Heartworm Disease


Heartworm disease is a serious and sometimes fatal disease caused by parasitic foot long worms and it is spread by mosquitos. Heartworms live in the heart, lungs, blood vessels and cause severe heart disease/failure, severe lung disease, and damage to other organs.


Heartworm disease affects many types of animals, including dogs, cats, ferrets, racoons, opossums, sea lions, seals, foxes, wolves, coyotes, tigers, lions, and others.  There have been a few rare cases of human heartworm infections, however they do not usually cause signs of illness.

All dogs and cats are at risk, even indoor animals!


Heartworm disease is present in all states and many countries throughout the world. Mosquitos can enter houses and indoor buildings.


How Heartworm Disease is Spread:

In an infected animal, adult female heartworms produce baby heartworms, called microfilaria, which live in the bloodstream of the infected animal. Mosquitos then take a blood meal from the infected animal ingesting the microfilaria. The microfilaria grow into the infective stage within the mosquito called a larval stage; at this time the larvae can be transmitted to another animal when the mosquito feasts again. These larvae develop into adult heartworms within 6 months. The first half of the larvae development takes place in the animal’s body with the end destination of the blood vessels of the lungs. During the second half of their development they grow into adults, which damage the blood vessels, decrease the heart’s ability to pump blood effectively; this results in severe lung and heart disease.

If the animal has a male and female adult worm they will mate and produce more microfilaria. The microfilaria cause a robust immune reaction within the animal’s body, which damages many organs. Adult heartworms are capable of living for 5-7 years within the dog, and several months to years in cats. Cats can have a reaction causing sudden death.

How heartworm disease is diagnosed:

Your dog will receive a yearly heartworm blood test during his or her annual exam. This tests for the presence of female adult heartworms. Since adult heartworms take around 5-7 months to develop once an animal is infected with the larvae, sometimes a negative test result occurs in an infected dog. Antibody and antigen tests, microfilaria testing, chest radiographs, echocardiograms, other blood tests may be needed and recommended for diagnosis and prior to or in combination with treatment.  In cats the diagnosis is more difficult and can be inconclusive.

Early in the course of the disease your cat or dog may show no noticeable signs. As the disease progresses you may notice trouble breathing, coughing, lethargy, exercise intolerance, vomiting (especially with cats),

How heartworm disease is treated:

Heartworm disease is progressive, meaning the earlier detected and treated the better the outcome for successful resolution. The goal is to eliminate the adult worms and microfilaria in a safe way as to not cause more organ damage and prevent anaphylactic reactions. The treatment period is over about 6 months and includes multiple oral and injectable medications to help kill the worms and reduce the inflammatory response of the body to the dying worms.  This treatment usually includes several visits to the veterinarian over those months and strict rest and confinement during the treatment time while at home.

There is no effective treatment that is safe for cats. The goal is to treat the signs usually due to the inflammation.
Surgery is a risky option, and often not performed, unless the case is very severe and necessary for survival of the pet.

How heartworm disease is prevented:

Heartworm disease is preventable. Heartworm disease is preventable. Heartworm disease is preventable.

You can prevent heartworm disease in your pet, including cats! There are topical and oral preventatives that are safe and effective. Many times, dogs think of their heartworm preventative as an anticipated treat. Prevention in our area of the country is recommended all year and throughout the life of your pet. We do not recommend stopping during the winter. The preventatives do not kill adult heartworms and will not eliminate heartworm infection.

The preventatives do not kill adult heartworms and will not eliminate heartworm infection. The preventatives do not kill adult heartworms and will not eliminate heartworm infection.

It is recommended to test your pet for heartworm disease prior to using preventatives and also 6 months after to ensure your pet was not infected prior to treatment. This testing is then done every 6-12 months. Nothing is 100% effective and there are cases where pets have been on year-round prevention and still contracted the disease; thus, the need for annual testing.