Causes of Heartworm Disease
Heartworms are transferred from infected to non-infected dogs by adult mosquitoes. Infection begins when a female mosquito feeds on a dog whose blood contains the immature offspring of adult heartworms, called stage 1 larva, or “L1 microfilariae.” The microfilariae go through several stages of development inside the female mosquito, molting from larval stage L1 to L2 and then L3. It is only during the brief L3 stage, which occurs between 1 and 2½ weeks after they are ingested by the mosquito, that the larvae can infect dogs. When a mosquito containing L3-stage microfilaria bites a dog, the immature heartworms are transferred into the dog at the site of the bite. They burrow into the skin and molt from L3 to L4 larvae between 1 and 12 days after the dog is infected. After another 50 to 60 days, they molt into L5 stage larvae. Then, the worms enter the dog’s blood vessels and lodge in the pulmonary arteries and right heart chambers, where they mature into adults. The pulmonary arteries connect the heart to the lungs; they are essential for transfer of oxygen and other nutrients throughout the body, and for removal of carbon dioxide and other waste products from circulation.
Heartworms reach adulthood about 6 or 7 months after a dog is inoculated with L3 larvae. Adults can be 15 inches long. Heartworm infection usually becomes apparent when adult worms reproduce and release microfilariae. A single mature female can produce up to 5,000 microfilariae in one day, each of which can survive in the dog’s bloodstream for years, continuing to produce more microfilariae.
Dead heartworms can be as problematic as living ones. When a dog is given drugs that kill adult heartworms (adulticides), the dead and dying worms can block blood flow. They also can wrap around heart valves and mechanically interfere with heart function and circulation.
Preventing Heartworm Disease
Heartworm disease is almost always preventable. Dogs can be screened for infection at 6 to 7 months of age. If they test positive for the heartworm antigen, a more sensitive test can be performed to detect whether microfilaria are in circulation. Heartworm preventatives should not be given to an infected dog without strict veterinary supervision. Dogs in endemic areas probably should be on heartworm preventatives for life, although only a veterinarian can recommend the appropriate prevention protocol. Fortunately, heartworm preventatives are convenient for owners to administer. Most are given monthly either orally or topically between the shoulder blades. Some also control fleas, roundworms, hookworms and/or whipworms.
Other preventative techniques include bringing dogs indoors in the late afternoon and early evening, when mosquitoes are out. Ponds, puddles and other sources of standing water should be avoided. Areas around the house and yard can be sprayed to eliminate mosquitoes.