Submitted by:
Michael List, D.V.M.

CATS AND HEARTWORMS­COMMON QUESTIONS ANSWERED

HOW SERIOUS IS HEARTWORM DISEASE IN CATS?

Some cats infected with heartworm disease show no signs of disease through the entire life cycle of the parasite. Others experience acute respiratory distress that mimics asthma, or die suddenly.

HOW COMMON IS HEARTWORM DISEASE IN CATS?

Heartworms have long been thought to be very rare in cats for several reasons:

--Heartworms in cats rarely produce larvae (microfilariae), so tests for them are nearly always negative.

--Tests for adult worms are usually negative because cats usually have fewer worms than do dogs.

--Post mortem examination is often the only definite way to diagnose heartworms in cats. Routine post mortems often do not reveal heartworms unless the examiner strongly suspects the disease.

However, recent reports suggest that feline heartworm disease may be much more widespread than previously assumed. We canąt help but be skeptical, since we have never succeeded in diagnosing the disease in cats, but we must admit that we may have seen cats with heartworms and failed to prove it.

CAN FELINE HEARTWORM DISEASE BE TREATED?

Mild cases may respond to corticosteroid drugs, but the treatment for seriously ill cats is difficult and even more dangerous than in the dog. Since a definite diagnosis very often cannot be made on a living cat, the decision whether to treat these cats can be very difficult.

CAN FELINE HEARTWORM DISEASE BE PREVENTED?

Yes. A monthly heartworm preventive medication has now been approved for use in cats. The same drug is very effective in dogs. The active ingredient, ivermectin, has been used at much higher doses for years to treat earmites in cats, so we assume the heartworm preventive form will be safe.

DO CATS NEED TO BE TESTED BEFORE STARTING HEARTWORM PREVENTIVE?

No. Since circulating larvae in infected cats are usually few or absent, there is very little risk of adverse reactions due to mass dying off of larvae.

DO WE RECOMMEND HEARTWORM PREVENTIVE TREATMENT FOR CATS?

This is a tough question. We find it difficult to recommend that all our feline patients routinely take heartworm preventive when we have yet to diagnose a case of the disease. However, we would feel terrible if we failed to make the product available and then had patients die of a disease we could have prevented. It is very hard to balance the risk of using the medication (which we assume to be very small) against the unknown risk of not using it. So our recommendation is this: If you, after reading the above information, wish to give your cat heartworm preventive, we will be happy to provide it to you. As far as we know, the product should be safe when used as directed.