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TESTING FOR HEARTWORMS

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 preventive 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.

It doesn't make any difference at all if you wait the extra three days. It is slightly safer to treat dogs by injecting one injection of Immiticide (Rx) and then waiting a month because the first injection kills off the weaker worms, spreading out the effect of the medication. The two followup injections kill the stronger heartworms (usually the young females). It is OK to treat just by giving two injections one day apart as the whole treatment but since that kills all the worms at once it makes the possibility of complications associated with worm death higher. You don't want your dog to have heartworms longer than necessary to ensure safe treatment but a few days either way isn't a big factor.

It is physical exertion that complicates heartworm treatment. If your dog is calm then staying with the family is no problem. Some dogs are more active when they are confined (digging at the gates, crates, etc.). Those dogs need to be with their family instead of being confined. Other dogs jump up and down, on and off the furniture and generally get so excited around their family that confinement is the only way to help them through a heartworm treatment. You have to do what works best to keep your particular dog quiet. While I have not seen it published anywhere I can think of I am absolutely convinced that coolness is important too -- keep the air conditioning on!