Heartworms (Dirofilaria immitis) are parasites that are transmitted through the bites of mosquitos. Cats become infected when a mosquito first bites an infected dog, picks up microscopic larvae called microfilariae, and then injects them into a cat. The larvae move around the body going through several stages of development and may eventually mature into large adult worms that live in the heart. In cats, much of the damage associated with heartworm disease is caused by larvae in the lungs. Heartworm disease may also be called dirofilariasis.
Symptoms in cats can be somewhat vague and nonspecific, but vomiting, difficulty breathing, and loss of appetite and weight are possible. Definitively diagnosing heartworm disease in cats is difficult because many infected animals come up negative on the tests traditionally used to diagnose the disease in dogs. Many veterinarians will use a combination of an animal's clinical signs, blood work that tests for exposure to heartworms, chest x-rays, and cardiac ultrasounds to make a diagnosis of heartworm disease in cats.
The drugs that abruptly kill adult heartworms in dogs can be very dangerous for cats. In many cases, treatment for feline heartworm disease involves placing cats on medications that prevent more worms from developing and addressing the symptoms that arise as the existing worms live out their expected life spans and eventually die and are absorbed by the cat's body. The prognosis for any cat with heartworm disease is guarded. The good news is that many different types of heartworm prevention are available that effectively protect cats from this potentially devastating disease.
Written by: Jennifer Coates, DVM
Last reviewed: October 2, 2008