Canine heartworm is widespread throughout the world, but mainly occurs in the tropics, subtropics and some temperate areas. It has been reported in all 50 of the states. It is found in dogs, cats, foxes, wolves and other wild carnivores as well as in sea lions and humans. Because of this, there is no reason to test puppies less than 6 months old. The two basic methods for detecting heartworm infection are microfilaria testing and antigen testing.
Antigen tests look for the presence of adult female worms.
These tests are almost 100 percent specific for Dirofilaria immitus , but may be falsely negative if there is a low burden of worms. Microfilaria tests are done by Knott tests or membrane filtration tests. These tests are also very specific, but may be negative if the adult worms are not producing offspring or if the body is destroying the offspring. Recently it has come to light that dogs on monthly heartworm preventative medications will become microfilaria-negative if they are infected with heartworms before or during preventative administration. Therefore, all animals on a monthly preventative should be tested by antigen tests.