Not A Veterinarian Disclosure (wide)

Heartworms are dangerous parasites that enter the bodies of animals by way of a mosquito bite. They are just what they sound like – long white worms that find their way into an animal’s circulatory system and feed on the nutrients in their host’s blood. Yuck! These nasty worms are most common in dogs, but unfortunately, can be found in cats as well.

Title Photo Credit: Eric Sonstroem via Flickr

Home Is Where the Heart Is Lungs Are


Photo Credit: Tarek Mahmud via Flickr

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 layers of flesh below the cat’s skin. They feed on the nutrients in the cat’s blood and develop as they go. Then, the larvae settle into the cat’s pulmonary arteries in the lungs.

The heartworms become sexually mature about 6 months after the initial infection. In cats, heartworms rarely ever make it to the adult stage. The lifespan of a heartworm in a cat is only about 2- 3 years (it is 5- 7 years in dogs).

 Cats Vs. Dogs

Practicing the Death Grip

Photo Credit: S. Carter via Flickr

Dogs are a natural host for heartworms while cats are not. The strong response of a cat’s immune system keeps most of the heartworms from completing their life cycle. Due to this response,  only 20% of cases of heartworm-infected cats will have any microfilariae in the bloodstream. Cats’ immune systems can often fight off the heartworms without medical intervention.

An infected cat will likely only have 1-3 adult heartworms. An infected dog often has between 20 and 50 adult heartworms. Heartworms are smaller, fewer in number, and have a shorter lifespan in cats than in dogs.

The way heartworm affects a cat is much different than the way it affects a dog. In dogs heartworms mainly affect the heart. In cats, the inflammatory immune response to the presence of the heartworms causes the most problems.  There are 2 points in the heartworm infection that are particularly dangerous in cats:

  1.  When immature heartworms migrate to the pulmonary arteries.
    These immature worms can block circulation and cause swelling in the lungs, which leads to difficulty breathing.
  2. When the heartworms begin to die.
    This causes widespread inflammation that can affect many organs of the cat’s body including the kidneys, digestive tract, and nervous system.

It has been theorized that they dying of heartworms can cause long term damage to the cat’s lungs. The name given to this problem is Heartworm Associated Respiratory Disease (HARD). However, this is a tad bit controversial in the veterinary community. There are studies which suggest that there are no differences in the lungs of cats that have never had heartworm disease and cats that had been naturally exposed and naturally cleared of the heartworm infection.

Symptoms of Heartworm in Cats

The symptoms of heartworm in cats are very non-specific. Symptoms of respiratory distress are most common. Contact your veterinarian if you have any concerns about your cat’s health or believe that your cat may have been infected with heartworms. Potential symptoms could include:

  • Difficulty breathing
  • Rapid breathing
  • Vomiting
  • Diarrhea
  • Collapse
  • Convulsions
  • Rapid heart rate
  • Lethargy
  • Anorexia
  • Weight loss
  • Sudden death


Austin Community College Vet Tech Program

Photo Credit: Austin Community College via Flickr

There is no specific heartworm test for cats and none of the tests is foolproof.  Tests used for dogs are not very useful when diagnosing cats. Tests used on dogs look for proteins found only in adult female heartworms (antigen test) and heartworm offspring in the bloodstream (Microfilaria test).

Infected cats often have few to no adult heartworms and the few they have are not always female. The antigen test  requires the presence of at least 2 adult female heartworms for an accurate result. As for the Microfilaria test, there are not usually enough heartworm offspring circulating to be picked up.

With cats, many veterinarians opt to do an antibody test (looking for the cat’s immune system response to heartworms) along with other forms of testing. Antibody tests should not be done alone because a cat that has been infected in the past, but is not currently infected, will test positive. Other tests performed may include:

  • Physical examination
  • Complete blood count
  • X-rays
  • Ultrasound
  • Angiocardiography

Fight Like A Cat

There are no approved treatments for heartworm in cats. Treatments for heartworm in dogs are toxic to cats. The trouble is that killing the adult heartworms can cause a reaction by the cat’s immune system that can kill the cat. Veterinarians treat the symptoms that the cat has and monitor closely the life cycle of the heartworms. Drugs to control the immune system response are common.

Prevention is the best way to solve the problem of heartworms. A cat’s immune system does a great job of killing off heartworm, so taking measures to boost your cat’s immune system is very helpful. Keeping your cat indoors and away from mosquitoes is helpful as well.  If you are in an area where heartworm is prevalent,  ask your veterinarian about the heartworm preventative treatments that are available for cats.

Have you ever known a cat that had a heartworm infection?