Heartworm Disease

April is Heartworm Disease Awareness month, so it’s a great time to talk about all things heartworm! Heartworms are a parasite spread by mosquitos, and infection can cause serious disease or even death in our pets. Heartworm disease occurs all over the world and has been diagnosed in all 50 states. It affects domestic dogs, cats, and ferrets, but can also occur in wildlife such as wolfs, coyotes, sea lions, and foxes.


The heartworm disease life cycle requires mosquitos both for development and spread of disease. The route of transmission of heartworm disease to our pets is as follows:

  1. A female mosquito bites an infected dog, fox, wolf, or coyote and ingests microscopic baby worms called microfilaria.
  2. The microfilaria develop within the mosquito, eventually becoming infective larvae.
  3. The infective larvae are released when the mosquito bites another susceptible mammal.
  4. Once an animal is infected, the larvae grow to maturity over 4-6 months and start reproducing. Adult worms live in the heart and the large blood vessels of the lungs. The worms eventually restrict or clog blood flow in the heart and lungs which in some cases can lead to death of the infected animal.

Clinical Signs:

Signs of infection with heartworm disease are very different between dogs and cats. Dogs are a natural host and can become infected with 30 or more worms at a time. Many dogs infected with heartworms do not show any early signs of disease, sometimes for up to two years. Signs usually start to develop over time as damage occurs to the heart and lungs but can happen more quickly if they are infected with a large number of worms. The most obvious signs include a soft, dry, chronic cough, shortness of breath, weight loss, weakness, listlessness, and loss of stamina. Many of these signs are most noticeable following exercise. Unfortunately, by the time the signs are seen, the disease is usually well advanced.

Cats are not natural hosts of heartworms. When cats are infected, they may only have a few adult worms, but often none of the worms reach the adult stage. Even though many infected cats do not have adult worms, immature worms can cause severe inflammation and damage to their lungs and airways. This can cause signs such as coughing, asthma like attacks, or in severe cases, sudden collapse, or death.


Perhaps the best news about heartworm disease is that there are many effective heartworm preventatives on the market for both dogs and cats. Preventatives come in many forms, including topical treatments, oral tablets and chews, or injectables. Please talk to your veterinarian about the best option for your pet.

All preventatives work by eliminating the immature microfilaria stage of heartworm disease but are not effective at getting rid of adult worms. Adult worms can develop in as little as 51 days, which is why it is extremely important to stay on schedule with heartworm prevention. Topical and oral preventatives are recommended once a month year round. The injection is recommended once every 6 months.


Heartworm tests are recommended once a year for all dogs. This is a blood test which tests for antigens from adult heartworms. It also tests for three of the most common tick-borne diseases- lyme disease, ehrlichiosis, and anaplasmosis.

While you can also screen for heartworm disease in cats, it is more difficult because they often do not have adult heartworms even when infected. A combination of the antigen test and an antibody test is recommended if heartworm disease is suspected or if your cat is not on monthly heartworm prevention.


If your dog tests positive for heartworm disease, there is an effective treatment protocol available. The heartworm disease diagnosis is usually confirmed with a second test to make absolutely sure the pet is positive prior to starting treatment, since it can be a long and expensive process.

Treatment typically includes a combination of antibiotics, steroids, and 3 injections of an anti-parasitic medication called melarsomine. The treatment process usually takes around 90 days to complete. Exercise restriction is recommended throughout treatment and for the first 6-8 weeks after completion to minimize risk for treatment side effects such as embolisms (clots) and severe immune reactions.

Unfortunately, the antiparasitic injection we use to treat dogs is not safe to use in cats. Prevention remains the most effective way to protect cats from heartworm disease.

Additional Information:

As a veterinary hospital, we are always happy to answer any questions or concerns you have about heartworm disease in general or within our community. Research on heartworm disease is ongoing, and we strive to adjust our recommendations and protocols based on the latest information available. For more details and the most current research on heartworm disease, please refer to the American Heartworm Society, a group of veterinarians and scientists whose mission is to lead the veterinary profession and the public in the understanding of heartworm disease. Their website can be found at https://www.heartwormsociety.org.