When Heartworm Tests Don’t Agree
Bennie is a case of a dog with heartworm tests that can’t agree with each other.
Bennie’s owner was shocked and amazed a few weeks ago when he got a diagnosis of a positive heartworm test from his regular veterinarian. Bennie had gotten his heartworm preventive on schedule every month, and was using the heartworm preventive his doctor had recommended. “How could this be?” his owner asked in frustration.
Several scenarios fit Bennie’s case.
One is that all medications have a failure rate, and heartworm preventives in general have a failure rate of between one and two percent. Likewise, we all know people whose children were conceived while on birth control medication.
Another scenario is that sometimes pets don’t actually swallow their heartworm preventives.
Third, conflicting test results may be explained by a “low worm burden.” The heartworm burden is the number of adult heartworms present in the body.
Heartworms, like people, come in male and female genders. Heartworm tests, in order to be highly accurate and specific, “look” for molecules from the adult female heartworm’s uterus. If none are found, a negative test results. It is generally accepted that most heartworm tests require between one and three female heartworms in order to generate a positive result. If a dog has 20 adult male heartworms and no females, we will still get a negative result on a blood test because none of those heartworms has a uterus. If there are but one or two female heartworms some tests may give us a positive result, while others don’t.
That’s probably what is happening with Bennie.
After the positive result at the other veterinarian’s office, Bennie’s dad asked us to perform a test, too. Ours was also positive. We proceeded to explain that sometimes the above scenarios play out and we should begin the process of heartworm treatment, which starts with routine blood and urine testing as an overall health screen, and a chest X-ray to determine the amount of damage to the heart and lungs.
We were not too surprised when Bennie’s chest X-ray looked good, as he is asymptomatic and knew he probably had a low worm burden. He had a little enlargement of the heart, which could have been heartworm-related or age and obesity-related, or a combination of causes.
What did surprise us was that the heartworm test that was part of the panel or “package” we submitted was negative. We backtracked and discovered that the brand of test we used was different from the brand the first veterinarian used and both of those were different brands from the one the laboratory used.
Our conclusion is that Bennie has a low worm burden, probably two or three female heartworms with a small but unknown number of male heartworms. Our course of action was to abort the heartworm treatment, keep him on heartworm preventive and retest him in six months.
Oh, and begin to deal with the obesity problem in the meantime.
See you tomorrow, Dr. Randolph.