Does your dog have a “Touch” of Heartworm?

Does your dog have a “Touch” of Heartworm?

By Dr. Ed Mapes
Stonebridge Animal Hospital
McKinney, Tx

When a lady has a pregnancy test done, and the results are positive, she is never termed to be a “little bit pregnant”. Likewise, positive tests for canine heartworm disease are just as positive; there’s no such thing as having a “touch of heartworm”. The degree of color change in the test does coincide with the amount of antigen present, but has far less bearing on the potential severity of disease from the infection. In that sense, these tests are either positive or negative without regard for how strong the color change happened to be; and all dogs testing positive should be assessed for their fitness to be treated to eradicate the parasites.

Tests for canine heartworm disease detect the presence of heartworm antigen – proteins on the worm to which the reagents in the test kit react to cause a color change. A positive test though, no matter whether it is a “strong” or “weak” indicator, does not indicate the severity of disease that can develop. A dog that tests positive without a strong indication can still have significant damage in the heart and lungs. These dogs should still undergo treatment to eradicate the adult heartworms and their offspring (microfilariae) from the bloodstream for two important reasons:

1) They serve as continual reservoirs of infection; a mosquito that bites and takes a blood meal from them will then be able to pass that infection along to another dog or cat.

2) Many of the symptoms seen in dogs with heartworm are actually due to their own body’s response to the worms. The immune system detects foreign proteins on the surface of heartworms and mounts a response. The body reacts in several ways; antibodies are produced in attempts to kill the invader, cells in the blood stream are mobilized to attack the worms in an attempt to eradicate them. So the degree of disease that results is not only due to the worm load present, but how the body reacts to those worms.

The many components of an immune response causes inflammation, fluid accumulation in lung tissue, a flood of various cells to the region, discharges into small airways, attempts to wall off the worms, and a host of other activities. These reactions are largely responsible for symptoms such as coughing, decreased ability to exercise, fever, secondary bacterial infection, decreased appetite, and lethargy.

The degree of immune reactions to invaders varies between individuals; dogs with small worm burdens can become severely ill with profound symptoms because of a particularly strong immune reaction. Appropriate testing and treatment can be more important in these cases than in some other with “strong” positive tests.

Whenever we have a positive heartworm test, the next step taken is to verify that result with a second test using different reagents. We also look at blood under the microscope to identify the presence of microfilariae. After confirmation, blood testing is done to assess for disease that can arise in other organs.
We also do chest X-rays to assess pathology in the heart and lungs; this is sometimes followed up with an EKG and blood pressure assessment. This information allows us to quantify the degree of illness and assure that the patient is medically stable for heartworm treatment to eradicate adults from the heart and lungs.

Any lady testing positive on a pregnancy test should be taken seriously, professionally evaluated, and then receive appropriate pre-natal care. Likewise, any dog infested with heartworm, regardless of the degree of color change, deserves to be evaluated and, if medically stable, treated to eradicate the worms.

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