Heartworms are a mosquito-borne parasite whose natural host is the dog. This means that the parasite's normal life cycle includes development and reproduction in the canine heart and blood vessels. However, the heartworm parasite can also infect cats and cause serious illness.

Cats infected with heartworms show predominantly respiratory illness, such as coughing and labored breathing. They may also vomit, lose weight, and act weak and lethargic. Unlike heartworm disease in dogs, treatment of cats is very difficult. In general, the prognosis for heartworm disease in cats is poor.

Cats are not as susceptible; however, 5 out of 100 exposed cats will be infected. Because some cats are at risk and if infected may become fatally ill, strong consideration is now being given to use of heartworm preventative in cats at risk - cats that go outdoors in areas known to have mosquitoes or households with heartworm-infected dogs. A major pharmaceutical firm is seeking FDA approval for a heartworm preventative in cats.

Currently, certain canine heartworm preventatives may be used in cats under veterinary guidance. Although it is not required, it is advisable to run a heartworm test on cats over 6 months of age prior to use of this medication. Dose recommendations for cats differ from those of dogs and incorrect dosing could be dangerous for your cat; therefore DO NOT try to give your cat heartworm preventative without consulting a veterinarian.