Heartworm Disease

Heartworms live in the right side of the heart and the large arteries running from the heart to the lungs (the pulmonary arteries).

Heartworm disease causes damage to the pulmonary arteries which eventually leads to heart failure. If you have any acquaintances with chronic heart failure, ask them how much joy there is in their lives. Heart failure robs the dog of its ability to play comfortably. Eventually it becomes difficult to just participate in normal activities. Serious damage begins to occur in other organ systems affected by the heart failure. Treatment of the symptoms alone fails to resolve the problem over the long term and the dog eventually dies -- after having been miserable for some time. Some dogs do manage to live almost normal lifespans despite infection with heartworms but they are very lucky. A rough estimate from our practice experience: 80% of dogs affected with heartworms probably die from the heartworm disease before something else causes them to die and 90% eventually show some or all of the symptoms of the disease.

Heartworm Disease Testing

There are several ways to test for heartworms. The first method developed was to put a drop of blood on a microscope slide and look for baby heartworms, which are microscopic and are known as microfilaria. This works if there are adult heartworms producing babies and if the dog's immune system is not killing the babies. It is probably an accurate test for adult heartworms in about 50 to 60% of infected dogs (just an educated guess on the percentages). The accuracy of this type of testing was improved by using "concentration" techniques --- methods of increasing the likelihood of finding microfilaria if only a few were present. Originally this involved centrifuging blood but later special microfilters were developed to allow filtering of baby heartworms out of a bigger blood sample. This was a better test but still was inaccurate if the heartworms weren't producing babies or if the dog's immune system was killing them. Estimates of the inaccuracy of this testing varied widely but the general consensus was that it missed between 20 and 30% of heartworm cases and I personally think it was probably higher than that. Testing for microfilaria was critically important when daily heartworm medications were in use. Ironically, the presence of microfilaria was actually more important than the presence of the adult heartworms when using these preventatives because reactions could occur that could cause death when daily preventatives were given to dogs with microfilaria in their circulation. There was not much incentive to get more accurate in testing for heartworms in order to use preventative but a number of dogs still had clinical signs of heartworm disease and yet tested negative on available blood tests. Tests for antibodies to heartworms were developed. These were problematic because they indicated exposure to the heartworms, so dogs remained positive on these tests even if heartworms had died naturally or after successful treatment. Finally, tests were developed that test directly for adult heartworm antigen -- but only from adult, sexually mature female heartworms. So now there are two situations in which the testing remains inaccurate -- all male heartworm infections and heartworm infections in which there are female heartworms present that are less than 6 or 7 months of age. There is also a small possibility of a false positive test with the antigen test kits -- estimates are that it is about 1 in 1000.

So how does this apply to your dog? The first test could be accurate and the second test a false positive. The odds of this are low but it is possible. The presence of clinical signs makes the odds very very low, in fact. The test may have been a filtration or concentration test for microfilaria and your dog may not have microfilaria despite having adult heartworms. The test may have been an antigen test and your dog may have been infected at the time of the first test but for less than 6 months -- so there were no adult females to make the test positive. Now, months later, the females have grown up and are causing problems. In any case, treatment is now indicated.

Heartworm Treatment

Heartworm treatment should be much safer for dogs now that a new medication, Immiticide (rx), is available to veterinarians. This medication has fewer side effects and kills a higher percentage of heartworms than Caparsolate (rx), which has been the only medication available until now.

Heartworms live in the heart and the large blood vessels which go to the lungs from the heart. When they are killed by medications, a danger of embolism results if the dead worms block the flow of blood to the lungs. This is a common cause of death during a heartworm treatment. This risk still exists with Immiticide, but to a much lesser extent, due to differences in the timing of heartworm deaths. In addition, Immiticide does not appear to damage the liver or kidneys, which was sometimes a problem with Caparsolate.

If your dog has heartworm disease and you have been putting off treatment due to the higher risk of complications associated with Caparsolate, it would be a good idea to call your vet and discuss this new treatment. Dog feel better after heartworm treatment and their expected life span is the same as a dog that was not affected with this parasite if treatment is successful. Even though there is a small risk of death during treatment, it is still usually a much better choice than not treating for heartworms.