Heartworm Disease in Cats

Dirofilaria worms use mosquitoes as the intermediate host. Mosquitoes ingest immature heartworm larvae (known as microfilaria) from the blood of an infected animal, usually dogs or wild animals such as coyotes, wolves, foxes, and sea lions.

The microfilaria develop inside the mosquito and are deposited as larvae in the tissue when the mosquito feeds on a cat.

Treatment of heartworm disease in cats is very different from protocols for dogs. Use of antiparasitic medication to eliminate the adult worms is usually avoided in cats as the dead worms can trigger fatal inflammation and blood clots in the lung tissue.

Prognosis varies with the severity of infection. Many cats are able to clear the infection spontaneously and require minimal treatment but 20% of cases involving a live adult worm are fatal. Cats with suspected heartworm require prompt veterinary attention but cats presenting with symptoms associated with severe infection such as breathing difficulties, or collapse, require emergency treatment.

The distribution of heartworm used to be isolated to the southeastern United States, but it has migrated gradually up the coast and is now found throughout North America. Cats living in any region where there are populations of mosquitoes may be at risk, but the highest rates of prevalence correspond to the migration path of the disease down the eastern coast over the last century.

Heartworm disease is an uncommon and potentially fatal condition in cats. Severity varies depending on the stage and extent of the disease.

Symptoms vary depending on the stage of the disease.

Stage 1 symptoms result from live adult worms developing in the blood vessels and heart. The immune system causes inflammation resulting in mild, non-specific symptoms such as coughing and lethargy.

Stage 2 symptoms are caused by death of the heartworms. The dead parasites trigger widespread inflammation, blood clots, and severe symptoms such as breathing difficulties, collapse, and sudden death.

Heartworm disease describes infection with the parasitic worm, Dirofilaria immitis as a result of a bite from an infected mosquito. The larvae migrate to the right side of the heart and pulmonary blood vessels in the lungs where they develop into adult worms over 6-8 months.

Symptoms of heartworm vary depending on the stage of infection:

Chronic cases often have no symptoms.

Investigation of heartworm involves:

• Physical examination
• Blood work
• Diagnostic imaging

Diagnosis of heartworm is often challenging in cats as false negative test results are common. Tests are often repeated on several occasions.

Treatment is challenging as it depends on the life stage of the parasite. Killing adult worms with antiparasitic medication can trigger blood clots and inflammation of the lung tissue in cats, which can be fatal. Treatment protocol is very different in cats than it is in dogs.

If the infection is caught during the larval stage, antiparasitic medication is considered safe and is used.

Treatment of adult worms may include antiparasitics as a last resort. In some cases adult worms may be removed from the blood vessels surgically.

Cats are not the natural host for Dirofilaria immitis and many patients clear the infection spontaneously. The immune response of the patient to the worms is often the cause of symptoms. Treatment is often conservative and involves steroids to reduce inflammation of the lung tissue, and close monitoring of the patient for signs of deterioration.

Prognosis varies depending on the extent and progression of the disease.

Cats with larval stages of heartworm are rarely diagnosed with the condition, but response to antiparasitic treatment is good and prognosis is excellent.

Cats with adult worms sometimes present with sudden death. 21% of cats diagnosed with adult worms died within 4 years.

Prevention of heartworm is safe, effective and widely available.

• Year round use of a veterinarian-prescribed antiparasitic medication
• Use in all cats in endemic areas, including indoor cats
• Start from 8 weeks old

Other prevention strategies involves the control of mosquitoes:

• Mosquito-proof barriers in the house
• Control of habitat such as stagnant water

Heartworm is a common disease in dogs in areas where Dirofilaria immitis is endemic. It is less common in cats, but is increasingly thought to be underdiagnosed. These areas include:

• USA, southern Canada
• Southern Europe
• Central and South America
• Australia
• Japan

Heartworm is more common in cats that don't receive regular heartworm prevention medication

• Steroids
• Antiparasitic medication for treatment and prevention of larval stages
• Surgical removal of adults