Cat with blue eyes laying on her back.

Heartworms in Cats

As frightening as it might seem, the incidence of this disease, once thought limited to canines, is on the rise in cats as well. Dirofilaria immitis, the same mosquitoborne organism that causes canine heartworm disease, also causes the feline disease.

Most cats that become infested with heartworms develop less than 10 worms within the heart, yet even this low number can damage the heart and lead to lung and kidney disease as they do in the dog.

The main reason for this is that cats produce a more potent immune response to the parasites than do dogs, which can result in greater damage to the tissues involved. Male cats allowed to roam outdoors are at greatest risk of contracting this disease.

Many cats infested with heartworms will show no clinical signs whatsoever, and the disease will be identified incidentally when these cats are brought to the veterinarian for other reasons. In more advanced cases, lethargy, breathing difficulties, coughing, vomiting, and sometimes even blindness can occur. Special blood tests can be used to help detect heartworm disease in cats.

However, because the worm burden in some cats is so low, some infections may be missed by these tests. As a result, many veterinarians base their diagnosis on chest radiographs (X rays) and ultrasound findings, which usually reveal changes in the heart and lungs characteristic of heartworm disease.

Treatment, if performed, is similar to that used for dogs. However, because of the comparatively small size of the feline heart and lungs and the intense feline immune response to these parasites, killing the adult worms contained within the heart can actually do more harm than good.

As a result, cats diagnosed with this disease, yet not showing any clinical signs, are often not treated but placed on heartworm preventive instead to prevent additional infestation. Unfortunately, those cases exhibiting clinical signs of heartworm disease carry with them a poor to grave prognosis for recovery.

Because of the difficulty in safely treating heartworms in cats if they come down with the disease, cat owners should consider giving their pets preventive heartworm medication on a monthly basis. Such medication is available through veterinarians.