Heartworms: Prevention and Treatment

by Barbara Murray
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Heartworm disease is a serious condition that is potentially fatal, especially for canines. Pets that suffer from this condition include dogs, cats and ferrets. Other mammals vulnerable to heartworm infestation include coyotes, foxes, sea lions and wolves. There are also reports, albeit rare, of heartworms infecting humans.

Heartworms begin life as tiny parasitic worms called microfilaria. If left to mature, these tiny creatures can grow up to a foot long in canines. They thrive in the heart, lungs and blood vessel connected to these major organs and can create serious problems in other organ systems.

A dog's internal body, unfortunately, is the perfect habitat for heartworms to live to maturity and reproduce. In cats, the parasites typically do not reach the adult stage but can still cause serious health problems for your felines when large numbers are present.


Graphic of the heartworm lifecycle.Heartworm disease is NOT contagious from one mammal to another. The parasites use mosquitoes to travel from one host to the next. When a mosquito sucks blood from an infected animal, immature worms are taken up with the blood. When the carrier mosquito then bites a second animal, the tiny worms make their way into a new host through the bite wound and then into the animal's cardiovascular system.

Heartworms mature from these tiny worms to the adult stage in about six months and can live up to seven years in the host if left untreated.

Signs, Symptoms and Stages of the Disease

The earliest stage, Stage 1 of heartworm disease, is often asymptomatic in dogs, meaning your pet may be carrying these deadly parasites and outwardly appear in good health.

In what is classified as Stage 2 of this disease your pet may suffer from excessive fatigue following moderate activities. Periodic coughing is also common in Stage 2.

Stage 3 symptoms include no desire to exercise, excessive fatigue, trouble breathing and the cough becomes persistent. The dog's abdomen may swell from excess fluid buildup.

If heartworm disease is allowed to progress to Stage 4 it becomes life-threatening. You dog can suffer from a condition called caval syndrome, in which large numbers of heartworms invade the right side of the heart and block blood flow.

Whether or not your dog suffers caval syndrome, the heartworms will eventually cause major organ failure if left untreated.

In cats, unfortunately, the first symptom of heartworm disease may be sudden collapse and impending death. Cats that do exhibit symptoms earlier may suffer from periodic vomiting, loss of interest in food, which leads to weight loss.


Prevention is the key in protecting your pets from heartworm disease. Preventive medications are readily available and your veterinarian can advise you on prescription medications. In addition, it's always a good idea to ask for guidance from your pet's vet before using over-the-counter products for prevention.

It should be noted that on February 17, 2017, the U.S. Food and Drug Administration announced the agency's approval for the first generic medication to treat heartworm disease in dogs. This may help reduce the cost of preventive treatment.

Some pet owners choose not to use monthly medications because of adverse side effects commonly reported. In this case, regular testing is necessary, as early detection is vital in treating this condition. Also, preventive medications should NEVER be given if heartworms are already present as this can cause serious and sometimes fatal events.

It is recommended that you have your pets tested annually, if at all possible. Your veterinarian will do a blood test and if heartworms are found, advise you on the appropriate treatment plan.

Remember, we can all do a small part in fighting this disease through reducing the mosquito population. Read the second part of this post, "Reducing the Mosquito Population," for ways to eliminate mosquitoes from around your home.

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