Pet Factsheets

Heartworm disease

This potentially serious parasitic disease can cause heart failure and other complications in dogs but is primarily a respiratory disease in cats. In most countries where the disease occurs, preventative treatment is given to pet dogs and cats to ensure they do not become infected.

What is heartworm?

The disease, unlike the name suggests, is caused by a parasitic worm called Dirofilaria immitis which lives primarily in the pulmonary vessels and not the heart. The life-cycle of this parasite involves dogs (where it can cause serious disease) and mosquitos, which act as a vector, transmitting worm larvae and spreading infection.

Heartworm is an important disease in several parts of the world, including:

  • Parts of Europe (not UK, except in imported pets)
  • USA, Central America and South America
  • Australia
  • Japan

How can my cat be infected?

The tiny immature forms of the worm (larvae) enter a cat via a mosquito bite. The mosquitoes acquire heartworm larvae from infected dogs, coyotes, foxes and wolves. The larvae are carried in the blood and settle in the pulmonary arteries. Here most of the immature worms die where they cause inflammation which can lead to lung damage. Occasionally a worm survives and grows into an adult worm. The eventual death of the adult worms in this location can lead to a serious allergic type reaction that can lead to death in 20% of cases.

Approximately 75% of cats exposed to infective heartworm larvae become infected.

How do I know if my cat is affected?

Most cats with infections have less than 6 heartworms. 1-2 worm infections are common. However, cats are highly sensitive to heartworms and, unlike dogs, do not need to harbour adults worms to become ill. The larvae can trigger a severe immune reaction called heartworm associated respiratory disease (HARD). This syndrome occurs in an estimated 50% of heartworm infections in cats.

Infected cats may have difficulty breathing, a chronic cough or vomiting (like an asthma-like lung disease).

How will my vet confirm the disease?

Heartworm in cats is difficult to diagnose with blood tests as false negative test results are common. Your vet may recommend an X-ray or ultrasound examination to make a diagnosis.

Can heartworm be treated?

Unfortunately, there is no approved treatment for adult heartworms in cats.  HARD is treated similarly to asthma.

How can I prevent my cat getting heartworm?

Preventative medication (ivermectin, milbemycin, selamectin or moxidectin), usually given monthly under veterinary prescription, is used to guard against heartworm infection. The American Heartworm Society ( recommends year-round heartworm prevention but currently less than 10% of cats in the US are on heartworm prevention.