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
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 dog 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.