As a pet owner it is devastating to find out your dog is infected with heartworms. This article sets out to help you understand heartworm test results and also to explain why some dogs still test positive after undergoing treatment. If this is a worry for you, please read on as understanding the mechanics of heart worm testing will help you understand why this may sometimes happen.

With any type of laboratory testing, certain conditions may exist that will cause laboratory tests to fail. In many cases, test results can indicate a false positive or false negative result.

This is true for human and animal laboratory testing. However, these false results are rare. False results typically occur due to human error in testing such as improperly following manufacturer guidelines, or the specimen is contaminated.

Veterinarians follow strict protocols for testing. In fact, most veterinarians will request an X-ray if a dog tests positive as this helps confirm initial test results.

Currently, the most accurate heartworm test for dogs is the antigen test. The test will detect the presence of specific antigens of the adult female worms. Most of these tests will detect the antigens of female worms that are at least seven or eight months old. If the worms are less than five or six months old, the antigen test may indicate a negative result. In addition, if the dog is infected with larvae (baby heart worms); the antigen test may be negative. However, as the worms mature, your dog may test positive in the future. If you would like to read more about antigen testing, use the link to visit an article on the American Heartworm Society web site.

Many veterinarians use a pre-screening test to detect larvae in the dog’s blood. The dog’s blood is smeared on a glass slide and viewed under a microscope. Your dog may test positive for larvae but the antigen test may be negative. Larvae may be detected in the blood, however due to the life cycle of the worms, they may not be mature enough for the test to be able to detect the antigens. If your dog is positive for larvae, your veterinarian will recommend re-testing in six or seven months. At this point, the adult worms will be mature enough to be detected by the antigen test. If at this time the dog tests positive, your veterinarian will probably request an X-ray to confirm the infection. Once confirmed, your veterinarian will determine if your pet is a candidate for heart worm treatment.

My Dog’s Heartworm Test is Still Positive After Treatment!

Once your pet has undergone treatment, another antigen test will be performed. It is possible the test will be positive. However, this does not mean the treatment was unsuccessful. The current Immiticide treatment for heartworms has a high success rate.

The problem with the antigen test is it does not determine whether live adult worms are still present. It can only detect the antigens and these may be present in the blood stream for 9 to 12 months after treatment.

If the heartworm test is positive after treatment, it is unlikely your veterinarian will recommend that your dog undergo treatment again. The typical recommendation will be to keep your dog on preventative heart worm medicine. This will keep your dog free of larvae and reduce the potential of developing adult worms. If you keep your dog on preventative medicine, it is likely the next antigen test will be negative.

There are several documented cases of dogs that continued to test positive for the heart worm antigens three to four years after treatment. The dogs did not have adult worms; however, the antigens continued to be present. These cases are uncommon, but they do occur. It is important to distinguish the difference between a heart worm and its antigens. Just because antigens are present after treatment, it does NOT mean the treatment was unsuccessful.

Depending on the severity of the infection, treatment will eliminate 90 to 98 percent of the adult worms. If your dog has two or three worms, the treatment will likely be 100 percent successful. A dog can have two worms or it can have a hundred worms. This depends on how long the dog has been without preventative medicine. It also depends on how long the dog has been without the benefit of a veterinarian examination. Most veterinarians perform testing annually during the dog’s annual check-up. If your pet visits the veterinarian annually, it is unlikely your pet will have a severe infection. In most cases, dogs will have a few worms instead of hundreds.

If you have had your dog treated for heartworms and the antigen heartworm test results are still positive, discuss treatment options with your veterinarian. Your veterinarian will probably recommend annual testing after the initial treatment. If you follow your veterinarian’s instructions and keep your dog on heartworm preventatives, it is likely your dog will test negative within the year after treatment. While a positive test after treatment can initially be very worrying, I hope this article has explained why this does not necessarily mean your dog still has heartworms. Follow your Vet’s advice and be diligent with your dog’s preventative medicine and your beloved pet should have a long and happy life.