The horse's immune system is a window to its world. The biochemical analysis of blood components can give the veterinarian valuable information about disease exposure and immune responses. When a horse is exposed to an antigen (a disease-causing substance, often a microorganism, that stimulates the immune system), the immune system responds by producing antibodies to block infection. For example, B lymphocytes are specialized white blood cells that secrete antibodies responsible destroying invading microbes. They produce immunoglobulin proteins (antibodies) such as IgG.

Antibodies destroy foreign infectious agents including viruses, bacteria, fungi, parasites, and foreign proteins. The type of antibody can be measured to evaluate the horse's disease-fighting capability. A quantity of particular antibody is referred to as a titer. Measuring the quantity of IgG is a common procedure in the neonatal foal to assess the degree immunoglobulins the foal has received through the colostrum.

Monitoring for disease exposure is commonly done using serum biochemical analysis. An example of this is when a horse has a Coggins test to verify equine infectious anemia (EIA) status. The veterinarian collects a blood sample and submits the serum (a portion of the blood containing proteins, including antibodies and nutrients for the body) for EIA antibody analysis. If the horse has been exposed to--or is infected with--the EIA virus, the test will show a positive response for antibodies. The two tests used for this analysis are agar-gel immunodiffusion (AGID) or the enzyme-linked immunosorbent assay (ELISA).

The ELISA test is fast, very sensitive, and has a high probability of detecting the disease, but it also can give a false positive result (tells you the disease is present when it isn't). The AGID test takes longer and is less likely to yield a false positive. It is often used to confirm a positive ELISA for EIA.

Sensitivity and Specificity

Based on principles of sensitivity and specificity, you can be reasonably sure that a diagnosis is accurate.

A sensitive test will detect a high proportion of a diseased population. In a population of 200 horses where 100 are diseased, a test with 98% sensitivity will detect 98 positives and miss two (two false negatives).

Specificity is the probability of a negative test among patients without disease. In a population of 200 horses where 100 are sick, a 98%-specific test will diagnose 98 healthy horses and 102 sick horses (two false positives).

Historically, some test methods have proven more accurate than others in detecting the actual presence or absence of disease. Therefore the clinician might elect to submit your horse's serum to a particular laboratory for a particular testing method.

An example of this is testing for equine protozoal myeloencephalitis (EPM). As the disease has become more recognized, the testing method that shows the most reliability in detecting the it is the Western Blot test (or immunoblot). Not all commercial laboratories run a Western Blot for EPM, so a veterinarian might send the sample to a lab that specializes in this test.

The polymerase chain reaction (PCR) test is in the news because this antigen testing method is developing as a fast and accurate way to identify the DNA of specific infectious organisms. As scientists identify DNA primers or specific DNA sequences for the more prevalent disease organisms, PCR testing might become more accessible and offer a faster, more accurate diagnostic option. This method offers another significant advantage because it can be used on readily accessible samples, such as blood and saliva, versus cerebrospinal fluid or post-mortem tissues.

Take-Home Message

When faced with managing disease risk, it is useful to be armed with the vocabulary and basic theory of immunology. Realizing one test might not depict a disease in black and white might help an owner make sound decisions for the individual horse and the rest of the population. The study of disease and immunochemistry is moving at a fast pace, all the while providing significant benefit to the horse.

About the Author

Kimberly Peterson, DVM

Kimberly Peterson, DVM, is an AAEP member and assistant professor in the Department of Veterinary Technology at Morehead State University in Morehead, Ky. Her husband, Eric, is an equine practitioner, and their family lives in Lexington.

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