Equine Infectious Anemia (EIA) And The Coggins Test

The Coggins Test diagnoses the presence of a past infection of the non-infectious disease called Equine Infectious Anemia (EIA) caused by a retrovirus of the same name in horses.  It is commonly called “Swamp Fever,” which debilitates the horse with a high fever, anemia, limb swelling and often death.  Some horses get a less severe form that causes recurring fever, weight loss, anemia and swelling of the limbs and sheath.  Surviving horses will be exercise intolerant due to chronic anemia, and fever will come and go without cause. Unfortunately, they will also become a reservoir that can become the source of infection to other horses.

This virus is not naturally transmitted from horse to horse and is not contagious like the flu virus.  Transmission does occur through vectors such as biting and blood-sucking insects (horse flies, deer flies and stable flies), which directly transmit infected blood bearing the virus into the disease-free horse.  Using the same needle or surgical instruments in multiple horses can transmit the disease.  Some transmission between mare and foal in utero or milk to nursing foals is possible but less likely.  The same is true for reproductive transmission.  It can affect asses and mules, but the severity of the disease is less.

Horses surviving the disease will test positive without showing signs, but they will still be able to transmit the disease by shedding the virus to other horses without protective immunity.  In other words, the horse with the virus will always be a reservoir for the disease throughout the horse’s life.  The positive test is caused by the formation of antibodies by the adaptive immune system when a horse is first exposed and then mounts a defense against the disease.  However, this antibody response comes days to weeks after contracting the virus.  The innate immune system (front-line defenders) fights off the new infection and causes tissue damage and fever.  In about two weeks, the lymph nodes provide long-lasting protection through antibody development specific to the disease, which is what the Coggins test discovers.  Recurring fevers later in life are evidence of virus variant formation and is also why a vaccine is ineffective.

A positive Coggins test proves that the horse has had the disease in the past, based on protective immunity. But unfortunately, the retroviruses, a reverse transcription type RNA virus, can take over the horse’s DNA in the cells producing an altered DNA recognized by the horse as its own.  Thus horses that recover will have the new DNA for the rest of their lives, with the occasional symptoms from viral variants and the ability to shed the variant virus to others.

Horses that test positive for EIA are either euthanized to protect the herd or isolated with a visible brand at least 200 feet from any other negatively tested horses.  The federal government controls this branding and isolation in the US and, I would assume, by governments in other countries.  EIA is a worldwide disease, and there is no known treatment. Only China has a vaccine that is not available outside their country.  The efficacy of this vaccine is unknown, but vaccines for retroviruses are usually not effective due to the nature of the virus-developing variants.

Every horse in the United States transported on a federally funded highway is required to have a negative Coggins test. In addition, any horse sold publicly or privately is also required to have a negative test.  

Now for some interesting stories.  In 1978 when I was an undergraduate at Cornell, I had a weekend job in the isolation labs.  Here ponies were isolated from the world and tested for infectious diseases.  One weekend, a disease that caused fevers broke the isolation barrier and jumped from one isolation barn to another.  I called the supervisor, and within an hour, the professors in charge were on-site.  One of those professors was Dr. Leroy Coggins (the other was Dr. Matt Keman, who later became an advocate for me – thank you!!!).  I was a little star-struck, but he had other things to do than pay attention to me.  He soon departed Cornell for North Carolina’s vet school, so I never got to meet him again.  However, I remember that weekend whenever I think of the “Leroy Coggins Test.”

Another story occurred when I treated a horse for severe pain at the withers.  Under anesthesia, I laid the horse down on the grass and removed a fractured piece of bone from the spinous process of one of the vertebrae that make up the withers.  The horse recovered beautifully, and then she sold him. Unfortunately, the horse tested positive during the sale for EIA, and she euthanized him.

This final story was from June 1972, when Hurricane Agnes dropped 20 inches of rain, causing the Chemung River in Elmira to flood.  This area was in my practice in 1984, and people still told stories about the great flood.  Horses were found alive in trees, unable to get to the ground as the waters receded.  More importantly, the biting insects flourished infected with the EIA retrovirus.  Biting flies are the primary transmitter of EIA, and many horses succumbed to it.  While I wasn’t there, two important things transpired from the many horse deaths still with us today.  The first was developing the EIA test by Dr. Coggins within a  year of this event.  The other was the introduction of single-use needles.  In 1973 when I started working with horses, veterinarians would use the same hypodermic needle for many horses, soaking them in alcohol between uses.  I saw this with my own eyes as I watched the farm veterinarian, a former president of the American Association of Equine Practitioners (AAEP).  By 1975, horse vets were using a fresh needle once in a horse and then throwing it away.  Surgical gloves also became popular, and the rise of PPE (personal protection equipment) started within the animal industry.

One more thought.  The retrovirus is a family of viruses that includes the Lentivirus, which causes EIA and the Human Immunodeficiency Virus (HIV, which is the cause of Acquired Immuno Deficiency Syndrome (AIDS). Unfortunately, like AIDS, there is no effective vaccination, and no effective treatment has been made for horses as there is no profit in it.  Thankfully an effective treatment is made for humans with AIDS, which is lifelong.  

The virus transmits through shared needles, blood, milk and body secretions.  The only effective deterrent in horses is constant monitoring for positive cases and isolation or euthanasia. Using sterile single-use syringes and medical equipment is mandatory to prevent the spreading from a shedding horse to a virus-free horse.  Controlling biting flies in the environment is an ongoing problem in the fly season.

In 2019 there were 89 positive cases of EIA in the USA.  It is NOT contagious to humans.  It is less severe in asses and mules, but they, too, are required to be tested for the disease.  For many, EIA is an unknown disease never seen in today’s horses – at least in America.  Let’s keep it this way. Test your horses.

Aren’t you glad you read through this article?  LOL, it’s fun to have “been there” and leave these stories.

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