Heartworm Infestation

Heartworms are the most life-threatening canine worms, for they reside in the dog's heart and pulmonary arteries, causing heart failure and eventually death. Adult worms are 10 to 30 cm in length and about 1 mm in diameter.

Transmission and life cycle

Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog, it passes him the worms’ larvae that start to migrate until they reach their final site (heart and pulmonary arteries) in about 3-4 months; here they grow to maturity (microfilaria) within a further 3 months and start producing larvae (microfilaria) which can survive for about 2 years in the bloodstream.
When a mosquito bites an infected dog, it picks up these larvae and can transmit the infection to other dogs.


Over time, the presence of the adult worms in heart and pulmonary arteries causes an inflammation and thickening of the walls of the blood vessels which leads to an increase in blood pressure and in the cardiac effort to push the blood in these vessels. As a consequence, the dog may develop heart failure, which can eventually lead him to death. Clinical symptoms usually appear only when the disease has reached a very severe stage (usually 3 to 5 years from infection). First signs consist of sporadic coughing and tiredness; as time passes, cough becomes chronic and is accompanied by difficult respiration, particularly during and after exercise. In advanced cases, the dog may collapse after even light physical exertion. Most dogs eventually develop congestive heart failure.

Treating Heartworm Infection

Treatment of heartworm infection is a long and risky process. Dying heartworms as well as their larvae may determine shock and embolism. During therapy, dogs have to be strictly controlled for side effects, and their activity restricted for a few weeks. Moreover, in advanced cases, health will not be restored even after effective treatment.

For all these reasons it is now clear why prevention is so important.

Heartworm Prevention

In contrast to therapy, most heartworm prevention is safe, easy and effective. Before prophylactic treatment is started, dogs should be tested for heartworm infection; those found to be infected have to be treated against adults and microfilariae, before a prevention program is started. Preventatives are usually administered monthly, starting within 1 month from the beginning of the mosquito season (or the exposure to a possible infection, e.g. during traveling to heartworm countries), until 1 month after the end of exposure.

Some preventatives are also effective against all other common dog worms thus ensuring a complete protection of your pet. Be careful using a "complete protection" in case your dog is sensitive to some of the ingredients. Also note that a preventive that prevents everything is loading your pet with a lot of toxic ingredients.