Merck Manual

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Pet Owner Version

Heartworm Disease in Cats


Clarke E. Atkins

, DVM, DACVIM, College of Veterinary Medicine, North Carolina State University

Reviewed/Revised Jul 2018 | Modified Oct 2022
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Heartworm disease is a potentially fatal, but preventable, infection caused by a worm parasite, Dirofilaria immitis. The organism is transmitted by mosquitoes, which carry the heartworm larvae (called microfilariae) from an infected animal host to a new animal host. Once the larvae arrive in a new host, they grow into adult worms in several months and live in the blood vessels leading from the heart to the lungs. In advanced infections, the heartworms may enter the heart as well. The presence of parasites stresses the heart, blood vessels, and lungs. In addition, severe complications are possible when the heartworms die.

Heartworm infection, cat

Heartworm infection, cat

Cats are somewhat more resistant to heartworm infection than dogs. Therefore, a lower percentage of exposed cats develop infections of adult heartworms, and there are often only 1 to 3 worms present. Heartworms have been known to migrate to areas outside the heart, and they can even invade the central nervous system.

Both indoor and outdoor cats can be infected. Other infections in cats, such as those caused by the feline leukemia virus or feline immunodeficiency virus, do not appear to increase the risk of heartworm infection. The critical factor in heartworm disease is being bitten by a mosquito carrying the infectious heartworm larvae.

The development of a heartworm infection in cats follows a pattern similar to that in dogs. However, because cats have smaller blood vessels and hearts, the severity of the damage is greater. In addition, cats react more to heartworms than dogs, and the lung inflammation associated with heartworm infection leads to poor oxygenation of the blood. Cats are more likely to die of heartworm infection than are dogs.

Signs and Diagnosis

Infected cats may show no signs or exhibit only mild signs. Weight loss is common in cats with heartworm infection. The signs (such as coughing or trouble breathing) often resemble those of feline asthma. Young worms often die once they reach the blood vessels within the lungs. Their death, however, causes a life-threatening condition called heartworm-associated respiratory disease. Cats harboring mature worms may have intermittent vomiting, lethargy, coughing, or occasional shortness of breath. The death of heartworms in a cat can lead to sudden respiratory distress and shock. This situation may be fatal.

Diagnosis of heartworm infection in cats is based on history and physical findings, chest x-rays, echocardiography, and appropriate blood testing. Multiple blood tests may be needed.


There is no current satisfactory treatment for heartworm infections in cats. The death of the heartworms leads to a reaction in cats that is often fatal. Restricted activity and corticosteroids might help ease signs in some cats. Even with medication, however, timing becomes an issue as heartworms live for approximately 2 years in cats. It is estimated that 25% to 50% of infected cats survive heartworm infection. Heartworms can be surgically removed from cats that have a large number of worms living within the heart.


Heartworm infection is preventable. Heartworm preventive medication is recommended for all cats, whether they live indoors or outdoors, in areas where the disease is common. Because heartworm infection can be fatal or severely debilitating for cats, your veterinarian will likely prescribe a preventive medication for all cats, including kittens. This program should be continued for the cat’s entire life.

The most important thing that pet owners can do to protect their companions from heartworm infection is to be absolutely sure their pet receives the prescribed dose of medication at the correct time. Because the most common preventive drugs for both dogs and cats are given only once a month, pet owners might forget to administer the medication. Take advantage of e-mail or other reminder systems offered by your veterinarian or other sources to ensure proper dosing. Neglecting to give the medication can have serious consequences. If you miss a dose, you should contact your veterinarian regarding administration of the medication and the need for retesting your cat to determine if an infection has occurred.

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