• Dr. Lyana Buckwold

Keeping Your Pets Safe from Heartworm Disease

With warm weather on the way, we start to see more mosquitoes, making April the perfect month to talk about heartworm disease. Heartworm disease is caused by Dirofilaria immitis. This worm lives in the blood vessels of dogsand cats and can cause severe heart and lung disease and even death. In addition to infecting dogs and cats, heartworms can infect wolves, coyotes, foxes, ferrets, and sea lions. Wherever mosquitoesare, heartworm disease can be also.

Life cycle/Pathology

Dirofilaria immitis needs two animals to survive: the dog or cat and the mosquito. The youngest larvae (L1) course around an infected dog’s blood stream until a mosquito sucks one up while feeding from the dog. The larva then matures inside the mosquito, migrates to the mosquito’s salivary glands, and waits the mosquito’s next feeding. When the mosquito feeds from another pet, the larva leaves the mosquito and attaches itself under the dog’s skin. It takes about 6-7 months for a larva (now called L3) to make its way from under a dog’s skin through the muscle and into the blood stream, this period is called the tissue phase. During this time, the dog will appear healthy, and only very rarely will react to the worm and have seizures or become blindness or lame. Once in the blood stream, the larvae are called microfilaria and they continue to grow until they finally take residence inside the heart and in the blood vessels of the lungs, and meet another Dirofiliaria to have “kids” (aka L1 larvae) of its own. The heartworms damage the blood vessels it inhabits causing the vessels to become larger and thicker, the blood pressure around these vessels to increase, and the heart to enlarge and malfunction. The damage cause by these worms is heartworm disease.

Clinical Signs

The severity of heartworm disease depends in part on how many heart worms are present in a pets blood vessels and on a dog’s activity. Early signs of heart worm disease include coughing and quickly becoming tired during exercise. As numbers of worms are within the blood vessels around the disease will progress. Signs that you may see include difficulty breathing, weight loss, collapse, pale or yellow gums, heart murmur, fluid in the abdomen, kidney disease, congestive heart failure, and acute death.


Currently we have two ways to prevents dogs from getting heart worm disease. The traditional and best way is to kill the heartworm during the tissue phase. There are multiple preventatives on the market including: Interceptor Plus, Heartgard Plus, Revolution and Proheart. Each medication works slightly differently in how it kills heartworms and some can cause illness if given to a dog with a high worm load in their blood stream. For this reason, we require that a dog be tested for heartworm disease prior to being placed on medication. Dogs under 6 months of age are exempt from this rule, as they are too young to have heart worms circulating in their blood and can be started on a heart worm preventative as young as 2 months of age. It is necessary to give heartworm preventative every month even during the winter.

The second way to prevent heartworms it to prevent mosquito bites. Two good mosquito repellents for dogs are Vectra 3D and Advantix. These products also protect against fleas and ticks. In a recent research by McCall et al, it was found that placing animals on both a heartworm preventative and a mosquito repellant provided the best protection against heartworm infections.


We recommend yearly screening for heartworms to monitor for any signs of disease and to make sure your heartworm preventative is working. Most veterinarians use a blood test that detects the presence of a female, adult heart worm inside of your pet. If this test is positive, additional testing will need to be performed to confirm the stage the severity of your pet’s disease prior to starting treatment.

Increasing Risk of Heartworms in New Hampshire

Heartworm disease is on the rise in New Hampshire. The reasons for this are multifold. There are more dogs being rescued from the South and other endemic areas. This has in turn increased the number of mosquitoes infected with heartworms. The winters are becoming more mild and in turn mosquitoes are present for longer periods of time. In addition, bats, a predator of mosquitoes, are dying from White-nose Syndrome. Their diminished numbers have caused an increase in the mosquito population. As the number and duration of infected mosquitoes continues to grow, we will likely be seeing even more heartworm disease in New Hampshire.

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