Heartworm Blood Test

Pet Tales
by Barry B. Burtis  D.V.M.

Heartworm Blood Tests

Over the next few weeks, many dogs in our area and throughout Ontario will be giving blood. It will be only a small amount of blood, usually about 2 1/2  ml. or one-half a teaspoonful. Most dogs will be very brave, complain very little with the painless collection procedure and it will be all over in minutes. It's too bad we can't explain to them the important information we can learn from the sample they donate.

I am referring to a test done on a portion of the blood collected to check for the presence of canine heartworm. Dirofilaria immitis is the formal name for this parasite that has posed a health threat to dogs living in this part of Ontario for the past 25 years. I'm sure most dog owners have heard about the disease. The parasite is spread between dogs or other members of the canine family by mosquitoes. The adult worms live in a dog's heart and anyone can imagine the seriousness of having 5-10 cm. long worms in the chambers of the heart.

But, let's get back to the blood testing. If a dog becomes infected, a blood test is the best way to detect the infection. Hopefully, if heartworms are present, the positive blood test alerts us to the fact before serious heart disease results and while treatment can still be safely considered. Most dog owners probably are not familiar with the details concerning this very important blood test.

The blood is collected through a sterile, disposable needle and syringe from a peripheral vein - usually the cephalic vein that runs up the top surface of a dog's front leg between the carpus (wrist joint) and the elbow joint or from the jugular vein located at the front of the neck. It is then transferred to a sterile glass collection tube. The tube will be identified with the dog's name, its owner's surname and the date of collection.

What happens next will differ between veterinary hospitals. Some clinics may run the blood test in house; others will send the sample to an outside laboratory. Vita-Tech is one of those laboratories frequently chosen by veterinarians to do heartworm testing as well as a large number of other diagnostic laboratory procedures. With facilities in Markham, Ontario and Dorval, Quebec, Vita-Tech has been supporting the animal health community with veterinary diagnostic services since 1984. It is a full service laboratory that employs a diverse team of highly qualified professionals including pathologists, microbiologists, parasitologists, molecular scientists, veterinarians and laboratory technologists. It is respected for its high standards of quality and accuracy.

Vita-Tech provides twice daily pickup of laboratory specimens for veterinary hospitals in our area.  When the carefully labeled sample arrives at the lab it is further identified with a bar code to prevent errors in tracking. When a sample for heartworm testing arrives in the hands of the Vita-Tech technologist actually doing the test, an antigen ELISA test is performed on the sample. This is now the most common type of test used to check for heartworms. ELISA stands for enzyme-linked immunosorbent assay. The test makes use of natural reactions between antigens or immune stimulating proteins and antibodies, the immune system's response to an antigen. In the heartworm test, the protein identified is produced by adult heartworms, and in greatest quantity by adult female heartworms. At the lab, blood samples are tested by exposing the sample to heartworm antibodies. A blood sample containing heartworm antigen reacts with the heartworm antibodies and a positive test results. No reaction between the sample and the test materials means no antigens in the sample and the test is negative. When tests are positive, Vita-Tech examines the sample for evidence of microfilaria, the microscopic sized, infective stage of juvenile or immature heartworm that circulates in the blood. It is a filtration test that allows a determination of the number of microfilaria per milliliter of blood. This is one of the factors necessary for veterinarians to determine how to best treat an infected dog.

In 2005, Vita-Tech tested over 160,000 blood samples from dogs living in Alberta, Manitoba, New York State, Quebec and Ontario for evidence of the heartworm parasite. There were 244 positive cases from 98 communities in Ontario. The greatest concentration of infected dogs lived in the Golden Horseshoe region of Lake Ontario. Dog owners should be sure their pet is not infected and take action to protect them during the upcoming heartworm season.