Heartworm Disease in Dogs

The prevention and symptoms of canine heartworm disease from transmission by mosquitoes to death.


Canine Heartworm Disease is not a new problem. It is a preventable problem that has been around for several decades killing thousands of dogs every year.

Heartworms are caused by mosquitoes, which have previously bitten an infected dog or cat. When the mosquito bites an infected animal, a small amount of blood is taken. Within this blood are tiny, microscopic, immature heartworm larvae. When this mosquito bites another animal, the larvae are mixed injected into a new dog’s tissue. They then migrate to the bloodstream where they will continue to develop over the next six months.

Adult heartworms lodge in the right side of the heart disrupting the efficiency of the heart itself. Each individual adult heartworm can be up to 18 inches long taking up space that normally is filled with blood. The heart, in turn, has to pump harder to push the same amount of blood through the body. Depending on the size of the heart, there can be as many as 100 heartworms lodged in the heart causing permanent damage to both the heart itself and the lungs, until death occurs. In fact, damage can occur with only one adult heartworm inhabiting the heart.

Symptoms of heartworm disease in dogs are vomiting, difficulty in breathing, coughing, lethargy, murmurs, and heart failure. It can also cause swelling of the abdomen or extremities and sudden death.

Regardless of popular belief, indoor dogs CAN become infected with the disease. Another misconception is that heartworms only occur in the southern part of the United States. The simple fact is, heartworm disease has become a prevalent problem in all 50 states.   

Canine heartworm disease is easy to diagnose. A blood sample is drawn from the patient and the results of tests can be had within minutes.  Sometimes, a drop of blood is placed on a slide, covered with a coverslip and checked under a microscope.  If the dog has a heavy infestation, micro-filaria (baby heartworms) is often present. They appear looking like hundreds of tiny worms moving all over the field of vision.

Another way to check is by using a Difil test. This involves mixing the blood with a special solution. It is then pushed through a special filter screen that a purple dye is added to later. This is then looked at through a microscope and the worms appear to be strands of hair caught by the screen.

Both of these tests check for micro-filaria. Sometimes, there is none present and the animal can still have heartworms. Then it is time for a test that checks for heartworm antigens. The blood is mixed with solutions then placed into a test device. This device works similar to a home pregnancy test. It is sensitive enough to pick up antigens even if only a couple of heartworms are in the heart.

A lack of micro-filaria can be for different reasons. The most common is that the adults haven’t been in the heart long enough to make the babies. Another reason can be the sex of the worms involved. As with almost any other living thing, males and females are required for reproduction. It isn’t unheard of for all the inhabiting worms to be one or the other. Regardless of the reason, treatment is necessary to prevent physiological damage to the dog.

Treatment involves an extensive battery of blood tests. It is very stressful for the dog so tests are done to check the heart, liver, and kidney functions to give the veterinarian an idea of the chances of survival.
Treatment is not a guaranteed thing. Although rare, the death of the patient can occur.

When the treatment is commenced, the dog is hospitalized for at least three days. During this time, injections are given on a daily basis to kill the adult heartworms. The injections are formulated to do this over an extended period of time so all the worms won’t die at once and plug the heart. The adult heartworms die, pass from the heart to the lungs where they dissolve over several weeks.

Three weeks after the in-hospital treatment, the dog comes back in to receive an oral dose of the drug Ivermectin. This drug is used to kill the micro-filaria. Many veterinarians will want another follow up in two more weeks to run tests making sure all the adults and micro-filaria are gone. If they are, preventative medication is then prescribed and a schedule of a once a month dosage is implemented to ensure the dog a longer, healthier life.