Most equine veterinarians are aware of the rules and regulatory requirements for testing for equine infectious anemia virus (EIAV). Veterinarians usually complete this routine testing during preventative care appointments in the spring prior to show and event season.
However, this well-cared-for group of horses is not necessarily the source of most EIAV infections. The current predominant transmission is due to iatrogenic causes rather than natural insect vectors. It is particularly prevalent in racing Quarter Horses.
A positive diagnosis in a horse has serious repercussions. The virus is significant in that, much like its Lentiviral cousin, the human immunodeficiency virus (HIV), there is no vaccine and there is no cure. A horse diagnosed positive for EIAV is either euthanized or placed under extremely strict quarantine conditions at least 200 yards away from other equids for the rest of its life.
How Do Horses Contract EIAV?
Blood is the culprit in causing an infection from one horse to another. Historically, natural transmission occurs when virus is transferred from an infected horse through biting flies, like horseflies, deer flies, or stable flies. EIAV does not replicate in insect tissues. The infective dose is limited by the amount of blood carried on a fly’s large mouthparts. EIAV survives for less than four hours on fly mouthparts.
A horsefly interrupted in its feeding will attack a new host. The further away the potential new host, the more likely the fly is to return to its original victim. Studies show that if a new horse host is tethered at distances of 50 yards or more, it is predominantly (although not entirely) immune from attack as the fly usually returns to the original host. Most authorities recommend larger distances such as 200 yards to err on the side of safety. Yet, it is important to consider that in congregated stabling areas, paddocks and pastures, horses reside in much closer proximity than 50-200 yards away. Consequently, testing is important to minimize the risk of exposure.
However, a different mode of transmission has become more commonplace in recent years. A disconcerting situation has developed in the USA. Between 2009-2013, of the 1.5-2 million horses tested yearly, an average of 40+ cases tested positive for EIAV. But, a recent uptick in positive cases has occurred due to an increased number of iatrogenic cases. These cases are particularly in Quarter Horse racehorses and unsanctioned Bush Track racehorses.
Prior to 2013, EIAV cases of iatrogenic transmission were not recognized as significant contributors to the number of positive horses. In 2013, about 25% of annual EIAV infections were caused by iatrogenic transmission, and in 2014, 54% of EIAV positives were traced back to an iatrogenic origin, which was preventable. In 2021, of 103 positive cases, 82% were related to iatrogenic transmission in current or former racing Quarter Horses.
How does an iatrogenic infection occur? It comes through contaminated blood or blood products and through shared needles. Transmission can also occur from blood contamination of hands, dental or tattoo equipment, or other instruments that contact other horses. Residual blood volume in a used syringe and needle can be 5,000- to 10,000-fold greater than that carried by even the largest horsefly. The virus remains viable for at least four days. Horsefly transmission pales to near insignificance compared to the current human-related spread (iatrogenic) that perpetuates EIAV.
Is EIAV on the Rise?
While EIAV is an insidious disease, it can be controlled. This is demonstrated by the declining number of cases in the U.S. over past decades due to surveillance. When the Coggins test was developed in the early 1970s, approximately 4% of all equids tested were positive for EIAV antibodies, with 10,375 EIAV-positive horses in 1975. Removal of these test-positive subjects from the population by the beginning of the 1980s resulted in about 0.5% test-positives. The current incidence is less than 0.01%.
In 2001, there were 534 EIA-positive horses identified. The numbers continued to decrease from there. By 2015, only 69 horses of nearly 1.5 million tested were confirmed positive for EIAV. However, the number of cases is rising again. Out of the more than 1.4 million horses tested in 2021 in the USA, 103 were positive for EIAV.
Epidemiologic investigations indicate that the majority of EIAV-positive horses participate in Quarter Horse and/or Bush Track racing. These horses have potential exposure to high-risk practices such as sharing of needles and other medical equipment or the use of contaminated blood products.
Sources of Iatrogenic Infection
Specific sources of iatrogenic infection are plaguing Quarter Horse and Bush Track racing horses:
- Reuse and sharing of needles and syringes on different horses.
- Contamination of multi-dose drug vials by inserting a used needle and/or syringe that deposits virus-laden blood into the bottle. Contamination of the drug vial results in disease spread with subsequent drug administrations to additional horses.
- Re-use of intravenous (IV) tubing—when connecting to an IV catheter, blowback of blood into the tube cannot be cleaned out with just a rinse or antiseptic solutions.
- Improper cleaning and sterilization of lip tattoo equipment.
- Blood doping, particularly of Bush Track horses, which takes blood from one horse and gives it to another via IV transfusion.
- Use of unlicensed blood or plasma products illegally imported from other countries, including Mexico or South America.
- The possibility of aerosol transmission subsequent to the power washing of facilities housing EIAV-positive horses, especially if hemorrhage from a sick horse contaminates the floor or walls.
What the Virus Does
There are three phases of infection with EIAV—acute, chronic and an inapparent carrier. The highest blood-associated titers occur during clinical episodes, particularly when there is high fever. Following resolution of clinical signs, these levels drop significantly. During a febrile episode, viral titers can reach 1 million infectious particles per milliliter (ml) of blood. With resolution of fever, this drops to fewer than 100 infectious particles per ml.
Incubation of EIAV takes from a few weeks up to 90 days post-exposure. Testing is usually done 60 days following exposure. Both agar gel immunodiffusion (AGID) and the c-ELISA test are used to identify more recent transmission.
Most infected horses develop some level of anemia while still looking fit and able to perform as intended. However, those that develop an acute crisis experience severe hemolytic anemia and are likely to die. Most infections, however, seem to progress to a relative state of remission. Yet these individuals still provide a reservoir for viral infection either via insect vectors and/or iatrogenic sources. The underlying anemia might decrease a horse’s resistance to contracting another blood-borne disease, such as equine piroplasmosis. It is thought that stress and/or age-related degeneration of the immune system amplify the risk for an inapparent carrier to become an active source of infection to others.
Lentiviruses such as EIAV do not stimulate natural, fully protective immunity. In part, this is because of a high mutation rate, which is difficult for a vaccine to duplicate. This is why billions of dollars and many different vaccine approaches against Lentiviruses, including HIV and EIAV, have been mostly unsuccessful. Because of the difficulty in protecting against EIAV with immunization, the only successful approaches rely on a two-pronged strategy: 1) Education about the importance of prevention by eliminating potential contamination by blood products; and 2) surveillance and testing.
Surveillance and Testing Policies
EIAV infection is limited to equids, so it is theoretically possible to eliminate the virus from countries such as the United States where the incidence is comparatively low. However, the key factor is compliance, making this a political issue and not one strictly limited to science.
Within the U.S., most show horses are tested at least annually for EIAV because of out-of-state travel to events, clinics and shows where a negative Coggins test is required for entry. Such a clean population continues to be tested on an annual basis and so remains clean.
Racing practices are different from other equine disciplines. Because these horses tend to be accomplished athletes, when they finish their racing career, many go to jumping, eventing or barrel racing pursuits, as just a few examples. This results in an infected horse serving as a reservoir for horses in other disciplines that might otherwise be clean. Not all horse owners know their horse’s origin, so they might not be aware that their horse was once part of a Quarter Horse racing circuit. Racing Quarter Horses exposed to EIAV at a young age that go undiagnosed pose a risk once integrated into the rest of the equine population. Even if a horse has never participated in racing, EIAV cases from natural transmission still can occur through insect transmission from silent carriers.
Owners should be aware of this potential risk and arrange appropriate testing prior to the sale or purchase of a former racing Quarter Horse. This recommendation applies to any new horse purchase. Attrition of valuable racehorses due to infection means a significant economic loss to racehorse owners. So, there is motivation to implement preventive practices against iatrogenic sources of infection. In many cases, it is simply a matter of horse owner education, a role that equine practitioners can take on to mitigate this problem.
Each state oversees movement regulations of horses into their state. Most adhere to requirements for EIAV testing every 12 months, and in some states every 6 months. In northern regions of the USA, annual testing might be sufficient. In southern climates where natural transmission vectors flourish year-round, there might be a need to test more often.
Another significant point is that Mexico has no control program or testing for EIAV. With that in mind, it is important to continue with routine EIAV testing in the USA due to the high risk of infection sources from south of the border.
Importance of Educating Horse Owners
The increasing number of EIAV positives each year is recently attributable to iatrogenic sources. Iatrogenic transmission is preventable through horse owner educational efforts and annual surveillance testing. Vigilance to control EIAV relies in part on each horse owner’s responsible care. Equine veterinarians can counsel horse owners and horse farm managers to require EIAV testing for outside horses coming onto a property for boarding, events or clinics. This protects all horses from exposure to EIAV.