April is National Heartworm Awareness Month. When it comes to heartworm disease, dogs are probably the first pets that come to mind. Although dogs are the pet most commonly affected by heartworm, cats can get them too. Recent studies have shown that the incidence of heartworm disease in cats is much greater than previously thought. Cats are not a natural host for heartworms and are relatively resistant when compared with dogs, but we still see an infection rate of 5-20% of the rate of dogs within the same geographic area.
Cats become infected by heartworms the same way dogs do: through the bite of a hungry, infected mosquito. Many pet owners are surprised to learn that about a third of heartworm-infected cats are indoor cats. Think of how easily a mosquito can enter your home – through an open door as you enter or leave, or a small tear in a window screen. How many times have you swatted at a mosquito inside? Though your cat may be an indoor cat, that does not make her immune to heartworm.
Cats must be bitten directly by a heartworm-infected mosquito; heartworm cannot be transmitted from one cat to another, or from a dog to a cat. When mosquitos land on a cat and have a blood meal, they insert their sharp mouthpieces into the cat’s body. They then deposit microscopic heartworm larvae into the cat’s bloodstream as they feast on the cat’s blood. The larvae mature for several months, then migrate towards the heart and lungs through the bloodstream. They end up on the right side of the heart and in the pulmonary artery (the large vessel that carries blood from the heart to the lungs), where they mature into adult heartworms. About six months after initially entering the cat, the heartworms are mature enough to reproduce.
Cats can be particularly difficult to diagnose with heartworm. There are no specific clinical signs in cats, although they may show non-specific signs such as lethargy, lack of appetite or weight loss. The most common sign is the sudden onset of a cough and difficulty breathing, which can often be mistaken for and/or misdiagnosed as asthma. Sometimes, an apparently-healthy cat may be found dead or develop sudden respiratory failure leading to death, in which case heartworm disease may be diagnosed post-mortem.
Unfortunately, there is no medication to kill heartworms that is approved for treating cats, as there is for dogs. Treatments are usually supportive, focusing on managing symptoms of the disease and preventing the reproduction of new heartworms. Surgical removal of the worms is possible but is often reserved for cats with poor prognosis without surgery. However, because cats are not natural hosts for heartworms, the worms are often unable to complete their life cycle and die without reproducing. This means heartworm infections may be short-lived in some cats and resolve on their own.
The best way to prevent heartworm in your cat is to give your cat a heartworm preventive. In warmer climates where mosquitos are active year-round, cats should receive a preventive year-round. In colder climates where mosquitos are seasonal, cats should be on a preventive for at least 6 months of the year, starting before mosquitos become active. There are many forms of preventive, usually given monthly; they may be given orally or applied topically. Your veterinarian can suggest products that best fit your needs.
During National Heartworm Awareness Month, talk to your veterinarian about the best heartworm preventive plan for your cat. After all, an ounce of prevention is worth a pound of cure!