the main thing many U.S. horse owners think about the Coggins test is that it's something they have to have done before taking horses to a show, sale, or another state. Its often overlooked purpose of identifying horses carrying equine infectious anemia (EIA, often called swamp fever) is critical to controlling the disease. Yet EIA researchers are now recommending that we test many horses less frequently and focus more effort on finding untested carriers.
Equine infectious anemia is an often fatal blood-borne disease of horses, donkeys, and mules caused by a lentivirus that's closely related to the human immunodeficiency virus (HIV). Although some infected horses won't become ill, others will display fever, anemia (low red blood cell count), jaundice, depression, edema (fluid swelling), and chronic weight loss. The virus is transmitted by blood-feeding insects and sharing of blood-contaminated equipment such as needles between infected and healthy horses.
There is no treatment or vaccine for EIA, and infection is life-long. Owners of infected horses in many U.S. states have two unappealing choices: Quarantine the horse at least 200 yards away from all other horses for the rest of his life, or euthanize him. Many opt for the latter.
EIA By the Numbers
The good news is that extensive testing and euthanasia of infected horses over the last 40 years have made the outbreaks of the past just that: Past. In the United States, only 84 positive equids were reported during the last 2 years (2009-2010). The majority appear to be horses tested for EIA for the first time, often as a result of a required test for transfer of ownership. Although these numbers are not accurate indicators of the rate of infection in the general population (many horses are never tested), they do provide a useful monitor of the tested population that is reported. Few if any of these cases have displayed active clinical signs of equine infectious anemia.
Today in the U.S., nearly 2 million horses are tested annually for EIA. If owners pay about $30 for each test, then the horse industry pays about $60,000,000 annually to find less than 50 cases each year (more than $1,200,000 for each!). Is this surveillance for EIA adequate, excessive, or effective? This strategy has led to the detection of more than 100,000 EIA-positive equids since 1972, and the number of positives found each year has decreased from more than 10,000 per year to less than 50--a tremendous success story. But many would question the wisdom of continuing the same testing strategy in the face of nearly zero risk of acquiring EIA in the U.S. today.
There is no question that active surveillance should be continued and expanded, but the strategy could be modified to permit less frequent testing of equids not at risk for EIA and an increase of testing focused on finding the remaining untested reservoir population (horses that are infected but haven't been tested/identified). I have referred to this as "smarter testing" and have urged states to adopt this strategy (although changes in the existing rules must be made one state at a time, as the only national rules concerning horses and EIA involve moving test-positive horses between states). This philosophy is highlighted in a new EIA video released by the USDA and available online.
The recommended reduction in testing relies in large part on regionalization, or grouping of states into regions with similar risks of EIA infection based on historical testing results. Thus, with this approach you wouldn't need a new test just to move from state to state; you'd only need to test on your state's recommended testing schedule if you stay in your region, and test again if you move from region to region.
The video says: "Any new plan for EIA control at the national level will be driven by states, industry leaders, and sound research. We estimate that regionalization alone will save more than $10 million per year in testing costs. We are confident that when combined with focused testing in areas of high risk (those with higher rates of EIAV detection and/or lower rates of testing)--in other words, smarter testing--the risk of acquiring EIA will be lower and at a reduced cost to the industry... We believe a regionalized approach and uniformity in regulations between states is the best way to control EIA."
To identify the reservoir population, more outreach to veterinarians to test especially in higher-risk areas will be needed. Free testing clinics can help, and in our opinion, states should be working with the horse industry to devise better ways to find the remaining EIAV-infected reservoirs.
Smarter testing also means using the most effective tests possible to find the highest number of EIAV-infected equids. Today that means adopting a three-tiered testing strategy that most effectively uses the strengths of the available laboratory tests for EIA. These tests include the most accurate test, the agar gel immunodiffusion (AGID or Coggins) test, and the more sensitive ELISA-based tests in combination, by using the sensitive ELISA-based tests first instead of the Coggins (the first tier), and using the more specific Coggins to confirm positive ELISA results (the second tier). (More sensitive tests such as the ELISA in this case find more EIA cases, but also can yield more false positives, whereas more specific tests yield few false positives but more false negatives.) If test results between the first and second tier are mixed or unclear, a third tier of immunoblot testing can settle the question of whether the horse has EIA.
This strategy was endorsed by the U.S. Animal Health Association in Resolution 26 in 2008 from the Committee on Infectious Diseases of Horses. The state of Oklahoma has been very progressive and adopted this strategy, has the support of their industry for the change, and seems pleased with the results.
Recently, Italy conducted a national surveillance for EIA using this three-tiered testing strategy and found more positive equids than would likely have been found if only AGID (Coggins) testing was done. This prevented the release of the "false negative" horses into the surrounding equine population, where they could serve as a potential EIA threat to others.
Biosecurity is Essential
What else should we be doing for EIA control today? In my opinion, we must continue to urge the adoption of methods designed to reduce the impact of man on transmission of EIAV. In natural settings, EIAV transmission between horses by large blood-feeding insects is a relatively inefficient process, especially when compared to equine herpesvirus and influenza that are easily transmitted by aerosol or contact. Once humans enter the picture, however, all bets are off; shared, contaminated needles and syringes have proven to be thousands of times more efficient at transmitting EIAV than insects.
Throughout the world, we need to reinforce the necessity of limiting the transfer of blood from one horse to another by human intervention. Toward that goal, I have put together a short note on this subject and it is included in the DVD package of the USDA video EIA Testing: Refining Our Approach. The DVD package is available for distribution from the USDA and includes the 1996 video on EIA and additional practical information on control of EIA.
If the horse industry and legislators can work together to implement smarter testing requirements for EIA, and if horse people practice good biosecurity when taking and handling blood samples, we can save money and more importantly, continue to chip away at equine infectious anemia.