Heartworm in cats is different to heartworm disease in dogs. Usually, when cats are infected, they get fewer worms which are not only smaller in size, they also live for a shorter time (heartworms in dogs can live for up to seven years but it is very uncommon for worms to live for longer than three years in cats).
It seems that cats have some resistance to heartworm as it is believed that the percentage of larvae in an infected cat which develop into adult worms may be less than 25% whereas it might be up to 90% in dogs and is typically in excess of 40%.
In fact some cats have such a strong immunity to heartworm that they rid themselves of the infection without any intervention.
The downside to all this is that in cats, even juvenile worms and larvae can cause very serious damage to the lungs. This can be caused by the fact that in newly infected cats, the majority of the larvae die very quickly inside the pulmonary arteries and this causes serious inflammation and damage. This type of respiratory disease, precipitated by heartworm infection is known by the acronym HARD which stands for Heartworm Associated Respiratory Disease.
Cats with HARD are usually monitored with regular chest X-rays and given treatment with prednisone to relieve the symptoms in the hope that the cat will rid itself of the infection as there are no medicines available in the US which are approved by the FDA for the treatment of heartworm in cats. In more serious cases, treatment to help the cat is largely confined to good nursing care and relief of the symptoms with medications that help to expand the airways and support the heart function. In very serious cases, surgery may be carried out where the worms can be removed manually – however, this is obviously not without risks and is normally only attempted if the worm burden is causing severe obstruction of the blood vessels to the heart and liver.
This Video Explains The Problem of Heartworm In Cats Really Well
Preventive heartworm treatment for cats is available and the American Heartworm Society (AHS) recommends that all cats should receive tests for both antigens and antibodies before any heartworm prevention treatment is embarked upon.
The two FDA approved heartworm preventatives taken orally are Heartgard for Cats and Interceptor and the two topical (spot on) treatments are Revolution and Advantage Multi for Cats. The FDA provides helpful information on heartworm prevention in your pet and there is a downloadable guide in pdf format here which provides a handy references for any questions you may have on heartworm in cats and dogs.