This parasite — Dirofilaria immitis — lives in the right side of the heart and is spread by mosquitoes. It's most common in humid areas that attract mosquitoes, but it's found throughout the world. Left untreated, heartworms are killers, and the treatment is difficult and expensive. It's best to prevent them altogether.

The heartworm life cycle begins when infective larvae enter the dog's skin through the bite of a mosquito. The larvae burrow beneath the skin and eventually develop into immature worms. These immature worms enter the bloodstream where they're carried to the heart. They reach adulthood approximately six months after entering the dog's body. Adult heartworms can reach lengths of four to twelve inches. They are long and threadlike, with small mouths. Heartworms take only six months to grow to maturity and can live for as long as five years. If male and female worms are present in the heart, they reproduce, giving birth to live young known as microfilaria. The microfilaria can remain alive in a dog for up to three years. The next time a mosquito bites the dog, it ingests the microfilaria with the blood it takes in and the life cycle continues.

Heartworm Effects and Signs

What do heartworms do? They clog the pulmonary arteries of the heart, obstructing blood flow. They can entwine around the heart valves and interfere with the heart's pumping action. Some heartworms migrate to the veins of the liver, causing liver failure.

The signs of heartworm disease depend on the number of worms infesting the dog. A dog with few worms may not show any signs at all. A heavy load of worms can cause fatigue, a soft, deep cough, weight loss, and rapid breathing.

Diagnosis, Treatment, and Prevention

A blood test is required to diagnose heartworm disease. A chest x-ray helps the veterinarian determine the severity of the problem. Heartworms can be prevented with daily or monthly pills, as well as with a monthly topical (applied to the skin) preventive. Once the worms develop, however, a drug must be administered to kill the adult worms, followed by a second medication to kill any circulating microfilaria. In severe cases, the worms must be removed surgically.

Your pug can begin taking heartworm preventive at six to eight weeks of age. He'll need a blood test first to make sure no worms are present. Dogs that live in areas where mosquitoes are endemic should take heartworm preventive year-round, even if they spend most of their time indoors.

It's important for your pug to see a veterinarian on a regular basis, even if he's problem-free.