At first glance it might seem improbable that the average house cat is going to be bitten by mosquitoes and potentially infected with HWD. After all, the cat is in the house. Cats enjoy a lower exposure rate than dogs do. What’s the big deal? There are several problems with this theory. First of all, mosquitoes can fly. They are very good at invading homes through the smallest cracks and crevices. And who is home all day and night-the cat. Inside cats are also more likely to be bitten by mosquitoes in the summer and fall, the time of the year that the insects carry the largest number of immature heartworms. With little exposure prior to this time of the year, the cat is unlikely to have protective antibodies present to fight off the heartworm larvae. Outside cats that get exposed to steadily increasing levels of larval heartworms may actually have a better chance of fighting off heartworm infestation than inside cats. New research has shown that even cats that do not develop adult heartworms often develop permanent inflammation in their lungs that causes asthma type symptoms when infected. This difficulty breathing can cause serious stress to the cat and stress on its heart. Even though a cat may only become infested with 1-5 worms, the smaller size of the cat in relation to the parasite can result in a life-threatening situation.

The good news is there is a better way to protect our cats than letting them outside to take their chances. There are now two products approved for cats for the prevention of HWD. Since it is impossible to prevent the cat from getting bitten by the mosquitoes, heartworm preventatives work by periodically clearing the cat’s bloodstream of infective larvae.

Heartgard is a chewable, beef-flavored tablet that is given once monthly to the cat. It is absorbed quickly into the bloodstream where it kills the larval worms as they enter a certain stage of their development. The medication does not remain in the cats system for more than 48 hours-it kills any larval heartworms that the cat has been exposed to in the past 30 days.

Revolution is now available for cats that will not eat the chewable medication. This is a topical, spot-on product that is applied to the base of neck, where it is absorbed through the skin. Revolution also kills fleas, ear mites, roundworms, hookworms and some skin mites. As with Hartgard, this product is used once monthly, and kills any larvae that the cat has been exposed to for the past 30 days.

We emphasize prevention of heartworm infestations because diagnosing and helping cats with heartworm disease can be very difficult. Some cats with HWD may act normally and their infection is detected on a routine screening test. Yet, other cats may be ill and their HWD is detected as part of a larger workup. Heartworms live an average of 1-3 years in the cat, and the most dangerous time for the cat is the period when the worms start to die. The worms cause an almost "allergic" reaction to occur in the lining of the blood vessels in which they live. When the worm dies the allergic response may intensify, or pieces of dead worm may break off and clog blood vessels, cutting off blood flow to important tissues or organs. Heartworm positive cats may vomit, cough, have difficulty breathing, show decreased appetite, or a lack of energy. These cats often present with clinical signs of asthma. Acute allergic reactions can result in sudden death of the cat. Test results may be inconclusive and multiple tests may be needed to achieve a diagnosis.

Treatment of these cats is usually supportive and aimed at reducing the potential for allergic reactions as the cat’s body is working to destroy the worms. Prednisolone is often given to decrease inflammation in the airways and blood vessels. There are no approved medications for the removal of adult heartworms in the cat. The medication approved for usage in dogs has as much a chance of killing the cat, as does the heartworm itself. Heartworm preventatives, such as Heartgard and Revolution, will not remove adult worms from either infected dogs or cats; however, they are routinely used in heartworm positive cats to prevent further infestation.

Heartworm disease is here to stay, and we must all decide the best way to protect our cats from this potentially deadly killer.