Canine Heartworm Infection

What is heartworm disease?

Heartworm disease is caused by a parasitic worm, Dirofilaria immitis, that lives in the heart, lungs, and the large blood vessels between them. Adult worms can reach a foot in length and live for five to seven years. As they accumulate they damage the lining of the pulmonary arteries, strain the right side of the heart, and eventually cause heart failure. Untreated, heartworm disease is fatal.

How do dogs get heartworms?

Only through mosquitoes. A mosquito bites an infected animal, picks up microscopic larvae, and passes them on when it bites another dog. The larvae migrate through the tissues for several months, maturing into adult worms in the heart and lungs about six months after the bite. Dogs cannot catch heartworms directly from each other.

Heartworm has been found in every state, and Southern California has both the mosquitoes and the infected coyote and dog populations to sustain it. Indoor dogs are bitten too.

What are the signs?

Early infection has no signs at all, which is why testing matters. As the worm burden grows, dogs develop:

• A soft, persistent cough
• Tiring easily and reluctance to exercise
• Weight loss and a poor coat
• Rapid or labored breathing
• A swollen belly from fluid buildup as heart failure develops

A sudden collapse with pale gums and dark urine can mean worms are blocking blood flow through the heart (caval syndrome), an emergency requiring surgery.

How is heartworm diagnosed?

A small blood sample detects proteins from adult female worms. Dogs should be tested annually, even when on prevention, and before starting or switching a preventive. When the test is positive, we add chest X-rays, blood work, and sometimes an ultrasound to judge how much damage has been done and plan treatment.

How is it treated?

Treatment for adult heartworms uses a series of injections of a drug called melarsomine, given over about two months, together with antibiotics and a preventive to clear younger stages, and sometimes steroids to control inflammation. The dying worms break apart in the lungs, so strict rest with no running or play is essential for several weeks after each injection; exercise during this period can be fatal. Treatment is effective but expensive, uncomfortable for the dog, and carries risk, all of which is why prevention is so strongly recommended.

How is heartworm prevented?

A monthly chewable or topical preventive, or an injection given every six or twelve months, kills the larvae before they can mature. Because larvae can take months to reach the heart, the medicine works retroactively on the previous month's bites, which is why it must be given on schedule and year-round. Our climate never gets cold enough to reliably stop mosquitoes. Most preventives also control intestinal worms and some cover fleas.

Puppies can start prevention as early as eight weeks of age. A dog that has missed doses should be tested before restarting. See our Dog Heartworm Prevention page for the options we carry.

See also: Feline Heartworm Infection