Heartworm, the parasite Dirofilaria Immitis, can be a very serious problem for both dogs and cats, but it is more common in dogs. Transmission of heartworm depends upon the mosquito population of an area. About 70 species of mosquitoes are capable of transmitting the disease. The more mosquitoes in an area, the greater the chance of heartworm transmission. Heartworms, left untreated, will be fatal. Click mosquito life cycle for more information on the mosquito life cycle and advice on reducing mosquitoes around your home.


This is a dog's heart infested with heart worms.


Cats typically have fewer and smaller worms than dogs and the life span of worms is shorter, approximately two to three years, compared to five to seven years in dogs. In experimental infections of heartworm larvae in cats, the percentage of worms developing into the adult stage is low (0% to 25%) compared to dogs (40% to 90%). Dogs are considered the definitive host for heartworms. However, heartworms may infect more than 30 species of animals (e.g., dogs, coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well.

Heartworms are the most life threatening of the canine parasites. They reside in the pulmonary arteries and right ventricle of the dog's heart, causing heart failure and eventually death. The adult heartworm is 6-14 inches in length, thread-like, and white in color. When adult male and female heartworms are present, mating occurs. The female releases large amounts of small, microscopic "microfilariae" (pronounced: micro-fil-ar-ee-a) into the bloodstream. The circulating microfilariae can live up to two years in the dog's bloodstream. Several microfilariae are ingested by a mosquito when it bites an infected dog. The mosquito serves as an intermediate host as well as vector (the transmitting agent) for the disease. The mosquito spreads the disease to another dog by injecting the microfilariae at the time of the bite. In order for the microfilariae to become infectious, they must develop inside the body of the mosquito. This development occurs only under certain environmental conditions. Two weeks of temperature at or above 70 degrees F. is required. As a result of this temperature requirement, transmission of the disease is limited to the warm months.

After the microfilariae have gone through their development, they are ready to infect a new victim. During a blood meal (mosquito bite), the mosquito injects the microfilariae into a new dog. These small, microscopic worms migrate under the skin and eventually enter the dog's blood stream. About 6 months after the initial mosquito bite, the microfilariae arrive at the heart. The final maturation and the mating of the heartworm occur in the pulmonary arteries. The adult worms live in the pulmonary arteries and right side of the heart, where they can survive for seven years.

Adult heartworms cause inflammation and thickening of pulmonary arteries. As time passes, more arteries become inflamed and clots begin to appear. The blocked pulmonary vessels lead to an increase in blood pressure. This increase in pressure places a strain on the right ventricle of the heart. Eventually, heart failure occurs.

Clinical symptoms of heartworm disease develop very slowly. Often, symptoms are not noticeable until 3 years after the initial infection. Most of the symptoms are due to problems associated with increased workload for the heart. Lack of energy and exercise intolerance are early symptoms. Chronic coughing and difficulty breathing are both common symptoms associated with heartworm disease. As the disease progresses, most dogs develop congestive heart failure and ascites. Dogs often collapse in the final stage of the disease.

Heartworm is a serious deadly disease. Diagnosis is made through regular blood tests, and heartworm preventative is prescribed for dogs with negative results (no worms present). Heartworm treatment for positive test results can be very traumatic to the host animal, and is a fairly involved veterinary process. Before heartworm treatment begins, laboratory tests and x-rays are generally required. An animal must be healthy enough to undergo treatment.

Prevention of heartworm is highly recommended. A year-round preventive program is most effective to keep pets free of heartworms.

Heartworm infection in humans is extremely rare as humans are unnatural hosts. Mosquitoes transmit heartworm, not pets, so a dog with heartworm poses no threat to human health.

Lakeside Veterinary clinic assumes no liability for injury to you or your pet incurred by following these descriptions or procedures.