Heartworm Disease in Cats - Treatment

This handout provides information about the treatment of heartworm disease in cats. For more specific information on the causes and transmission of heartworm disease in cats, as well as testing procedures, see the handouts “Heartworm Disease in Cats” and “Testing for Heartworm Disease in Cats”.

What is heartworm disease?

Heartworms are a blood-borne parasite called Dirofilaria immitis that reside in the heart or adjacent large blood vessels of infected animals. Female worms are 6 - 14 inches long (15 - 36 cm) and 1/8 inch wide (3 mm). Males are about half the size of females. Heartworm disease is much more common in dogs than in cats; however, recent studies of cats with heart and respiratory diseases have found an incidence of heartworms that is far greater than previously thought. Cats are relatively resistant to heartworm infection when compared to dogs, with the infection rate in cats reported to be 5-20% of the rate in dogs in the same geographic location. Typically, cats have fewer adult worms than dogs, usually less than six. Many pet owners are surprised to learn that approximately 1/3 of infected cats live indoors only.

How are heartworms transmitted to a cat?

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The life cycle of the heartworm is complex and requires two host animals in order to complete it. Heartworms require the mosquito as an intermediate host. As many as 30 species of mosquitoes can act as this host and transmit heartworms.

Mosquitoes ingest microfilariae (immature heartworm larvae) by feeding on an infected cat or, more commonly, an infected dog. The microfilariae develop further for 10 to 30 days in the mosquito's gut and then enter its mouthparts. When an infected mosquito bites a cat, it injects infective larvae into the cat. The larvae migrate into the bloodstream, ending up in the right side of the heart and the pulmonary arteries. There, they mature into adult heartworms capable of reproduction in about six to seven months. Shortly thereafter, around eight months after infection, they begin to produce a new crop of microfilariae that will live in the cat's blood for about one month.

As cats are resistant hosts, few circulating microfilariae are generally found. It is necessary for a cat to be bitten by an infected mosquito in order to become infected with heartworms. Heartworms are not transmitted directly from one cat to another or from a dog directly to a cat.

How is heartworm disease in cats treated?

There is no drug approved for treating heartworms in cats. Melarsomine (Immiticide®), one of the drugs for treating dogs, has been used in cats but it causes significant side effects and is considered toxic to cats. To complicate things further, when the adult heartworms die during this treatment, they pass through the pulmonary arteries to the lungs where the reaction to the dead and dying worms can cause sudden death. Thus, there is a dilemma when a cat is diagnosed with heartworm disease. One of three choices must be made:

1. Treat with melarsomine. However, this drug can have serious side effects in cats including acute pulmonary (lung) failure and death. For this reason, this approach is not recommended.

2. Treat the symptoms of heartworm disease and hope that the cat outlives the worms. Since heartworms live in a cat for about two to three years (as opposed to five to seven years in dogs), several months of treatment are needed. When cats are in a crisis, they are treated with oxygen and corticosteroids to reduce the reaction occurring in the pulmonary arteries and lungs, and, if needed, drugs to remove fluid from the lungs (diuretics). When the cat is stable, it is treated continuously or periodically with corticosteroids. In many cats, this treatment will reduce clinical signs and improve their quality of life but the threat of an acute crisis or sudden death always exists.

3. Surgical removal of the heartworms is currently the recommended treatment for cats with severe signs of heartworm disease. This procedure must be performed by a specialist, often at a college of veterinary medicine. Studies have shown that up to 40% of cats may die during or after this procedure, so surgical heartworm removal is typically reserved for those cats who have severe disease and a poor prognosis without surgery.

Is there a way to prevent heartworm disease in cats?

Veterinarians strongly recommend that all cats receive year-round monthly heartworm preventative in areas where mosquitoes are active all year round. Cats that live in colder areas, where mosquitoes are seasonal, should be on monthly preventives for at least six months of the year. There are excellent heartworm preventives available for cats, making prevention of heartworm disease safe and easy. The reasons that heartworm prevention is now recommended for all cats include:

• Diagnosing heartworms is not as easy in cats as it is in dogs.
• Heartworms are not nearly as common in cats as they are in dogs; they are probably more common than we realize. As we look more aggressively for heartworms in cats with better tests, we expect to find that the incidence is greater than previously thought. University studies have shown that up to 15% of all cats in certain locations, regardless of whether they are indoor or outdoor cats, have been exposed to heartworms.
• There is simply no good treatment for heartworm-infected cats. Effective drugs are not available, and cats that seem to be doing well may die suddenly. Treating heartworm infections in cats is risky at best, and not treating these cats is just as risky. It will take about two years for the parasitic infection to be eliminated in the cat, and serious clinical signs can suddenly appear at any time during this period.
• Cats given heartworm prevention drugs have not shown signs of toxicity. There is a wide margin of safety, even in kittens as young as six weeks of age.
• Indoor cats get heartworms too. Exposure to mosquitoes is required for transmission. Cats do not have to be exposed to cats or dogs infected with heartworms. Obviously, cats that go outdoors are more likely to be exposed; however, an infected mosquito can easily get into the house and infect the cat.

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