Feline Heartworm Disease

Heartworm disease is far better known in dogs, but it is also a serious — and frequently overlooked — threat to cats. The parasite responsible, Dirofilaria immitis, is spread entirely by mosquitoes, which means no cat is truly safe simply because it lives indoors.

Cats are what parasitologists call "resistant" or atypical hosts. Many larvae that enter a cat never reach adulthood, and those that do tend to be few in number and live shorter lives than in dogs. This sounds like good news, but it has a dangerous flip side: feline heartworm infection is hard to diagnose (standard antigen tests often miss it), and the cat's strong immune response to the worms — and especially to worms dying off — can cause sudden, severe lung inflammation. This syndrome is known as HARD, heartworm-associated respiratory disease, and its signs are easily mistaken for feline asthma.

The most sobering fact about feline heartworm is that there is no approved, safe treatment to kill the adult worms. Melarsomine, the drug used to clear infections in dogs, is dangerously toxic to cats. Veterinary care therefore focuses on managing inflammation with steroids and supporting the cat through respiratory crises while waiting — sometimes two or three years — for the worms to die naturally. In some cats, the first and only sign of infection is sudden death.

Because treatment options are so limited, prevention is overwhelmingly the priority. Safe, effective monthly preventives are available, and the American Heartworm Society recommends year-round protection for all cats, indoor and outdoor alike.