Heartworm disease is a serious and fatal disease of the heart and lungs caused by a parasite, Dirofilaria immitis, that is transmitted by mosquitoes. Historically, heartworm disease in cats has been given less consideration than in dogs because the incidence is so much lower compared to dogs and the diagnosis is more difficult.

Age is not a risk factor. Adult cats of any age can be affected, with cats as young as 1 and as old as 17 having been diagnosed. Indoor and outdoor cats can get infected, although outdoor cats have a higher prevalence. However, up to 33 percent of reported cases are in cats who are described by their owners as "strictly indoors." Males are a bit more likely than females to be affected.

Transmission occurs when a mosquito bites an infected dog or cat and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

What to Watch For

Symptoms don't usually occur until damage has already occurred to the heart. Cats can have a wide range of symptoms, with some cats being completely asymptomatic (no symptoms at all). Others may show vague, generalized clinical symptoms. A small percentage, however, may show severe life threatening symptoms. Symptoms usually occur because of heart failure. These include:

Difficulty breathing

Rapid breathing rate

Coughing

Hemoptysis (coughing up blood)

Vomiting

Fainting

Lethargy

Anorexia

Weight loss

Decreased ability to exercise

Neurologic signs such as blindness, seizures, circling, incoordination