Dogs are not the only ones that can get heartworm. Cases of heartworm in cats are not as prevalent as they are in dogs, but they can be just as devastating. All cats are susceptible to heartworms. Since heartworm in cats must be transmitted via mosquito, cats that are exposed to more mosquitos are more likely to become infected. About 2/3 of heartworm cases in cats are in outdoor cats.

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The lifecycle of the heartworm, or Dirofilaria Immitis, is very complicated. Heartworms start out as immature larvae (microfilaria) in the blood stream of a host animal. Then a mosquito comes along and sucks up a bunch of the larvae as it takes in its blood meal. The larvae will continue to develop inside the mosquito’s gut and mouth parts for 10 – 30 days. The next time the mosquito bites a cat, these more developed larvae will be injected into the cat along with the mosquito’s saliva.

Once inside their new host cat, the larvae work their way through the cat’s sucutaneous tissues feeding on the nutrients in the cat’s blood and developing as they go. About 3-4 months after the initial mosquito bite, the larvae settle into the cat’s pulmonary arteries (mostly in the lungs) and develop until they are sexually mature. Around 8 months after the initial bite, the female worms will reach the adult stage and release new microfilaria into the cat’s blood stream. The lifespan of a heartworm in a cat is only about 3 years (it is 5- 7 years in dogs).

Cats Vs. Dogs

Cases of heartworm in cats are far more rare than in dogs. Cats are very resistant to heartworms. The strong response of the immune system keeps most of the heartworms from completing their life cycle. In only 20% of cases of heartworms in cats will there be any microfilaris in the blood stream. Heartworms are smaller, fewer in number, and have a shorter lifespan in cats than in dogs. Cat’s immune systems can often fight off the heartworms without medical intervention.

The way heartworm affects a cat is much different than the way it affects a dog. In dogs heartworms mainly affect the heart, but in cats heartworms mainly affect the lungs. There are 2 very dangerous points in a heartworm infection for a cat: the initial entering of the larvae into the blood stream and the dying of the adult worms. The cat’s immune system reacts with a very strong response to these events. The immune system causes swelling of the pulmonary arteries around the location of the heartworms which in turn allows for fluid to build up in the lungs.

How Do You Know?

Diagnosing heartworm in cats is very difficult as the symptoms are very non-specific. The most common symptoms are respiratory difficulties (i.e. coughing, rapid breathing, etc) and vomiting. Some cats never show any symptoms and suddenly fall dead. Often heartworm is misdiagnosed as asthma or allergic bronchitis instead of Heartworm Associated Respiratory Disease (HARD).Lists of the symptoms are divided into “acute symptoms” and “chronic symptoms.”

Blood Sucking Parasites - Heartworms

Heartworms Photo courtesy of

Acute Symptoms

  • Difficultie breathing
  • Rapid breathing
  • Vomiting
  • Diarrhea
  • Collapse
  • Convulsions
  • Fainting
  • Rapid heart rate
  • Sudden Death

Chronic Symptoms

  • Difficulty breathing
  • Vomiting
  • Anorexia
  • Weight loss
  • Lethargy
  • Fluid in the chest cavity

There are several tests that a veterinarian can run to help diagnose heartworm. Unfortunately, there is no specific heartworm test for cats and none of the tests is fool proof. The tests your veterinarian may run include (but are not limited to):

  • Physical examination. Harsh lung sounds found during this test can be indicative of a heartworm infection.
  • Antibody blood test. This test can determine whether or not the cat’s immune system has been exposed to heartworms. However, cats can spontaneously rid themselves of heartworm infections, so a cat could test positive for this and not currently have heartworms.
  • Antigen blood test. This test can positively determine that adult female heartworms are present. However, there must be at least two female heartworms for this test to be positive. Also, if the infection has no adults or only male adults this test will not indicate them.
  • Complete blood count. This test can show anemia, immature red blood cells, and increased numbers of certain types of white blood cells.
  • X-rays. Veterinarians can use x-rays to see if the pulmonary arteries are enlarged or not well defined.
  • Ultrasound. This can help the veterinarian to see heartworms in the heart and sections of the pulmonary artery that are close to the heart. However, this is not good for seeing non-adult worms or worms that are located further into the extremities of the artery.
  • Angiocardiography. For this test the veterinarian injects a special liquid into the heart and uses an x-ray to examine it. This is not often used because it is very invasive.
  • Microfilaria testing. This test can detect the presence of microfilaria in the cat’s blood, however, heartworms rarely make it to this stage in cats. If the test is positive you can be sure the cat has heartworms, but if it is negative that is not a sure sign the cat does not have heartworms.

Fight Like A Cat

Unlike the other posts in this Blood Sucking Parasites series, this one will not have a Do’s and Don’ts section. There is no treatment for cats with heartworms approved for use in the U.S. Treatments for heartworm in dogs have not proven to be safe in cats. The trouble is that killing the adult heartworms can cause a reaction by the cat’s immune system that can kill the cat suddenly. That being said, veterinarians treat the symptoms that the cat has and monitor closely the life cycle of the heartworms. Gradually decreasing doses of Prednisone can be used to treat lung disease. Supportive therapies are used for other severe symptoms.

Heartworm prevention medications are availble for cats. These medication include Revolution, Advantage Multi for Cats, Heartgard for cats, and Interceptor. Any of these products are readily available through most veterinarians.

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