D319 - Canine heartworm
(Dirofilaria immitis)

Description:

Canine heartworm infection causes disease primarily in the small pulmonary arteries and secondarily in the heart. Dirofilaria immitus adult female worm is 6 to 14 inches (15.2 to 35.6 cm) long and 1/8 inch (3 mm) wide; the male is about half the size of the female. One dog may have as many as 300 worms. The adult worms live in the peripheral branches of the pulmonary arteries and produce large numbers of microscopic offspring (microfilaria), which circulate throughout the bloodstream. Heartworm disease is caused by the body's immunologic reaction to the presence of the adults and offspring. As the body produces a number of different substances to destroy these foreign proteins, the vasculature system of the lungs is damaged.
The heartworm life cycle begins when a dog with circulating microfilaria is bitten by a mosquito. The mosquito takes up microfilaria with its blood meal which then mature into infective larvae within the mosquito. When the infective mosquito bites a dog, larvae are injected into the dog's skin. Once in the dog, the larvae migrate and mature into adult worms in the blood vessels of the lungs. The adults (male and female) produce microfilaria after about six months. Canine heartworm is widespread throughout the world, but mainly occurs in the tropics, subtropics and some temperate areas. It is found in dogs, cats, foxes, wolves and other wild carnivores as well as in sea lions and humans.

Diagnosis:

Heartworms must reach maturity (about six months from infection) before tests for heartworm antigens or microfilaria become positive. Because of this, there is no reason to test puppies less than 6 months old. The three basic methods for detecting heartworm infection are microfilaria testing, antigen testing and PCR testing.
Antigen is a test performed on a blood sample. It is the most widely used test because it detects antigens (proteins) produced by adult heartworms. It will be positive even if the dog does not have any microfilaria in the blood; this occurs about 20% of the time. Dogs with less than five adult heartworms will not have enough antigen to turn the test positive, so there may be some false negative results in early infections. Because the antigen detected is produced only by the female worm, a pure population of male heartworms will give a false negative, also. Therefore, there must be at least 5 female worms present for the most common test to be positive.
Blood test for microfilariae: a blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. The number of microfilariae seen gives us a general indication of the severity of the infection. However, the microfilariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Approximately 20% of dogs do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm. Also, there is another microfilarial parasite which is fairly common in dogs; on the blood smear, these can be hard to distinguish from heartworm microfilariae.
Using a DNA-based assay capable of sensitive and specific identification of the Dirofilaria immitus genetic material in blood specimens. PCR (DNA) test has several advantages over other biochemical and serological tests. The test results are independent of the age or previous infections of the animals. DNA is very stable and only smallest quantities are needed. PCR assay is so sensitive that even a single heartworm cell can be detected and can be use for early detection of heartworm infection. This assay can be done in 24 hours. Positive DNA result confirms infection.

Sample:

Whole blood (3 ml) in a lavender top (EDTA) tube.

Special Handling:

Store blood sample at 4°C until pick up or shipment.

Test Code:

D319

1. Narine et al. (1999) Pulmonary presentation of Dirofilaria immitis (canine heartworm) in man. Eur. J. Cardiothorac. Surg. 16: 475-477.
2. Favia et al. (1998) Advances in the identification of Dirofilaria repens and Dirofilaria immitis by a PCR-based approach. Parassitologia. 39: 401-402.
3. McCall J.W. (1998) Dirofilariasis in the domestic ferret. Clin. Tech. Small. Anim. Pract. 13: 109-112.