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The American Heartworm Association has deemed April as Heartworm Awareness Month in hopes to educate pet owners about the importance of year-round heartworm prevention and annual testing.

MosquitoHeartworm disease is caused by a parasitic worm that lives in the pulmonary artery as well as the right side of the heart. It is a serious and potentially fatal condition that affects dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and even humans.

The disease is caused by a nematode (roundworm) called Dirofilaria immitis. All ages and breeds of dogs AND cats are susceptible to heartworm disease, which is found in all 50 states with the warmer southern states being more endemic. The route of infection is through the bite of a mosquito.

Lifecycle of Heartworm

The lifecycle of heartworm is slightly complex but important in understanding heartworm testing and prevention. Adult female heartworms release their larva called microfilaria into the animal's bloodstream, which is then ingested by a mosquito during a blood meal. The larva then takes about 2-3 weeks to mature inside of the mosquito to the infective state called L3. It can then be transmitted to a dog or cat through the bite wound of a mosquito where it matures in the tissue for about 2 months. Once mature, the adult heartworm migrates to the right side of the heart and pulmonary arteries. In dogs, the adult heartworm can live up to 7 years.

Heartworm Testing

Understanding the life cycle is important in determining accurate heartworm testing. The heartworm screening test for dogs that we offer at our hospital detects the presence of the heartworm antigen in the blood. Since it takes about 6 months for the heartworm to reach maturity and shed antigens in the blood stream, we must wait at least 6-7 months after a missed monthly dose of heartworm preventative before testing.

The American Heartworm Society recommends annual heartworm testing in all dogs. Routine screening tests are not used in cats as they are very inaccurate and difficult to detect.


The clinical signs in heartworm disease differ greatly between dogs and cats, however most clinical signs of heartworm disease may not be recognized in the early stages. Dogs typically show respiratory signs due to the obstruction of blood flow from the physical size and amount of the worms. Clinical signs include increased respiratory rate, persistent cough, lethargy, exercise intolerance, weight loss, and syncope.

In cats, clinical signs are due to the inflammatory reaction generated by the heartworms. Clinical signs are typically vague and often mimic other diseases such as feline asthma or allergic bronchitis. Signs in cats include vomiting, gagging, collapse, chronic coughing, weight loss, and lethargy.

Dr. Nicole Uranko is Chief of Staff at Pleasant Valley Animal Hospital in Quakertown, PA She has a special passion for feline internal medicine and nutrition.