Page 88 - Speedhorse, December 2018

disease in the Northeast is a good example of this,” Arbittier explains.
When trying to determine if a specific agent is causing disease, it is crucial to know how this pathogen affects the horse. For example, some bacteria cause disease by invading cells of a particular body system; they are intracellular organisms, making it difficult for tests to find them in the blood. Testing in these cases will focus on the response of the horse’s immune system, such as performing a serum titer for particular antibodies that have been made in response to the invading bacteria.
Even if these antibodies are detected, this doesn’t always mean that the horse has the disease. It may just mean the horse has been exposed to the organism at some point— either through vaccination or the actual disease and has created antibodies.
“The disease bloodwork falls into two categories,” says Wilson. “One is testing for antibodies against that disease, produced
by the animal after being exposed to that disease. The other category includes tests that look for the organism itself. By far, the majority of tests are simply testing to see if antibodies are there. It depends on the dis- ease, but in general terms this simply means that the animal had the disease at some point in the past and developed antibodies to fight it, or it could indicate that the ani- mal actively has the disease right now. With some diseases, it can sometimes be difficult to determine which scenario this is.
“To some degree, the level of increase
in antibodies may indicate recent infection versus long-term infection. Before new tech- nology and better tests were developed, we measured antibodies with a test done when we were first looking at the animal, and again two weeks later to see if the antibody level increased or decreased. If it increased, this would mean that a disease process was going on at that time,” explains Wilson.
In the real world, this is not very helpful, however, when trying to figure out imme- diately what’s wrong with the animal and treat appropriately, rather than having to wait two weeks and do another blood test. Today, sometimes different tests can be run for varying disease processes, and sometimes you have the option of multiple types of
tests for one disease process, according to Arbittier. “For example, there are many ways to look at antibodies. Some commonly-used methods in equine medicine are the enzyme- linked immunosorbent assay (ELISA), Western Blot, and indirect fluorescent anti- body testing (IFAT),” she says.
A different type of commonly used diagnostic test is a polymerase chain reaction (PCR). “This test does not look at antibod- ies, but instead uses amplification of DNA to diagnose bacterial and viral infections that are difficult to diagnose by other methods. Many disease processes have multiple types of tests available to help diagnose them and new literature is constantly being published as to which methods are most reliable.”
One common misconception is that a posi- tive blood test always means that the horse has the disease. “Many tests that have a positive result may indicate that the horse has been exposed to the disease and has mounted some kind of antibody response, but this does not confirm clinical disease,” Arbittier says.
“Two examples that we test for fre- quently in my area are Equine Protozoal Myeloencephalitis (EPM) and Lyme Disease. Two commonly run blood tests for EPM are the IFA and the ELISA. UC-Davis runs the IFA and explains that the tests look at anti- bodies to both S. neurona and N. hughesi and states that these tests, ‘provide quantitative indication of EPM infections by providing actual titers and a likelihood ratio of dis-
ease correlating to the titer.’ The second test commonly used is the ELISA, run by Equine Diagnostic Solutions in Kentucky,” she says.
“The ELISA test, when used to check for EPM, measures antibodies created against the antigens SAG 2, 3, and 4, which are uniquely expressed on the surface of S. Neurona. There are multiple other tests that may be used to aid in the diagnosis of EPM as well,” she says.
The use of tests may vary between practitioners, but this does not mean one test is better than another or that your vet- erinarian is right and another is wrong. “If there are multiple tests that vets are widely using, it is because no one test has proven to be clearly the most useful and reliable. Every test has advantages and drawbacks,” explains Arbittier.
Polymerase Chain Reaction (PCR) is a diagnostic test that uses amplification of DNA to diagnose infections such as tick-borne Lyme Disease.
It is very important to understand that
a positive result does not necessarily mean that the horse is suffering from EPM.
This simply means that the horse has had exposure to the causative agent. “The blood results must be correlated with the clinical picture of the horse to decide upon whether treatment is indicated,” she says.
Testing for Lyme disease must be per- formed with similar caution. “Antibodies present in the blood do not definitively show that the horse is suffering from infection with B. Bergdorferi. It simply shows that exposure has occurred,” she says.
Treating the horse in response to the
lab work without correlating the clini-
cal picture of the horse can confuse the
case and prevent a proper diagnosis from being reached. “Doxycycline is a common treatment for Lyme disease and it has non- specific anti-inflammatory properties as well as anti-microbial properties. It can act like an NSAID, giving your horse relief with- out curing the underlying problem. This improvement in the horse’s clinical picture can give the owner a false sense of having treated the problem when it may only be masking it,” Arbittier explains.
Running bloodwork is a very valuable and affordable way to rule in or rule out certain disease processes, monitor response to treatment, and have a healthy baseline for your horse. Care should be taken in interpreting certain test results, so the horse owner realizes that exposure should not be confused with disease.
“Running bloodwork is a very valuable and affordable way to rule in or rule out certain disease processes and monitor responses to treatment.”
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EQUINE HEALTH