By Tracy Dewhirst

Q: I adopted my black lab last year, and he was positive for heartworms. He had the treatment and has been on Sentinel preventive medication ever since. He just went in for his yearly exam and is heartworm positive again. How common is this? Are these parasites he had from last year, or did the medication not work? I'm just really confused.

A: This scenario is uncommon, but it does happen. It is important to know that the heartworm test can remain positive for four-six months following treatment even if all worms were killed, so review the time line of your dog's therapy. If it has been more than six months and the test is positive, we typically retest the dog using a different testing method to confirm the status.

If the second test is positive, then we can be certain heartworms are present. Here are some reasons for a positive test following treatment:

The majority of dogs clear all heartworms with the standard treatment, but treatment is not always 100 percent effective. The label reads 90-99 percent effective; this is due to variation in each individual animal. One female worm could survive and cause the test to be positive. If the worm burden was large before treatment then perhaps not all the worms died.

A new infection is possible if the monthly preventive was dosed incorrectly, or a dose was missed or accidently not ingested by the dog. Also, the monthly prevention should be started three weeks following the last treatment injection; if there was a delay a new infection is more likely.

Very few medications or vaccines offer complete efficacy; heartworm preventives are no different. The preventions have approximately 97 percent efficacy rate if taken properly, and although it is very rare to diagnose heartworm infections in dogs on the monthly preventions it can occur.

Another unlikely possibility is that the treatment protocol could be insufficient. Veterinarians do heartworm treatment routinely and calculations are simple, but the treatment drug is calculated based on weight and if there was a miscalculation it may not kill all worms.

The options for a dog that remains positive after treatment depends on the animal's health status and the stage of disease at the time of diagnosis. According to some veterinary cardiologists the treatment options are either to continue a monthly preventive for six months and retest, or do another round of treatment. If only one or two worms remain there is not much threat to the cardiovascular system and the monthly prevention may kill them slowly over time.

I recommend you confirm the positive result with a second test. Talk to your veterinarian about the risks versus benefit of retreating or waiting six months and retesting if your dog is truly positive. Also, switch to a monthly Ivermectin product until your dog either has a negative test or gets retreated; these products have a better slow kill rate and have fewer reactions in dogs with circulating microfilaria, the heartworm larvae.