With the advent of spring comes wildflowers, longer days, and more time outside with our best friends. While all of these things are welcome, something else that comes with warm weather is not: mosquitos and their bites.

Over 30 species of mosquitoes can now transmit deadly and serious disease to our pets – dogs and cats alike. You may have heard of heartworm disease, and although this was once thought of as a problem only in southern states, the truth is that the incidence of heartworm disease is growing in all 50 states and in Canada. Once infected, your pet faces permanent organ damage and even death.

Learning the facts about heartworm disease and how you can prevent it is the first step towards keeping your furry loved ones safe.

What is Heartworm Disease?

Heartworm disease is caused by the mosquito borne parasite Dirofilaria immitis. Once mature, the adult heartworms live in the heart and pulmonary arteries of pets and wildlife (coyotes are a main carrier).

Pets get heartworm disease with these basic steps:

  • An infected animal (pet or wildlife) is bitten by a mosquito, which takes up the heartworm larvae, or microfilariae, into its body
  • The microfilariae develop inside the mosquitoes body for between 10-30 days
  • The same mosquito bites a different pet, injecting the microfilariae into that pet’s body.
  • In the new pet’s bloodstream, the baby heartworms circulate and develop further over the course of several weeks.
  • The microfilariae reach the pet’s heart and lungs where they mature into foot long adult heartworms capable of reproduction.
  • After about 6-7 months of infection, the adult heartworms reproduce and send baby microfilariae into the pet’s bloodstream. The pet is now a carrier and can spread the disease with the next mosquito bite.

Signs of Heartworm Disease

To complicate matters, heartworm symptoms can resemble other disease symptoms, and are different in dogs and cats. Signs may be subtle at first, and diagnosis is rarely made based on clinical signs alone.

Signs include:

  • Soft, dry cough
  • Vomiting
  • Exercise intolerance or lethargy
  • Weight loss
  • Rapid breathing
  • Bulging chest
  • Collapse


In dogs, heartworm diagnosis may be made based on a combination of tests. Cats are resistant hosts to heartworms, which makes diagnosis difficult if not impossible. Cats typically have only between 1 and 3 adult worms at a time, but even this small number can cause heartworm associated respiratory disease (HARD) and other life threatening complications.


In dogs, treatment consists of injections that kill the adult heartworms over a period of several weeks. Dead and dying worms pose a serious risk to the dog, as fragments can break apart and block oxygen and blood flow to the heart and lungs. Most dogs are hospitalized during treatment to minimize risks of respiratory distress or sudden collapse, and all should be on exercise restriction during the entire course of treatment.

Sadly, the same drug is not approved for use in cats. The drug can cause complications in cats that include pulmonary failure and sudden death. Some choose to treat the cat anyway and hope there is no reaction. Others prefer to treat the symptoms of heartworm disease in cats with a combination of oxygen therapy, corticosteroids, and other medications with the goal of supporting the immune response to the worms.


You may have arrived at the conclusion that prevention of heartworm disease is the way to go, and we are in 100% agreement! Prevention is simple, effective, and cost effective compared to the costs of treatment and management of complications.

We can help you choose a preventive that is right for your and your pet. Once we do, giving it on schedule year round is essential. Microfilariae mature about every 51 days, and interrupting this process is important to avoid organ damage and complications of the disease.

If you have any questions about heartworm disease or keeping your pet healthy, please give us a call at Huntington Veterinary Hospital.