F310
Heartworm Disease
(Dirofilariasis)

Introduction: Heartworm disease is found all over the world. In the U.S.A., the disease has been found most commonly in the East, South, and Midwest areas. With time, the disease can spread to every area of the country where mosquitoes are present.

Causative Agent: Heartworm disease is caused by the parasite Dirofilaria immitis. The parasite begins its life cycle as microscopic organisms called microfilariae. Microfilariae enter a cat’s circulatory system through the bite of an infected mosquito. Once the infective larva enter the tissues and blood stream of the cat, they grow and mature into adult stage worms. After this phase, they begin to migrate to the heart and lungs.

Clinical Signs: The most common signs of heartworm disease are coughing, weight loss, fever, and a lack of energy. Additionally, signs of more progressed disease include difficulty breathing, fluid in the abdomen, and swelling of the extremities. If the worm has progressed to the heart, severe weakness, collapse, and sudden death can occur. In some cases, heartworm disease may cause damage to the kidneys; consequently, an animal will have an increased thirst and need to urinate. Sudden death with no other warning may occur.

Disease Transmission: Heartworm disease is indirectly spread from animal to animal through the bite of a mosquito. Any other method of sharing blood or blood products between cats, such as blood transfusions, may also cause disease transmission. Heartworm disease is not transmitted by direct animal to animal contact unless blood is exchanged.

Diagnosis: If any animal is suspected of having heartworm disease, a simple test may be performed in the clinic. See page D308 for information on this test. In some cats with heartworm, these tests can produce a negative result. If this occurs, additional testing may be necessary. Because treatment is most successful if the diagnosis is made early, routine screening of healthy pets for heartworm disease is highly recommended.

Treatment: Cats have an inherent ability to withstand and fight off heartworm infection on their own; because of this and because of the greater risk in treating cats for heartworm disease as compared to dogs, treatment plans for cats infected with heartworm differ greatly from treatment plans for dogs.

In cats, the treatment for mild to moderate cases is mainly supportive, with frequent monitoring of the cat’s heartworm status. The hope is that supportive therapy will keep the cat healthy, while the cat’s own immune system eliminates the infection. Supportive care consists mainly of anti-inflammatory and diuretic therapy (prednisolone and furosemide), along with drugs to kill the larval worm stages (ivermectin).

In more severe cases, where it is apparent that the cat will succumb to the disease if the adult worms are not destroyed, a drug is used to kill the existing worms in the heart. Such treatment is dangerous, however, and can result in death of the cat. Treatment is also given to destroy the larval worm stages, and strict rest is recommended for several weeks. Careful monitoring under the watchful eye of a veterinarian is essential during any form of treatment for heartworm disease.

Prevention: Once a cat is determined to be heartworm free, any of the preventative products may be recommended. These products are very safe for most animals. See pages C348 and C710 for more information on heartworm prevention.