Although far likelier to infect dogs, cats can also become infected by heartworms. These parasitic worms take up residence inside the right side of the animal's heart, along with the large blood vessels between the heart and the lungs. Once in place, these sizable worms cause inflammation, damage to the blood vessel walls, and restrictions to blood flow that can cause symptoms such as coughing, shortness of breath, vomiting, weight loss, and lethargy. In severe cases, the cat may go into respiratory distress that can be fatal.
Don't assume that only outdoor cats get heartworms: Around one-third of infected cats are indoors-only. All it takes is the bite of a mosquito for your cat to become infected, which can happen indoors or outside.
Because it is far more difficult to treat heartworms in cats than in dogs, the best course of action is prevention. Here's what you should know about the symptoms, causes, treatment, and prevention of this dangerous parasite.
What Is Heartworm Disease?
Heartworm disease is caused by an infection with the parasitic roundworm Dirofilaria immitis. Heartworm larvae enter the cat's body when an infected mosquito bites the cat. Because cats are not ideal hosts for heartworms, (dogs are the ideal host) the larvae may become weakened and succumb to the cat's immune system before causing any problems in the cat. This is why it's somewhat uncommon for cats to develop full-blown heartworm disease. Many show no symptoms at all.
However, if the immature heartworms can survive long enough to travel to the arteries of the lungs, they create an inflammatory response generated by the cat's immune system. This can cause serious complications like heartworm-associated respiratory disease (HARD), which leads to coughing and trouble breathing. It can be even worse when an adult worm dies, however. When this happens, toxins are released that can damage the lungs. The death of just one adult worm can be enough to cause sudden death. Other cats may develop chronic lung problems.
Symptoms of Heartworms in Cats
Not all infected cats will appear ill. Some will have no symptoms at all, while others show only vague signs that are common to many disorders. Sadly, a small number of cats will show no signs until they suddenly collapse or die.
In cats that show symptoms, there are two points during the infection that they are likeliest to appear: when the immature worms reach the arteries in the cat's lungs and heart and when adult heartworms die.
The following are some of the many symptoms it's possible you'll see in a cat with heartworms.
Symptoms of heartworms in cats can resemble many other illnesses, including feline asthma or viral respiratory infections. That can make it tricky to recognize the signs of this potentially very serious infection.
If immature worms take up residence in the cat's lungs and associated arteries, respiratory symptoms are the most obvious sign of infection. The cat will typically cough and gag, may breathe more rapidly than normal (tachypnea), and can appear short of breath or as if laboring to breath. If the inflammation is severe enough to cause respiratory distress, the cat may suddenly collapse or die.
Cats with mature heartworms can develop chronic, vague symptoms of illness, including intermittent vomiting or diarrhea, loss of appetite that leads to weight loss, and general lethargy or fatigue.
Causes of Heartworms
Heartworms, more properly called Dirofilaria immitis, are parasitic worms that reach impressive lengths. Females can be as long as 14 inches and up to 1/8-inch in width. Males are generally much smaller, but still large in relation to the small blood vessels in a cat's heart and lungs.
Heartworms require two hosts to complete their lifecycle: an intermediate host and a definitive, or primary, host. The intermediate host is always a mosquito. The definitive host for heartworms is most often a domestic or wild dog, but cats and ferrets can also serve this role, although not nearly as effectively.
In the usual heartworm lifecycle, female worms infecting a dog release their larva, called microfilariae, into the dog's bloodstream. If a mosquito bites the dog and ingests its blood, it also takes in the microfilariae, which over the next two weeks or so develop into infective larva inside the mosquito. At this point, if the mosquito bites another dog, it passes the larva back into the animal, where the worms mature into adults over the next six to seven months, eventually mating and producing more microfilariae. In a dog's heart, the worms can live as long as seven years.
In a cat, however, far fewer worms survive to mature into adults. Those that do take on average a month longer to produce microfilariae than in a dog. Typically, 80 to 90 percent of dogs with heartworm disease have microfilariae in their bloodstreams, compared to around 20 percent of infected cats. Heartworms also don't generally live as long in a cat as in a dog; two to four years is typical. And overall, infected cats generally have far fewer adult worms than do dogs. An infected cat might have as few as one or two worms, compared to an average of 15 worms in a dog and potentially hundreds of these dangerous parasites. Nor do adult heartworms grow as large within a cat's heart as they do in dogs.
Diagnosing Heartworms in Cats
It is more difficult to definitively diagnose heartworm disease in cats than in dogs. An antibody test may be performed, but this only verifies that the cat has been exposed to heartworms. It cannot confirm—nor does it rule out—the presence of immature worms in the cat. Another blood test that's often used is the heartworm antigen test. This detects adult female heartworms very accurately, but only if there are at least two adult female worms present, which is often not the case with cats. A cat that has positive results on both of these tests is confirmed to have heartworms, but it is quite possible for an infected cat to have negative results on one or both of the tests.
Another blood tests looks for the presence of microfilariae in the cat's blood. If positive, that establishes a diagnosis of heartworms. However, since the majority of infected cats don't have larva in their bloodstream, a negative result cannot rule out infection.
Chest radiographs (X-rays) and echocardiography can reveal damage to the lungs and heart, consistent with heartworm disease. However, lung changes can look much like asthma and do not necessarily confirm the presence of immature heartworms.
Veterinarians use a combination of testing along with the cat's clinical signs to determine whether or not heartworm disease is present.
Unfortunately, there is no specific treatment available for heartworm disease in cats. The treatment for dogs with heartworm disease is not suitable for cats, as it can cause serious side effects.
Supportive care is generally recommended for cats with heartworm disease that are not showing severe symptoms. This generally involves corticosteroids either intermittently or continually to reduce inflammation. If the cat suffers an attack of respiratory difficult, oxygen therapy is usually prescribed until the cat is once again stable. Diuretics to remove fluid from the cat's lungs might also be needed during a flareup of symptoms. Routine monitoring is also important to assess the cat's lungs and overall health.
Surgery to remove heartworms may be recommended in severe cases. Unfortunately, surgery is risky; up to 40 percent of cats will die during the procedure or recovery period.
Prognosis for Cats With Heartworms
Unfortunately, the prognosis for a cat with heartworms is poor. Up to half of felines with this parasite will die from the infection, some suddenly without showing prior signs of illness.
How to Prevent Heartworms
Prevention is key when it comes to heartworm disease in cats. It is very difficult to treat an established heartworm infection in a cat, but there are several effective options to prevent infection before it begins. Many veterinarians recommend heartworm prevention for all cats, whether indoor-only or outdoors. And don't assume that your cat is safe just because you live in an area where heartworms aren't as prevalent as they are on the Atlantic and Gulf coasts of the United States; heartworms have been found in every state.
Typically, heartworm preventatives are oral medications that are given just once per month. Talk to your veterinarian about the best option for your cat.
Are Heartworms Contagious to Humans?
While in rare cases, humans can catch heartworms from the bite of an infected mosquito, your cat cannot directly transmit heartworms to you, nor can it directly infect other pets in the home.
Heartworm Disease in Cats. Merck Veterinary Manual.
Heartworm Disease in Cats. VCA Animal Hospitals.
Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease. U.S. Food and Drug Administration.