seal  Purdue News

May 24, 2004

Feline heartworms more deadly, difficult to treat

WEST LAFAYETTE, Ind. – Many pet owners don't realize that protection from heartworms is as important for cats as it is for dogs.

Steven Thompson,
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"For a long time, veterinarians didn't know cats were getting heartworms because we didn't see heart disease like we see in dogs," said Dr. Steven Thompson, who specializes in canine and feline practice. "Heartworm disease in dogs presents itself as heart failure, with worms stuck in the pulmonary artery increasing the blood pressure and causing enlargement of the heart or other symptoms. Pet owners might notice coughing or exercise intolerance. The dog is the primary host, so many heartworms will be found in the heart during necropsy (an autopsy for animals)."

Cats, on the other hand, are typically only infected with one or two heartworms. Their larval migration to the pulmonary artery stimulates a profound immune response, he said. Because of this difference, heartworms in cats create asthma-like symptoms and breathing problems.

"When one or two adult heartworms make it to the heart, they cause changes in the pulmonary arteries, but rarely heart failure," Thompson said. "While the worm is traveling to the heart, it can trigger an asthma-like condition that can be debilitating or even fatal. Even more critical, when the worm dies a year or two later in the artery/bloodstream of a cat, it can block an artery, called pulmonary thrombosis, and can cause death very quickly. The standard treatment after infection in dogs is to kill the worms, but this is not a good option for cats."

Heartworm preventives available from a veterinarian act the same in dogs and cats. The medication eliminates the worms under the skin during the early larval stages prior to their migration to the heart: a process that starts in the mosquito and takes at least eight months inside a cat.

"Often a cat's own immune system kills the larvae, but it's like playing Russian roulette to not use a heartworm preventive for your cat," said Saralyn Sharp, a registered veterinary technician at Purdue's Pet Wellness Clinic who focuses on community education. "Who knows whether their cat will be the one who can fight it?

Several misconceptions about heartworm exposure in cats cause many pet owners to think they don't need to protect their cats from the potentially fatal parasite.

"A lot of people think their indoor cats can't get heartworms, but mosquitoes can get in houses," Sharp said. "All it takes is one bite from one mosquito carrying heartworm larvae. People also think thick fur will protect their pet from mosquito bites, but it doesn't."

Thompson said that although indoor cats may be less likely to encounter mosquitoes, (studies have demonstrated heartworm exposure rates are 50 percent that of outdoor cats) their immune systems may not be as well-equipped to fight these parasites once they are exposed.

"One study suggests that the more mosquito bites a cat receives, the more heightened is the immune response to this worm," he said. "Indoor cats with lower exposure may not eliminate the larval stage, allowing it to reach the arteries in the heart to become an adult."

Dogs infected with heartworms may develop heart failure, and symptoms may not be apparent for years after an infection. Infected cats are more likely to die suddenly or develop chronic breathing problems. A minority of cats with heartworm disease manifest intestinal problems like vomiting.

"Heartworms evolved to live in the heart of a dog, so when they are inside of a cat, they tend to migrate to different parts of the cat's body, still seeking the dog's heart," Thompson said. "In infected cats, 10 to 15 percent have larvae that migrate to the eyes and brain, which can cause seizures or vision abnormalities."

The best tests to determine exposure to heartworms are different in dogs and cats. It remains quite difficult currently to confirm a cat has heartworms with any single test.

Monthly preventive medications remain the best way to battle heartworms since, after infection, about 25 percent of cats die from the parasite. Preventive medication dosages vary for cats and dogs, so proper selection should include approved prescription products in consultation with a veterinarian.

Thompson said feline heartworm research is ongoing, and that companion animal veterinarians are constantly gaining new insights on the diagnosis, treatment and prevention of this condition in cats.

Writer: Reni Winter, (765) 496-3133,

Sources: Henry Green, assistant professor, veterinary clinical sciences, (765) 494-1123,

Dan Hogan, assistant professor, veterinary clinical sciences, (765) 496-6743,

Steven Thompson, (765) 494-1107,

Saralyn Sharp, (765) 494-1107,

Purdue News Service: (765) 494-2096;

Related Web sites:
American Heartworm Society

Purdue School of Veterinary Medicine Wellness Clinic

Related release:
Spring brings insects and parasites dangerous to pets

Steven Thompson, veterinarian and director of the Pet Wellness Clinic at Purdue University's School of Veterinary Medicine, inspects Kit Kat's fur for evidence of parasitic insects that can carry diseases harmful to animals and humans. During the warmer months, it is vital to protect dogs and cats from fleas, ticks and mosquitoes that carry diseases and parasites that can be harmful to animals and humans. Cats as well as dogs can get heartworms from mosquitoes. Prevention is the best approach to heartworms, especially for cats whose bodies have a lower survival rate than dogs if infected. (Purdue News Service photo/David Umberger)

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