Heartworm Infection in Cats

What is heartworm disease?

Heartworm disease is a parasitic health condition caused by a blood parasite, Dirofilaria immitis. The condition is characterized by moderate to severe heart and lung damage, often resulting in serious clinical signs and can be fatal if not detected early and not treated appropriately.

This condition is more common in dogs and causes a wide range of symptoms depending on how heavy the worm burden is. Cats are considered atypical hosts of the parasites and rarely experience severe health problems, but the presence of infection can still result in considerable damage to the heart.

How do cats get heartworm disease?

Heartworms are vector-borne parasites, with mosquitoes being the main culprit in spreading parasites among cats and dogs. They require two hosts to complete their life cycle, and infection happens when a mosquito, the intermediate host carrying the infective larval stage of the parasite, takes a blood meal from the definitive host.

The immature heartworm, called microfilaria, will then stay in the host’s bloodstream where it will develop into an adult worm in about 5-7 months. Dogs are the main definitive hosts of heartworms, where most microfilaria that can infect the animal will become adults if not treated. In cats, however, microfilaria rarely survives to reach their adult stage.

Cats are considered atypical hosts for heartworms, and most microfilaria will not reach their adult stage inside the cat’s cardiovascular system. Most cats that are infected with heartworm will only have one to three adult heartworms in their system, unlike in dogs that can have enough adult heartworms to cause severe obstruction of blood flow or compromise their heart’s functions.

Adult heartworms will mate and produce immature heartworms on their own, but these larvae are not infective and will not develop into adults in the same host. It needs to infect the intermediate host where it will enter its infective larval stage, ready to infect another cat or dog.

Since the number of adult heartworms seen in cats is significantly less compared to infected dogs, the number of microfilariae it produces is also smaller. This results in a significantly slower spread of infection among cat populations compared to dogs. Technically, heartworm infection is not contagious, and an infected individual will not be able to transmit the parasite to other animals. Participation of the intermediate host (mosquito) is necessary for the spread of infection between two animals.

Symptoms of Heartworm Disease in Cats

Clinical signs associated with heartworm disease can be generally categorized into 4 classes, depending on how severe the symptoms are:

• Class 1: No symptoms or mild clinical signs such as occasional coughing can be observed in infected individuals.
• Class 2: Mild to moderate symptoms can be seen like more frequent coughing fits and mild exercise intolerance.
• Class 3: Moderate to severe clinical signs are present. Coughs are more frequent and forceful and general signs of illness such as lethargy and decrease in appetite can be seen.
• Class 4: Severe clinical signs such as persistent, forceful cough, breathing difficulties, lethargy, weakness, and inappetence are usually seen in animals with this type of heartworm infection. Also called caval syndrome, this degree of infection is often fatal to infected individuals even with treatment.

The whole range of symptoms can be seen in dogs infected with heartworm, but in cats where the presence of adult parasites is very uncommon, the clinical manifestation of the disease is usually limited to either class 1 or class 2 infection types. Heartworm infection will rarely cause severe clinical signs in cats, but immunocompromised individuals or those with other health problems are at a greater risk.

How can my cat be tested for heartworm?

The presence of heartworm in cats can easily be confirmed by testing their blood. Since heartworms mainly stay in blood vessels, blood can be tested for signs of heartworm infection such as the presence of antigen or microfilariae.

Antigen tests detect the presence of proteins released by adult female worms. However, since the presence of adult heartworms in cats is uncommon, there is a chance for false-negative results in heartworm antigen tests in cats.

Microfilaria is easily visible under the microscope, and its presence can easily be confirmed through microscopic examination of the cat’s blood. This often yields a more accurate result since immature heartworm larvae are more commonly seen in infected cats than adult worms.

Treatment and Prevention of Heartworm Infection in Cats

To date, the only approved and recommended treatment against heartworm infection is an injectable drug called Melarsomine dihydrochloride (under brand names Immiticide and Diroban). This is an arsenic-based drug formulated to kill adult worms and is injected into the muscles of infected animals. This has been the go-to medicine for heartworm treatment in dogs for the past years and is the only treatment approved by the American Heartworm Society.

Unfortunately, this treatment is only approved for use in dogs. Studies have shown that the use of melarsomine on cats can result in life-threatening complications and is not recommended for the treatment of heartworm infection in feline patients.

Treatment in cats comprises mainly symptomatic and supportive medications. Management of clinical signs is the main goal of therapy for heartworm-infected cat patients. Your vet will assess how heavy the infection is and will put your cat on appropriate medications to manage symptoms and control damage to the heart.

Thankfully, heartworm disease is completely preventable in cats. Topical medications containing ivermectin or other parasite preventatives are readily available in most veterinary clinics. These topical spot-on treatments can be administered monthly to help protect cats from heartworm and other parasitic infections and infestations.

It’s best to discuss with your vet which preventive medicine is most appropriate for you and your pets.

Read more:

Parasites That Cause Diarrhea in Cats

Safety Considerations for Indoor and Outdoor Cats

How to Prevent Flystrike in Dogs and Cats

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