Canine Heartworm Infection

Heartworms are parasites that live in a dog’s heart or pulmonary arteries. They are nine to 11 inches long and look like angel hair pasta. Heartworm infection, also known as dirofilariasis, is a serious and potentially fatal disease in dogs.

Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog, it deposits heartworm larvae into the body. The larvae migrate for several months before ending up in the right side of the heart or the pulmonary arteries. Once the parasites mature (about six months from the time they enter the dog's body), they begin to release immature heartworms, known as microfilaria. Microfilaria live in the dog’s blood for about one month and may be ingested by mosquitoes feeding on the dog. 

Because of the parasites' life cycle, it is necessary for a dog to be bitten by a mosquito to be infected with heartworms. Heartworms are not transmitted directly from one dog to another nor from a cat directly to a dog. 

Canine heartworm infection occurs all over the world. In the United States, it was once limited to the South and Southeast regions; however, the disease has spread and is now found in most U.S. regions and Canada, particularly in areas where mosquitoes are prevalent.

It can take a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in four- to eight-year-old dogs. Heartworms are seldom diagnosed in a dog under one year of age because the larvae take up to seven months to mature following establishment of infection.

Adult worms cause disease by clogging the heart and major blood vessels. They also interfere with the valve action in the heart. By clogging primary vessels, the blood supply to other organs—particularly the lungs, liver and kidneys—is reduced, often resulting in malfunction.

Most dogs infected with heartworms do not show any signs of disease for as long as two years, but this is entirely dependent on the severity of infection. Unfortunately, by the time signs are seen, the disease is usually advanced.

The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present and the degree of damage to the heart, lungs, liver and kidneys. The most obvious symptoms are a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness and loss of stamina. All of these signs are most noticeable following exercise.

Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation. Evidence of weight loss, poor condition and anemia may also be noted. Severely infected dogs may die suddenly during a period of exercise or excitement.

In most cases, diagnosis of heartworm infection can be made by a blood test. Further diagnostic procedures are essential, particularly in advanced cases, to determine if the dog can tolerate heartworm treatment.

Depending on the case, we will recommend some or all of the following procedures before treatment is started.

There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug most commonly used to treat heartworm infection contained arsenic. While this treatment was effective for killing heartworms, it often led to toxic reactions and illness in the dog. Today, newer drugs are available that do not have these toxic side effects, and as a result, we are able to more successfully treat canine heartworm infections. However, a strict protocol for medications and exercise restriction is essential for reducing risks during treatment.

Some dogs with advanced heartworm disease are more difficult to treat. An advanced heartworm infection means that the heartworms have been present long enough to cause substantial damage to the heart, lungs, blood vessels, kidneys and liver. A few of these cases are so far advanced that it is safer to simply treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months.

Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing symptoms. In most cases, the dog has a renewed vigor and vitality as well as improved appetite and weight gain.

Yes. The state of heart failure is taken into consideration and treated as described above. Some severe cases require surgery to actually remove the worms due to the risk of too many dying at once during the treatment and causing severe side effects or death. 

When a dog has been successfully treated for heartworms, he/she is still at risk for reinfection. Therefore, it is essential to begin a heartworm prevention program. Monthly chewable tablets can be used to prevent heartworm infection. These products are very safe and effective and should be started immediately after treatment is completed.