Heartworm Disease (HWD) refers to the condition caused by the parasite Dirofilaria immitis, carried by mosquitoes and affecting dogs, cats, and ferrets. In the United States, it is a major problem in the Southeast, East, and the Mississippi River valley.

The adult worm is large, up to 12 inches long, and lives predominantly in the pulmonary arteries (Pas, the large vessels that carry blood to the lungs from the heart). When an infection consists of both male and female mature worms, reproduction occurs with resultant microscopic circulating baby forms called microfilariae (L1). These are an important part of the life cycle because they allow infection of other animals to occur and, when found upon microscopic examination of the blood, allow the diagnosis of HWD. For transmission of heartworms (HWs) to occur, a mosquito sucks blood containing L1 from infected dogs. The L1 develops in the mosquito, becoming infective in about 2 weeks. The mosquito then transmits the infective larvae to another dog. Further development occurs with migration to the heart and PAs 3 to 4 months later. Adult HWs are thought to live for 5 to 7 years.

From 1 to over 200 HWs may reside in the heart and PAs. The PAs become thickened and inflamed, increasing the work of the heart as it pushes blood past the worms into the lungs. In addition, the lungs themselves become inflamed. Mild infestations may produce no signs. The earliest clinical signs are typically exercise intolerance, cough, and weight loss. More severe signs may include severe cough, labored breathing, and heart failure (usually manifested as abdominal swelling). Once HWD has reached this stage, the dog may die.

The diagnosis can be made by finding L1 in blood. An enzyme-linked immunosorbent assay (ELISA, a test that identifies proteins [antigens] produced by adult female HWs) readily detects infections with two or more adult females. A diagnosis may also be suspected on the basis of radiographs (x-rays).

After tests to ensure that a dog is healthy enough, arsenical drugs are used to kill adult HWs. A newer, more expensive agent (melarsomine) is safer than arsenamide (Caparsolate), allowing gradual destruction of HWs so that the lungs can gradually "clean up" the infection. Both drugs can cause irritation at the site of injection and could damage the liver and kidneys. By far the greatest concern is dead HWs, producing a severe reaction in the lungs 1 to 3 weeks after administration. This can be prevented or minimized with melarsomine given in three doses (one initially and two separated by 24 hours in 1 month) and by severely restricting exercise for at least 1 month after adulticidal therapy. Exercise restriction is imperative after adulticidal therapy!

Therapy with steroids may be needed to reduce lung inflammation and resultant cough but is typically discontinued before adulticidal therapy. Aspirin may be used to reduce the vascular damage caused by HWs but is controversial.

After killing the adult worms, the L1 forms should be killed, thereby lessening the risk to other pets. A dosage of ivermectin, milbemycin, or moxidectin can be given approximately 6 weeks after the adults have been killed. Although often effective, this treatment may produce severe reactions. An alternative is to use ivermectin at lower doses, thereby gradually eliminating L1 over about 6 months. This may be done before or after the adulticide and adverse reactions are rare. Ideally the pet is closely observed the day of the first dose with either method.

Heartworm infection is clearly better prevented than treated. Prevention is instituted at 6 to 8 weeks of age or as soon thereafter as climatic conditions dictate. Prevention of HWD can be accomplished by daily administration of diethylcarbamazine (DEC) or monthly administration of ivermectin, milbemycin, or moxidectin. Although each of these drugs is effective when given as directed, even brief lapses in DEC therapy may result in infection. The monthly drugs, however, provide protection despite lapses of up to a month. Both DEC and monthly drugs are extraordinarily safe if administered before infection but may produce severe, even fatal, reactions if administered to dogs with L1. Such reactions are more severe with DEC. Heartworm testing should be performed in all dogs older than 6 months of age (if there has been seasonal potential for exposure) before institution of preventative.

There are areas in the United States in which no HW preventative is necessary. In the deep South and California, preventative is typically administered all year. In the North, the season is shorter; DEC is used from the first mosquito sighting until 2 months after the first hard frost, and the monthly drugs should be administered from the onset of mosquito season until 1 month after the first hard frost. Your veterinarian knows the appropriate preventative schedule for you region.

Yearly testing is required for dogs receiving DEC because of the potential for adverse reactions in dogs that become infected and are restarted with preventative. The need for yearly testing with monthly treatment is less certain. Many veterinarians still advocate yearly testing because pets may not receive or may not swallow the necessary preventative. If it is certain that the medication is administered and swallowed for the entire HW season, then testing every 2 years is an option.