Heartworm Disease in Cats: Treatment

What is heartworm disease?

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. Heartworms live in the right side of the heart and the pulmonary artery (the large blood vessel that carries blood from the heart to the lungs). Cats become infected when they are bitten by an infected mosquito carrying infective heartworm larvae, and the larvae pass from the mosquito into the cat.

Heartworm disease is challenging to treat in cats because there is currently no approved medication that safely eliminates adult heartworms. As a result, treatment often focuses on managing symptoms and reducing the risk of sudden death while the infection resolves naturally, a process that can take several years.

How is heartworm disease treated?

There are no drugs approved for treating heartworms in cats. Melarsomine (Immiticide®) is used to treat heartworm in dogs, and has been used in cats, but it causes significant side effects and is considered toxic to cats. In addition, this treatment causes the adult heartworms to die and then pass through the pulmonary arteries to the lungs, where the reaction to the dead and dying worms can cause sudden death in cats.

Monitoring mild cases. If a cat is diagnosed with heartworm but is not showing clinical signs, all that may be needed is to monitor antibody and antigen testing, along with regular chest X-rays, while the heartworm infection is naturally eliminated. This can take several years, and the hope is that the cat outlives the worms.

Medical management. Medical options are available to manage the clinical signs associated with heartworm disease and eliminate the worms more quickly. These may include:

• Prednisolone and other corticosteroids can reduce inflammation and scarring in the pulmonary arteries and lungs.  
• An antibiotic called doxycycline can eliminate the Wolbachia bacteria that seem to help worms survive.
• A monthly dose of a heartworm preventive may weaken the worms and eliminate any microfilariae that are present.
• For a cat in crisis, oxygen support, bronchodilators (medications to open airways), and diuretics (medications to remove fluids from the lungs) may be needed.
• When stable, some cats continue to receive corticosteroids periodically or long-term. In many cats, this treatment will reduce clinical signs and improve quality of life, but there is always a threat of an acute crisis or sudden death.

Surgical treatment. Surgical removal of the heartworms may be attempted in cats with severe 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 surgery is generally reserved for cats with severe disease and a poor prognosis without surgery.

How can I prevent my cat from getting heartworms?

Veterinarians strongly recommend year-round monthly heartworm preventives for all cats. Where mosquitoes are seasonal, cats should be on monthly preventives for at least six months of the year. Preventives are recommended for all cats for several reasons:

1. Diagnosis is difficult. Diagnosing heartworms is not as straightforward in cats as it is in dogs. Diagnosis often requires several different tests.
2. Heartworms may be more common in cats than thought. Studies show that up to 15% of all cats in certain locations have been exposed to heartworms, regardless of whether they are indoor or outdoor cats.
3. There is no good treatment. There is no consistently effective treatment for heartworm-infected cats. Some infected cats may die suddenly even when they appear healthy. Treating heartworm infections in cats is risky at best, and not treating these cats is just as risky. It takes about two years for heartworm to be eliminated in the cat, and serious clinical signs can appear at any time.
4. Prevention is safe and easy. Heartworm preventives have an excellent safety record and can be used in kittens as young as six weeks of age.
5. Indoor cats get heartworms too. Cats only need to be exposed to mosquitos to get heartworm – they do not need direct contact with infected animals. Cats that go outdoors are more likely to be exposed, but an infected mosquito can easily get into the house and infect your cat.

Talk with your veterinarian about the best heartworm prevention program for your cat.