Heartworm testing is commonly done to screen patients prior to beginning or continuing a heartworm prophylaxis program or to determine infection in symptomatic animals. Antigen testing is considered by the American Heartworm Society's 2002 guidelines to be the most sensitive diagnostic method available. This paper reviews currently available microfilarial and serologic laboratory testing. There are several methods of detecting Dirofilaria immitis microfilariae in the bloodstream. They can be differentiated from Acanthocheilonema reconditum (formerly Dipetalonema) microfilariae by size, shape, and motility in a direct smear, wet mount, hematocrit tube (at the buffy coat), and by concentration methods. Antigen tests check for the presence of adult female worms. Animals with few worms or male-only infections will test negative. The specificity of current canine antigen tests ranges from 94% to 100%. False positives are most likely due to laboratory error. Sensitivity estimates vary from 30% to 94% in one study and from 61.9% to 100% in another. The high number of false negatives is attributable to factors such as early infection, presence of male worms only, low worm burden, or improper technique.
Antibody tests are also available for cats. Cats with heartworm disease rarely have microfilaremia, and often have low worm burdens, making antigen testing difficult. Antibody-positive cats may have a prepatent infection or an infection with adult heartworms, past exposure but none currently present, or a false positive result.
COMMENTARY: Testing for heartworm infection can be problematic and this is a helpful guide to troubleshooting. For further information, see the recommendations of the American Heartworm Society at www.heartwormsociety.org.
Update on canine and feline heartworm tests. Datz C. Compend Contin Educ Pract Vet 25:30-41, 2003.