What You Need to Know about Feline Heartworm Disease

Heartworm disease is an issue around the world caused by a parasite known as Dirofilaria immitis. It is most common in areas with high humidity and a high mosquito population.

Lifecycle of a Heart Worm

The disease needs a host. The mosquito normally serves as the host. The transmission and replication of the heartworm to other hosts happens with the mosquito.

The process starts when a mosquito feeds on an infected host. This is usually an infected domestic or wild mammal. When the mosquito bites an infected host, they ingest some microfilaria. Microfilaria is the juvenile circulating in the bloodstream. The microfilaria replicate and advance in the intermediate host. This process cannot happen without the mosquito.

After this, the heartworm develops into the first of three larval stages. These stages are called L1, L2, L3 inside the body cavity of the mosquito. When the larva enters into stage L3, it can advance into the mouth of the mosquito and be transmitted. The time frame of maturity is dependent on the temperature and humidity. When it is 80 degrees F with 80% humidity, the process takes about 10-14 days. If it is cooler with less humidity, the process is slower. When in the new host, the L3 molts and enters into L4. This final larval stage can occur between 2-12 days.

50-70 days after infection, the L4 develops into juvenile or immature adults and migrate through the body.

67-120 days after infection, the juvenile worms migrate to the pulmonary system. They average 1-1.5 inches in length. They are usually present in the pulmonary arteries, the right ventricle of the heart and the atrium.

120 days after infection, the heartworm reaches sexual maturity.

6-9 months after infection, the microfilaria are visible in the bloodstream

Dogs are the preferred host but cats and ferrets are able to develop heartworm disease. Dogs are the preferred host because they have the higher ability of the L3 continue to mature compared to other hosts. In dogs, 56% of L3 are able to continue to mature when only 6% are able to in the cat. Cats have a lower heartworm burden than dogs. Most infected cats only have 1-3 worms. Adult heartworms have a shorter lifespan in cats. They only live 2-3 years when they live to 5-7 years in dogs.

Symptoms in Cats

  • coughing

  • asthma like attacks

  • weight loss

  • vomiting

  • intermittent tachypnea (rapid breathing)

When infected with heartworm disease, cats type II alveolar cells are damaged. This causes hyperplasia (thickening) of the cells. This leads to the impairment of oxygenation across the cells. Due to this, cats normally show signs of lower respiratory disease that can mimic asthma and bronchitis. Clinical signs of heartworm disease are subtle in cats and can depend on the location of cell injury. Cats do not normally develop congestive heart failure or pulmonary hypertension from heartworm disease. They are more likely to die from compromised oxygenation due to heartworm disease.


Diagnosis a cat with heartworm disease can be difficult. The standard blood test that is not always reliable. The test requires at least one mature female to be present and this is not alway the case. This is because cats have a low worm burden and there may be only male heartworms present. Cats who show signs should be tested but the team should keep in mind that this could be a false negative.

The antibody test can detect both male and female heartworms and cats can have antibodies to heartworms for months before having an active infection. When an antibody test has a negative result, this tells us that there is a 90% probability that the cat does not have heartworm disease. The antibody test can also assess the cats' risk factor. Antigen tests should not be used in cats due to its low sensitivity.

Other diagnostic options:

Complete blood count (CBC)- Some infected cats may have elevations in eosinophils and basophils (types of white blood cells).

Serum chemistry panel. This blood test evaluates organ function for liver and kidneys, electrolyte and protein levels.

chest radiographs. Changes are less noticeable in cats than dogs. Radiographs can help rule out other conditions such as asthma, bronchitis, cardiac disease and neoplasia.

Echocardiogram. Sometimes in cats, heartworms can be visualized with an ultrasound of the heart.


Treatment for feline heartworm disease is supportive care. The veterinary team manages the signs with corticosterioids, bronchiodilators, and oxygen support at the time of accute event.

Because of the short life expectancy of worms, a spontaneous recovery is possible if reinfection is prevented. There is NO APPROVED ADULTICIDE FOR CATS. Melarsomine is not recommended in cats beccause it is toxic in small doses. If a worm is shown on echocardiogram, surgical removal is recommended.


All cats, indoor and outdoor cats, should be on a prevention regimen. Mosquitos can come indoors and cause an infection. Year round treatment is recommended in humid climates. Most preventatives can also treat gastrointestinal parasites. Consult with your veterinarian to determine what prevention option is best for your cat.


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