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About Heartworm Disease
Heartworm disease is caused by a parasite that lives mainly in the blood vessels of the lungs. Cats are not a natural host for heartworm, but tend to develop severe and potentially fatal disease even when only a single worm is present. Treatment is extremely difficult and risky, but preventative medications are available.
Cause of Heartworm Disease
Heartworm disease is caused by a roundworm called Dirofilaria immitis. Adult heartworms are long and skinny, and can be up to 12 inches long in dogs, though they don't reach this size in cats. They live mostly within blood vessels of the lungs, and sometimes in the heart. Heartworms cause irritation and inflammation in the blood vessels, leading to damage.
Canine heartworm disease is related mostly to damage and blockages in blood vessels. However, in cats, most of the problems arise from the inflammatory reaction to the presence of the worms, which can have serious effects on the lungs.
Heartworm Life Cycle
Adult heartworms reproduce and release their larvae, called microfilariae, into the bloodstream of infected dogs. When mosquitoes feed on a dog (or other animal) with microfilaria in the blood, they ingest the microfilariae, which undergo changes within the mosquito.
After this, the infected mosquitoes can transmit the microfilariae to other animals. The microfilariae migrate through the skin and tissues, and undergo further development, ultimately settling in the blood vessels of the lungs and growing into adult worms.
Cats produce a strong immune response that prevents most of the microfilariae from developing into adults. Also, because cats are not the natural host, the microfilariae may get "lost" and end up in unexpected places, such as the brain.
Heartworm can only be transmitted where temperatures are warm enough to allow the microfilariae to complete their developmental stages within mosquitoes. It is found in widely throughout the world, mainly in areas with relatively warm climates. Heartworm has been found in all 50 US states.
Risk Factors for Heartworm
Cats are more resistant to heartworm infections than dogs, so not all exposed cats develop infections, and those that do typically have fewer than six adult worms in their blood vessels (different from heartworm disease in dogs). However, cats can suffer from severe disease even with such a small number of worms.
Exposure to mosquitoes carrying microfilariae is the main risk for infection. Even indoor cats are at risk, because mosquitoes are notoriously difficult to keep out of the home.
Signs and Symptoms of Heartworm Disease in Cats
Symptoms often appear in cats in two phases. The first is when immature adult worms first arrive in the blood vessels of the lungs, when many of the immature worms die. The associated inflammation can lead to Heartworm Associated Respiratory Disease (HARD), which can easily be misdiagnosed as asthma/allergic bronchitis. Symptoms may subside, but when the mature worms die later on, symptoms (often severe) may reappear. In some cats symptoms of infection may be mild or absent, or intermittent. Signs of infection can include:
- Coughing
- Labored or rapid breathing
- Weight loss
- Vomiting
- Lethargy
- Rarely, other signs can be seen, such as fluid accumulation in the abdomen, fainting, and neurological signs such as incoordination and seizures.
- Sudden severe breathing problems, collapse, shock, and/or sudden death can occur in some cases
Diagnosis of Heartworm Disease
Diagnosis is difficult in cats. In dogs, diagnosis is usually made by a blood test to detect proteins produced by the worms ("antigen") in the blood. This test may be negative early in an infection, if no female worms are present, or only a small number of worms are present, making this test less reliable in cats because severe heartworm disease can occur under all these conditions.
Testing for antibodies to heartworm is also available, but difficult to interpret alone. Radiographs (xrays) can detect changes in the lungs typical for heartworms, and sometimes the worms can be visualized on ultrasounds. A combination of these tests is usually used to make a diagnosis.
Cats rarely have circulating microfilariae, so testing for microfilariae is not as useful.
Treatment of Heartworm Disease
Killing the adult worms with medication can produce a severe and often fatal reaction, so is usually reserved as a last resort. In mild cases, prednisone can be used to diminish inflammation caused by presence of the worms (until the worms die off naturally, since the worms can only live 2-3 years in a cat).
Cats who have a severe response to the worms need more intensive supportive care, including oxygen therapy, medications to open airways, anti-inflammatories, and intravenous fluids to combat shock.
Occasionally, it may be possible to surgically remove worms. Care must be taken to remove the whole worm, however, because cutting a worm can also cause a severe reaction. Because the potential health risks of heartworm infections are so severe in cats, and treatment is so difficult, prevention is much easier than dealing with the disease.
Prevention of Heartworm Disease
Fortunately, prevention of heartworm disease is usually straightforward. There are several preventative medications made specifically for cats (e.g., Heartguard®, Interceptor®, Revolution®, Advantage Multi®). These are given monthly and kill any microfilariae before they can develop into adults.
The American Heartworm Society recommends using a preventative year round, for the life of the cat (starting as a kitten) in areas where heartworm is found.
Many of these medications also protect against some other parasites as well, and your vet will help you choose the right medication for your cat.
Giving the medication at the proper intervals is very important. If your cat misses a dose of heartworm preventative check with your vet, because sometimes a missed dose can leave cats vulnerable to infection.
Please Note: this article is intended for informational purposes only. If your pet is showing any signs of illness, please consult a veterinarian as quickly as possible.