Heartworm disease in cats is a bit different than in dogs. Heartworms in cats do not live as long (average lifespan is only two to three years) or grow as long, and fewer of them mature into adults. Worm burdens are lower in cats, with the average being one to three worms. However, due to its relatively small body size, a cat with only a few worms is still considered to be heavily infected.
In cats, it takes infective larvae eight months to mature into adult heartworms and produce microfilariae. This is about one month longer than in dogs. The presence of microfilariae in a cat’s bloodstream is uncommon. Only 20 percent of cats with heartworm disease have microfilariae in the bloodstream, compared to 80 to 90 percent of dogs with heartworm disease. Also, the presence of microfilariae in the bloodstream is inconsistent and short-lived in cats.
It is harder to detect heartworm infections in cats than in dogs. Veterinarians generally use two types of blood tests in combination to check a cat for heartworms. However, negative test results do not rule out heartworm infection, and positive test results may or may not mean that there is an active heartworm infection. A veterinarian uses the results of both blood tests, along with the cat’s symptoms and the results of other tests such as x-rays and an ultrasound of the heart, to determine if a cat has heartworm disease.
Not all cats with heartworm disease show symptoms. Some cats are able to spontaneously rid themselves of heartworms without showing symptoms. However, some infected cats die suddenly from heartworm disease without ever showing signs of being sick. Cats with heartworm disease may show very nonspecific symptoms that mimic many other cat diseases. These nonspecific symptoms include vomiting, decreased activity and appetite, and weight loss. Cats with heartworm disease rarely show signs of heart failure.
In cats that show symptoms of heartworm disease, respiratory signs are the most obvious due to the lung damage caused by the heartworms. Cats typically show symptoms of heartworm disease at two time points – when the immature heartworms arrive in the heart and lung arteries and when the adult heartworms die.
The immature heartworms arrive in the heart and lung arteries about three to six months after a cat is bitten by an infected mosquito. Many of these immature heartworms die, causing a strong inflammatory response in the cat’s lungs. This response is called heartworm associated respiratory disease (HARD) because respiratory signs, such as trouble breathing, increased respiratory rate, and cough, are the most obvious. It may be difficult to distinguish HARD from feline asthma or feline bronchitis.
When the adult heartworms die, they release toxins into the cat’s bloodstream which cause lung damage, leading to respiratory problems or sudden death. Even the death of one worm can be fatal for a cat.
There is no FDA-approved drug for the treatment of heartworm disease in cats, although symptoms may be managed with medications. Surgical removal of adult heartworms may be a treatment option if the heartworms can be seen by ultrasound. But surgery is risky, and if the heartworms are not removed intact, there can be potentially serious complications, such as shock and death.
Several products are FDA-approved to prevent heartworms in cats. There are both topical and oral products for cats, and all are given monthly and require a veterinarian’s prescription. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas and ear mites).
Again, year-round prevention is best! Talk to your cat’s veterinarian to decide which preventive is best for your cat.
Ferrets can also get heartworms from the bite of an infected mosquito. Ferrets are similar to dogs in their susceptibility to heartworm infections, but their symptoms are more similar to those seen in cats.
Infected ferrets typically have low worm burdens, and microfilariae are seen in the bloodstream in only 50 to 60 percent of ferrets with heartworm disease. Symptoms of heartworm disease in ferrets include decreased activity level, coughing, trouble breathing, and overall weakness. Heart failure can occur in severe cases. Based on a ferret’s symptoms, a veterinarian may perform chest x-rays and an ultrasound of the heart to determine if it has heartworm disease. Blood tests to detect heartworm infections in ferrets are generally unreliable.
No drugs are FDA-approved for the treatment of heartworm disease in ferrets. And only one drug, Advantage Multi for Cats (imidacloprid and moxidectin), is approved to prevent heartworms in ferrets. Available only with a veterinarian’s prescription, it is a topical solution that is applied monthly. In addition to preventing heartworms, Advantage Multi for Cats also treats flea infestations on ferrets by killing adult fleas.
Again, prevention is the best treatment! Year-round prevention is recommended for all ferrets. Talk to your ferret’s veterinarian about preventing heartworm disease in your furry friend.