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What Is Heartworm Disease?

Heartworm disease is a serious, potentially fatal disease in dogs, cats, and other mammals. It is caused by parasitic worms which live in the heart and other adjacent large blood vessels of infected animals. Heartworm disease had been reported in all 50 states and is most problematic an areas where mosquitoes are prevalent.

Heartworm Life Cycle

Heartworm Disease in DogsDogs or other animals infected with Dirofilaria immitis are the reservoir for heartworm disease. The life cycle begins when a female mosquito bites an infected animal. Mosquitoes become infected with microfilariae, the larval form of the worm, when they take a blood meal.

Once in the mosquito, the microfilariae mature into the infective larval stage. Then when the mosquito bites another susceptible animal, the larvae is introduced into the new host.

A mosquito, the intermediate host for the worm, is required for transmission. It isn't spread directly from dog to dog. It takes about 2 months for these larvae to migrate into the animal's venous blood stream and to arrive to the vessels of the lungs.

It takes a total of 6 months for larvae to mature into adult heartworms which produce more microfilariae. Adult worms may live up to 5-7 years in the dog.

Clinical Signs

For dogs, there may be few, if any, clinical signs in early stages of infection. In heavily infected dogs, signs may eventually include a mild persistent cough, reluctance to exercise, fatigue with moderate exercise, reduced appetite, and weight loss. Microfilariae remain in the small blood vessels, partially blocking blood flow in them. They primarily affect the lungs and liver, causing a cough, or jaundice and anemia with cirrhosis of the liver. Cats may show non-specific signs including vomiting, gagging, respiratory difficulty, lethargy, and weight loss.


Your veterinarian can detect heartworm disease with blood tests for the heartworm microfilariae or antigen. Neither test is consistently positive until about 7 months after infection. For this reason, yearly tests are recommended as part of annual wellness visits for dogs over 6 months old. The antigen test will be positive even when there are no microfilariae in the blood. However, dogs with less than 5 adult worms may not have enough antigen to produce a positive test result, giving a false negative result. Also, the antigen is only produced by female worms, so an infection of only male heartworms will also give a false negative.


Once a dog is determined to have heartworm disease, treatment may differ somewhat depending on how advanced the disease is and worm burden. Other blood tests and radiographs will be needed before treatment if the pet's overall health and organ function have been affected. There are some risks involved with treatment. In the past, a drug containing arsenic was used which caused many side effects. Now, a safer drug is available and usually all but the most advanced cases can be treated by using a series of injectable medication, called an adulticide. The specific schedule will be determined by your veterinarian. An antibiotic may also be used against a bacteria (Wolbachia) inhabiting the worms, thereby weakening the worms. Hospitalization during this period is recommended. Once the dog is sent home, exercise should be limited to leash walks for 1-2 months to decrease the possibility of blockage of blood flow through the lungs by the dead worms. Reinfection is prevented by using a heartworm preventative which also can eliminate microfilariae.

Heartworm Disease Prevention

At Illiana Veterinary Hospital, the veterinarians and staff try to promote prevention of heartworm disease. This is accomplished by a yearly heartworm test for dogs to ensure that your pet doesn't already have the disease. Then heartworm preventative medication will be recommended to be given monthly all year round. Currently we carry Heartgard, a monthly beefy chewable heartworm preventative tablet, and Iverhart, a monthly preventative tablet.

Dr. Jeanette Asuncion is an associate at Illiana Veterinary Hospital in South Holland, IL. Originally from the Detroit suburbs, Dr. Asuncion earned a DVM degree from Michigan State's College of Veterinary Medicine in 1990.