Once a horse becomes infected, they can enter one of 3 stages of disease. The first stage is the acute stage. This is a sudden onset of the disease with symptoms such as high fever, anemia (breakdown of red blood cells), swelling of the limbs, weakness, and even unexpected death. The subacute stage is less severe, but with similar symptoms. The disease will tend to progress at a slower rate than the acute phase and will often enter the chronic phase. The chronic phase is also known as a carrier phase. The horse may seem fine, but still carries the virus. He may tire easily and may have a recurrent fever and anemia. Relapses into the acute or subacute phases can occur years after the original attack. Most horses that test positive for EIA are not showing outward signs.
The Coggins test is named after Dr. Leroy Coggins. He developed the lab test to diagnoses EIA at Cornell University in 1970. The procedure does not detect the virus, but it detects antibodies to virus. An antibody is a protein built by the immune system to attack foreign substances, known as antigens. The immune system only develops antibodies if an antigen is presented to it. In other words, if the horse has never been exposed to EIA, then it will not have antibodies against it.
The test that Coggins developed is called an Agar Gel Immunodiffusion test. What this means is that the serum (blood sample) is placed into a hole in a petri dish. If antibody (Ab) is present in the blood sample, as it diffuses out of the hole and into the agar gel, it will bind with the antigen (Ag) and form a precipitate. This is a positive test. If there is no antibody, then there is no precipitation, or the animal is negative. (See diagram below). This test takes about 2 days to complete. The USDA made it the official EIA test in 1973.
Does every horse need a Coggins test? Actually, no. While it is recommended, if you maintain a small backyard herd that does not travel, the test may not be necessary. Keep in mind, though, that if one of them develops a fever and lethargy, they should be tested.
While we don't see a lot of EIA anymore, it is still around. Canada saw a resurgence of the disease just last year, with 179 horses being affected. If you're travelling with your horses, it's important that you have an up to date coggins certificate. Also, make sure that any horses that are coming onto your farm have been tested (and were negative!) for EIA. It may be difficult to eradicate this disease, but we can all help to prevent its spread.