greycatHeartworm infection takes place when a mosquito carrying microscopic-size heartworm larvae, obtains blood from an infected animal and draws into its system the microfilariae which grow into the infective larval stage. These heartworms are injected into the new host as larvae and develop further as they travel through the cat’s body over a period of eight months. As they become sexually mature male and female worms (Difrofilaria immitis), they usually settle into the heart and lungs, and their arteries and blood vessels. Even as immature adults, the heartworms can mate and the females can then release their offspring (microfilariae) into the blood stream.

Cats, like dogs, are in danger of serious health issues when infected by mosquitoes with heartworm larvae. Fortunately, felines are resistant hosts even when injected by an infective mosquito. The percentage of microfilariae in cats developing into the adult stage is low (0-25%) compared to dogs (40-90%). The percentage of infected cats from which adult heartworms are recovered is generally high (61- 90%), but still lower than in dogs, where they are recovered in virtually 100% of those infected. The number of adult heartworms in infected cats is usually low, fewer than six, although there have been documented cases with as many as 45 in one male cat and 65 in another. Single sex and ectopic (adult worms in sites other than the heart, lungs, and associated vessels) heartworm infections are also more common in cats than in dogs. Although heartworms reach a substantial size in cats, they are smaller than those in dogs. The life span of heartworms in cats is approximately two to three years, compared to five to seven years in dogs.

cat_heartwormThe average time from when the microscopic parasites enter the host until the females develop into mature worms, producing microfilariae is approximately eight months and is referred to as the prepatent period. This is about one month longer than in dogs

Microfilaremia (the presence of heartworm offspring in the blood of the host animal) is uncommon in cats (usually less than 20% of cases). When present, it is inconsistent and short-lived. This indicates that cats are poor reservoirs of infection. There is no predictable age in cats for becoming infected with heartworms. Cases have been reported in cats from nine months to 17 years of age, the average being four years at diagnosis or death.

Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat’s body.

Clinical Signs

Dyspnea (difficulty breathing)Vomiting
Convulsions Dyspnea
Blindness Anorexia
Tachycardia (fast heart rate)Weight loss
Syncope (fainting)Chylothorax (lymphatic system backup in chest)
Sudden death

The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases. Diagnosis by clinical signs alone is nearly impossible. As with dogs, most heartworm infected cats are free of any clinical signs of disease and may exhibit chronic clinical signs, such as vomiting intermittently, coughing, asthma-like signs (intermittent difficulty in breathing, panting, open-mouthed breathing), gagging, rapid breathing, lethargy, anorexia, or weight loss. Some cats exhibit acute clinical signs, with disease often related to the organs where the adult heartworms are thriving.


Actual cases of feline heartworm infection have been difficult to estimate because diagnosis has been based mainly on clinical signs and tests developed for use in canines. The diagnostic plan for heartworm disease in cats can include a physical examination, X-ray, ultrasound, complete blood count (CBC), serologic testing (antigen and antibody study), microfilaria testing, and necropsy. Unfortunately, these highly specific tests lack the sensitivity to identify many infections in cats. They fail to detect infections of immature heartworms, infections with only male heartworms and some infections with only one or two adult female worms.



Currently, there are no products in the United States approved for the treatment of feline heartworm infection. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases can be monitored with chest X-rays every six to twelve months, as needed. Supportive therapy with small, gradually decreasing doses of prednisone is recommended for cats with radiographic or clinical evidence of lung disease.

tabbycatCats with severe manifestations of feline heartworm disease may require additional supportive therapy, and may benefit from intravenous fluids, oxygen therapy, cage confinement, bronchodilators (which expand the air passages of the lungs), cardiovascular drugs, antibiotics and nursing care.

Heartworm extraction with various surgical devices has been performed in cats in which the worms can be visualized with ultrasound at the tricuspid valve or in the right atrium (of the heart), and especially in those rare instances of caval syndrome (obstruction of blood flow affecting the heart and the liver).


It is generally recommended that all cats be tested for antigens and antibodies prior to administration of a heartworm preventive. There are three products approved by the FDA for use in cats, Heartgard® for Cats, RevolutionTM and Interceptor®. All of these products are considered effective in preventing the development of adult heartworms when administered properly on a monthly basis relative to the period of transmission.

For more information about heartworm disease, please contact the American Heartworm Society at