Heartworm Disease (Feline)

by Petside Team

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Feline heartworm disease is a disease caused by a parasite, Dirofilaria immitis, that lodges in the heart and primarily affects the lungs in the cat, as opposed to affecting the heart in the dog.

Common name: Feline heartworm disease
Scientific name: Dirofilaria immitis

Diagnosis

Signalment
All cats are susceptible to developing the disease if they live in areas where mosquitoes breed and there are dogs infected with immature worms (microfilariae). For cats to be infected, the mosquito must first feed on an infected dog and then, after a period of warm environmental exposure, feed on a cat.

Incidence/prevalence
The incidence of heartworm disease in cats is less than in dogs due to the number of cats that live indoors and the difficulty in diagnosing the disease in cats, but it ranges from 19 percent to 46 percent.

Geographic distribution
Heartworm disease is more prevalent in climates that are conducive to large mosquito populations for long periods of the year and areas where there are large populations of dogs that are not on a heartworm preventive.

Clinical signs (primary, most to least frequent, scientific term, synonyms)
Asymptomatic, Coughing, Vomiting, Dyspnea (respiratory difficulty), Anorexia (loss of appetite), Weight loss, Collapse, Convulsions, Tachycardia (increased heart rate), Syncope (fainting), Sudden death.

Clinical signs (secondary, most to least frequent, scientific term, synonyms)
Fever.

Cause (scientific, common term)
Dirofilaria immitis (heartworms).

Organ systems affected
Lung, Heart.

Diagnostic tests
Antigen/antibody test.

Differential Diagnosis
Asthma, Cardiomyopathy, Fungal infections affecting the lungs, Pneumonia, Neoplasia (cancer).

Overview

Feline heartworm disease is caused when a parasite, Dirofilaria immitis, lodges in the heart. It is caused by the same parasite as the disease in dogs, but it is a uniquely different disease in cats, manifested by lung involvement, as opposed to primary heart involvement in the dog.

All cats are susceptible to the disease if they live in areas where there are breeding mosquitoes and a large population of dogs that are not on the heartworm preventive. The disease is spread by mosquitoes feeding on an animal that has the immature worms, microfilaraie, circulating in the blood.

Since cats very rarely have the immature form of the parasite circulating in their bloodstreams, the disease is transmitted from an infected dog by a mosquito that then bites a cat. Dogs can have a very heavy worm infestation, but cats are usually infected by only one to three worms. The worms are often of the same sex so that no reproduction takes place, which is why there are no immature worms in the bloodstream of cats.

In cats, the parasites cause more problems in the lungs than in the heart, and the respiratory signs can be confused with the signs of feline asthma, pneumonia, fungal infections in the lungs and cancer.

Treatment

Home Care
Prevention of the disease by use of a preventive product prescribed by a veterinarian. Treatment of the disease must be done by a veterinarian.

Professional Care
It is often difficult to diagnose heartworm disease in a cat due to the small number of worms that are present in the cat's heart. The standard tests used in dogs often come up negative even when the disease is present because of the low number and same sex of the worms in cats. Radiographs can sometimes show cardiac and pulmonary abnormalities that point to the possibility of heartworm disease.

Because of this, diagnosis is often based on clinical signs and the ruling out of other diseases that could be causing the same clinical signs. The signs most often associated with the disease are coughing, difficulty breathing, vomiting, weakness, loss of appetite and weight loss.

Action
In the dog, there is a medication that can be used to destroy the adult worms and thus clear the heart of the worm burden. This medication is hazardous if used in the cat and is not recommended, unless the respiratory signs are life-threatening, since complications can include extreme respiratory difficulty and sudden death.

The only treatment available is to use the same medication that is used as the preventive. These preventives can be given as a monthly pill or as a topical, spot-on product. These medications prevent further infection and any possible maturation of the immature worms if the few adult worms have managed to reproduce.

Treatment is then to provide basic supportive care for the cat and treat the clinical signs while waiting for the adult worms in the heart to die, which can take as long as two years.

Outcome
With proper treatment of the respiratory problems and the monthly administration of the heartworm preventive, the outcome for most cats is favorable.

References/Additional Readings

Bowlin, C., Dillon, R., Eigner, E., Ford, R., Knight, D., Kevy, J., McCall, J., and Nelson, T., Alternatives: A Veterinary Clinical Update, Supplement to Compendium on Continuing Education for the Practicing Veterinarian, Vol. 21 (12L) 1999; 1-8.

Miller, M., Heartworm Disease in Cats. In: Tilley, L.P., and Smith, Jr., F.W.K., ed. The 5 Minute Consult: Williams & Wilkins, 1997; 637.

Aiello, S.E., ed, Heartworm Disease. In: The Merck Veterinary Manual, Merck and Company, 1997: 93-97.

Rawlings, CA, and Calvert CA, Heartworm Disease. In: Ettinger, SJ and Feldman, EC Textbook of Veterinary Internal Medicine, 4th Ed. Vol 1. W. B. Saunders Company, 1995; 1062

Author
Joyce Eisold, DVM

Editor
Steven Hansen, DVM, MS, MBA
DABVT, DABT