April is National Heartworm Awareness Month, and, while most people think about dogs when they think of heartworms, cats can get them, too! We don’t tend to hear much about heartworm disease in cats, probably because unprotected dogs are roughly 10 times more likely to get heartworms than unprotected cats. Nevertheless, dogs and cats are infected in the same manner: through the bite of an infected mosquito. After the bite, though, the disease progresses differently in cats than in dogs, and many cat owners may not realize what the problem is.

First thing first: whether you have a cat or a dog, heartworm disease is the result of a bite by a mosquito infected with a parasitic worm called Dirofilaria immitis. The worm’s larvae are transmitted by the mosquito bite, and they enter your pet’s bloodstream where they develop. Eventually (after about 6-8 months) the larvae move to the right ventricle of the heart and the lungs and arteries nearby. Normally, this is the point where the worms begin to cause significant problems for dogs, but in cats, problems can start much earlier.

So here we see how heartworm disease is different in cats than in dogs. While in dogs, it is a disease primarily of the heart, in cats, it is primarily a disease of the lungs. The larvae arrive in the arteries of the cat’s lungs 60-100 days after infection. At that point, your cat’s immune system reacts with a strong inflammatory response designed to destroy the invaders. Sometimes, the immune response does destroy the worm larvae, but at the same time, it damages the pulmonary arteries as well as the airways, causing the lungs to appear congested. As a result, your cat may display a number of different symptoms, from coughing and labored breathing to vomiting, anorexia, lethargy, or weight loss. There can be more dramatic effects, but the most common symptoms in cats mimic symptoms of other medical problems, such as asthma. In fact, many owners report that their cats’ coughs most resemble (unsuccessful) attempts to cough up hairballs.

Your cat’s response to heartworm infection is the result of the number of worms present in its body, the duration of the infection, and the immune response to the worms. As you saw above, it’s the immune response that can be responsible for some of your cat’s most common symptoms. While it is possible for some cats to spontaneously clear the infection, it isn’t safe to assume that will happen. Moreover, while there are treatments that can kill the worms that infect your dog, there are no such approved treatments for cats. The drugs that can cure dogs of their infections are fatal to cats. Therefore, monitoring and supportive therapy are necessary for cats displaying clinical signs of disease.

Luckily, the heartworms that infect cats tend to have much shorter life spans than those in dogs, making it possible for cats to outlive the worms. Of course, the best way to make sure your cats survive heartworm disease is to make sure they never get them! Because heartworms have been diagnosed in cats that live solely indoors as well as outdoors (mosquitoes go indoors, too!), it’s recommended you give your cats monthly preventatives year round, just like you do your dogs. If you have questions about preventatives or treatments, give us a call!