Heartworm in Dogs: Symptoms and Tests

Heartworm disease, so named because the adult worms live in the right side of the heart, continues to be a major problem for many species of animals. Heartworms are spread by mosquitoes, and thus are found throughout the world. In the United States the prevalence is highest along the southeastern Atlantic and Gulf Coasts, but heartworm has been found in all 50 states. The disease is less prevalent at higher elevations.

The highest infection rates (up to 45 percent) in dogs not maintained on heartworm preventive are observed within 150 miles of the Atlantic Coast from Texas to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States have lower incidence rates (5 percent or less) of canine heartworm disease, while some regions have environmental, mosquito, and dog population factors that combine for a higher incidence of heartworm infection. The infection rate in male dogs is as much as four times that of female dogs, and dogs housed outdoors are four to five times more likely to be infected than indoor dogs.

Although there are differences in frequency of infection for various groups of dogs, all dogs in endemic regions should be considered at risk and placed on prevention programs.

Heartworm Life Cycle

A knowledge of the life cycle of this parasite (Dirofilaria immitis)is needed to understand how to prevent and treat it. Infection begins when L3 infective larvae in the mouthparts of a mosquito enter the dog's skin at the site of a bite. The larvae burrow beneath the skin and undergo two molts that eventually lead to the development of small immature worms. The first molt (L3 to L4) occurs one to 12 days after the dog is bitten by the mosquito. The larvae remain in the L4 stage for 50 to 68 days, and then molt into the L5 stage (immature worms).

It is only during the brief L3 stage, 1 to 12 days after the larvae enter the dog's body, that they are susceptible to the killing effects of diethylcarbamazine. However, throughout the L3 and L4 stages the larvae are susceptible to three other drugs: ivermectin, selamectin, and milbemycin.

Immature worms make their way into a peripheral vein and are carried to the right ventricle and the pulmonary arteries. Approximately six months after entering the dog's body, they mature into adults. Adults can grow to 4 to 12 inches (10 to 30 cm) long and live up to five years. As many as 250 worms may be found in a heavily infested dog.

Sexual reproduction occurs if worms of both sexes are present. Females give birth to live young called microfilaria;5,000 microfilariae can be produced in one day by a single worm. Microfilaria are able to remain alive in the dog's circulatory system for up to three years.

Before the microfilariae can become infective to another dog, the Li larvae must go into a secondary host, the mosquito. This occurs when the mosquito bites the dog. The LI larvae in the mosquito molt to L3 larvae. In warm southern climates this process takes less than 10 days; in northern climates it can take up to 17 days. The L3 larvae then move to the mouthparts of the mosquito and are ready to infect a new host.