Every year, many dogs and cats become infected with heartworm. More dogs than cats are infected. It is found most commonly in outdoor pets, but indoor pets can also be infected. Heartworm is most prevalent in the mid-western, eastern, and southern parts of the United States, but there have been cases reported in all 50 states. Heartworm is a very dangerous condition with very few, if any, symptoms. By the time the pet begins exhibiting any signs of infection, it is often too late. Fortunately, heartworm is a very preventable condition.

What is heartworm?

Heartworm is a parasitic worm that is spread by female mosquitoes. Once an infected mosquito bites the pet, the larvae are implanted in the skin. They then burrow into the skin, move through the veins and eventually into the heart. After about three months, adult worms emerge and can live in a dog’s heart for up to five years and up to three years in a cat. Dogs can have as many as 40 to 250 worms living in them, while cats usually only have two or three. Male and female worms will reproduce, sending millions of offspring, called microfilaria, to live in the small blood vessels of the pet. This eventually leads to lung and liver problems caused by a blockage of blood flow.


Sometimes a pet will show no symptoms of being infected with heartworm and will die suddenly. The only way the owner knows what caused the pet’s death is by having an autopsy performed. Dogs may exhibit such symptoms as coughing, labored breathing, becoming tired easily, weight loss, abdominal swelling and bloody stool. Cats may also exhibit these symptoms, as well as diarrhea, vomiting and convulsions. If you notice any of these symptoms in your pet call your veterinarian immediately.

Diagnosis and treatment

To make a diagnosis, your vet may begin by doing a simple blood smear to look for microfilaria. If that is inconclusive, more blood tests will be performed. They may also find it necessary to perform diagnostic test such as x-rays, an angiogram or echocardiogram.

Once a diagnosis has been made, treatment begins immediately. Treatment can be very dangerous, but the majority of pets given the proper treatment will make a full recovery. For cats, the most common way of dealing with heartworms is to let the worms die off naturally. This can take years and there is a risk that the cat may become seriously ill or even die during this time. The other option is to kill the worms all at once (adulticide therapy), but this may have serious side effects. If adulticide therapy is chosen, the cat should be given an antigen test 12 weeks later to make sure all of the heartworms have been killed.

Treatment for dogs requires two steps. The first is to kill all of the adult worms with adulticide therapy. This is accomplished by intravenous or intramuscular administration of two very potent arsenic compounds. This will cause clotting of the dead worms in the circulatory system, accompanied by fever, vomiting, loss of appetite and coughing up blood. It is extremely important that the dog not be allowed to exercise for two weeks after treatment. Cage/crate rest is highly recommended. The second step in treatment is called filaricide therapy. This is to kill the microfilaria once the adult worms have been killed. This is done three to six weeks after the adulticide therapy and the dog is hospitalized due to the threat of coma or death.


Obviously, it is well worth it to make sure your pet is never infected with heartworms. Prevention is as simple as giving your pet a pill once a month. Before starting your pet on a heartworm prevention routine, it is important that they be tested for the presence of heartworm first. Some heartworm prevention medicines can cause serious illness or even death if given to a pet already infected with heartworms. Once it is determined that your pet is heartworm-free, it is safe to start them on a monthly medication. There are many heartworm prevention medicines to choose from, some of which even help with fleas and ticks. Ask your veterinarian what they recommend. In addition, if you live in an area that has a lot of mosquitoes, keep your pet inside during the time of day that the mosquitoes are most active.

For more information on heartworm, visit the American Heartworm Society (http://www.heartwormsociety.org/).