Canine Tick Disease

The first step

In order to make a diagnosis, one of the first things a vet will do is run a Complete Blood Count. A CBC can be very helpful, showing up things like reduced platelets or an increase or decrease in white blood cells. However, it's important to remember that a normal CBC does not mean that a dog is free of a tick-borne disease. The CBC alone is not enough to rule them out. In fact, a negative titer on an IFA or ELISA test is not enough. Tests are only part of a diagnosis.

Titers

The results of the IFA, the western blot, the PCR and ELISA tests are expressed as a titer. (Results from IDEXX tests for Lyme disease are expressed in actual units per milliliter, U/mL, values rather than titers.)

Dr. Tom Beckett explains the process this way.

"To do an IFA, the procedure is to initially dilute the serum sample to 1:20, and then make serial two-fold dilutions from that point (e.g., 1:20, 1:40, 1:80, 1:160, 1:320, and so on.) Beginning with the lowest dilution each dilution is tested until a dilution is reached that does not show a positive reaction. The highest dilution that *does* show a positive reaction is considered the end point, which is commonly referred to as the 'titer'." (Further information from Dr. Beckett on titers is here.)

Any titer number indicates that the dog has made antibodies to combat a given disease. However, a negative titer is not necessarily conclusive in determining that a dog doesn't have that disease. Mistakes in handling the serum sample can result in a negative titer...or a positive one, for that matter.

If tests are run too soon, it could be that the dog has not had time to make enough antibodies to be detectable.

If the disease is far advanced, the immune system may be failing and no longer able to make antibodies.

In the end, diagnosis often depends most on the knowledge, judgment and experience that your vet brings to treating your dog. Tests can often give him information he (or she) needs to identify and treat a disease but they are not "all there is" to a diagnosis.

If a test comes back negative but everything he sees when he looks at a dog is telling him this is a dog with Lyme, erhlichiosis or RMSF, most likely your vet will advise putting your dog on doxycycline. If the dog improves almost at once, in a matter of days, you have a presumptive diagnosis right there and you know to keep him on the antibiotic as long as necessary. (Babesiosis canis would probably be treated with Imizol.)

"The most reliable indices of response to therapy are clinical signs, CBC changes (i.e. anemia, platelet counts) and serum globulin concentrations. A reasonable approach is to monitor recovered dogs with a CBC and titer at six-month intervals as long as the dog remains clinically normal. If clinical signs develop or the CBC becomes abnormal, re-treat, using Imizol if doxycycline was used the first time around. The majority of Ehrlichia dogs will require only one treatment course, but owner and veterinarian should remain watchful in order to recognize a relapse." Suzanne Stack, DVM

Testing for Lyme Disease

IDEXX Laboratories has patented two tests which look for the C6 antibody the immune system creates in response to the C6 antigen which occurs only in active cases of Lyme disease. Unlike other tests, these do not react to the Lyme vaccine so there is no question of a false result caused by vaccination. Both are available only from IDEXX.

The Snap3DX is a test that your vet can do in-house. It is an inexpensive way to check for Lyme, Ehrlichiosis canis and heartworm. A positive for Lyme disease on this test is strong evidence that the dog actually does have it so, after a CBC, the Snap3DX should probably be your next step. From there, if the Lyme portion of the test is positive, you would go on to the Quantitative C6.

The Snap3DX is a screening test and may return a false negative if the UmL is below 1:256; in that case, an IFA could be run to clarify the result if Lyme is still suspected.

The Quant C6

Since the Snap3DX is a yes/no test which can only tell you if antibodies are there or not, your next step is to run a Quantitative C6 Assay Test. This is a more advanced test that gives you a baseline reading in units per milliliter of the number of C6 antibodies in your dog's serum sample. Knowing how many antibodies were found when you started treatment gives you a way to determine if treatment has worked when you retest 6 months later. If the number has dropped considerably, IDEXX says by 50%, you can safely assume it has.

This test is done only at IDEXX and your vet has to open an account with them to have it done.

How the Test Works

The IDEXX explanation of how their test works wasn't easy for me to understand so I turned to a friend, Nancy Campbell, a Registered Veterinary Technician, to help me understand what's going on. She explained it this way: The Lyme spirochete is adept at making changes (turnover) in its outer surface proteins (Osps) to escape detection. One portion of the surface of the Lyme spirochete, however, the C6 peptide, a distinctive compound of amino acids, always stays the same no matter how much the rest of the spirochete changes. This means that the antibodies created in response to the C6 peptide are also a constant and can always be identified. These are what the IDEXX test looks for.

Vaccines don't contain this peptide so the IDEXX tests won't give a false positive if the dog has ever been vaccinated. Other disease bacteria don't have this peptide either so, if one of those diseases is present, the IDEXX tests for Lyme won't pick up on any of them.

The IFA Lyme titer and the Western blot don't specifically pick out the C6 antibodies. Because they don't, because they look for Lyme antibodies in general and the immune system may keep churning those out long after it's necessary, it's difficult to determine from either of these tests if the spirochetes, the actual disease organisms, are still there or gone.

The Western blot may or may not be able to differentiate the Lyme vaccine from the actual disease.

"The c6 antibody tests are a real advance over previous tests.
Culturing for the Borrelia is a complex and pretty unrewarding exercise. PCR tests for Borrelia DNA are also complex and negative results are highly equivocal. The aggregate of antibodies measured by previously available tests tended to stay elevated whether the dog had a current infection or some previous brush with B. burgdorferi. Significant levels (>30 U/mL) of c6 antibody are highly specific for active infection."

Tom Beckett, DVM