You may have heard of heartworms in dogs, but did you know that cats can get heartworms too? Heartworm in cats is less common and quite different than it is than dogs. In fact, heartworm disease in cats can be even more dangerous.
What is Heartworm Disease in Cats?
Heartworm disease is caused by an infection of the parasite Dirofilaria immitis. Heartworm larvae enter the cat's body when an infected mosquito bites. 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 heartworm disease.
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). Cats may develop chronic lung problems that come and go. Others suffer acute distress and need emergency veterinary care. Though uncommon, some cats may collapse and die suddenly.
Signs of Heartworms in Cats
Not all cats will show signs of heartworm disease. Sadly, a small number of cats will show no signs until they suddenly collapse or die. Cats that do show signs of illness typically have respiratory issues like coughing, gagging, labored breathing, wheezing, and asthma-like episodes. Other potential signs include vomiting, diarrhea, loss of appetite, lethargy, and weight loss.
Causes of Heartworms in Cats
Cats get heartworms after being bitten by infected mosquitos. Although outdoor cats are more often infected, indoor cats are still susceptible to heartworms. A mosquito becomes infected after biting an infected animal and ingesting heartworm larvae. The infected animal is often a dog or cat. However, other mammals may be carriers of the heartworm parasite. In the mosquito, the heartworm larvae begin to mature. When the mosquito bites a cat, the heartworm larvae are injected into the cat where they further develop in the cat's tissues. The larvae migrate through the body, making their way to the lungs.
In dogs, the larvae migrate to the heart, develop into adult worms, and begin to reproduce. The adult heartworms eventually cause problems in the dog's heart, lungs, and circulatory system.
In cats, heartworm disease is primarily a lung problem. Since cats are not ideal hosts, the larvae are compromised by the cat's immune system. The presence of immature heartworms in the lungs causes the immune system to respond with inflammation. Cats may exhibit signs of respiratory illness due to this immune response. In addition, the death of the immature heartworms can cause ongoing respiratory signs and further complications like blood clots. If the cat survives the death of the larvae, leftover scarring and damage can remain for some time, leading to chronic respiratory problems.
Diagnosing Heartworm Disease in Cats
It is more difficult to definitely diagnose heartworm disease in cats than in dogs. This is because the primary test used in dogs, the antigen test, detects the presence of at least one adult worm. In cats, heartworm disease is caused by immature worms. If there are no adult worms present, the antigen test will be negative.
An antibody test may be performed, but this only verifies that the cat was exposed to heartworms. It cannot confirm the presence of immature worms in the cat.
Chest radiographs (X-rays) and echocardiogram can reveal damage to the lungs consistent with heartworm disease. However, these lung changes look much like asthma and cannot ensure the presence of immature heartworms.
Veterinarians use a combination of testing along with the cat's signs to determine whether or not heartworm disease is present.
Heartworm Treatment for Cats
Unfortunately, there is no specific treatment available for heartworm disease in cats. The treatment for dogs with heartworm disease is potentially fatal to cats and is rarely if ever administered.
Surgery to remove heartworms is rarely done but may be recommended in severe cases. Unfortunately, surgery is risky; many cats will die during the procedure or recovery period.
Supportive care is generally recommended for cats with heartworm disease. Your vet may prescribe some medications to treat symptoms and others to reduce inflammation caused by the heartworms. Routine monitoring is also important to assess the cat's lungs and overall health. Heartworms typically only live for two to three years in cats (as opposed to the five to seven years in dogs). If the cat can outlive the heartworms, there may be residual chronic lung disease.
How to Prevent Heartworms in Cats
Prevention is key when it comes to heartworm disease in cats. It's more effective to stop the heartworms from infecting a cat than to try to treat a cat with heartworms. There are a number of effective feline heartworm prevention products available. Most are given topically or orally once per month. Ask your veterinarian for information about the best heartworm preventive products for your cat.
Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease. U.S. Food and Drug Administration, 2020