Heartworm disease….Isn’t that just a dog problem?

Heartworm is a parasite (Dirofilaria immitis) which lives in the blood stream of the heart and lungs.

The natural host of this parasite is the dog and as most people with dogs know we have been treating dogs with heartworm preventative medications for decades.

Recently we have begun to realize that the heartworm parasite can infect cats as well as dogs. The symptoms are various and can range from mild to severe.

Quick Facts:

  • In some areas feline heartworm disease may be more prevalent than more commonly known cat diseases such as Feline Leukemia Virus (FeLV) or Feline Immunodeficiency Virus (FIV).
  • Heartworm disease is more difficult to diagnose in cats than dogs.
  • A single heartworm or even heartworm larvae can be fatal to cats
  • Heartworm disease cannot be treated in cats but it can be prevented.


Symptoms of Feline Heartworm Disease

  • Acute 
    • Difficulty breathing
    • Collapse
    • Seizures
    • Diarrhea/Vomiting
    • Increased heart rate
    • Blindness
    • Sudden Death!
  • Chronic
    • Coughing
    • Vomiting
    • Lethargy
    • Anorexia
    • Weight loss
    • Fluid in the chest
    • Difficulty breathing

Here’s how the heartworm life-cycle works:

  1. A mosquito carrying heartworm larvae bites an animal and injects some larvae into the animal.
  2. These larvae circulate in the bloodstream and once they have developed into adult worms they live in the pulmonary arteries of the lungs.
  3. The adult worms reproduce and release larvae into the blood stream.
  4. A mosquito sucks some of these larvae out of the blood stream
  5. That mosquito then transmits the larvae to another animal.

We now realize that more frequently than we thought, heartworm larvae can survive and develop in cats and can cause significant problems for infected cats. While cats are less likely than dogs to develop a heartworm infection after a mosquito, they are far more likely to develop serious (and potentially life threatening) symptoms if they become infected. Unlike dogs, the larvae do not even need to develop into adult worms to cause problems for cats.

Dogs with heartworm disease often times do not develop any symptoms unless there are a large number of adult worms present or they have been present for a long period of time. Most dogs are tested for heartworm disease annually and can be treated if they have become infected.

Cats are different. The presence of larvae or 1 or 2 adult heartworms can be fatal to a cat. Recent research has shown that the developing heartworm larvae circulating in the blood stream can cause a significant amount of disease. Many cats develop problems without ever developing adult heartworms. This is a syndrome known as Heartworm-Associated Respiratory Disease (HARD). In this syndrome, significant changes to the vessels and the tissue of the lungs occur. These changes may be present even in cats with no visible symptoms. This syndrome is primarily a result of an inflammatory response to the presence of heartworm larvae in the cat’s lungs. In fact, resent research has shown that the presence of heartworm larvae alone can cause as much damage to the lungs as adult worms in the cat.

Figure 1: Lung tissue from a normal cat*

Figure 2: Lung tissue from a cat with heartworm larvae*

*Images courtesy of Pfizer Animal Health

One reason that feline heartworm disease has only recently been recognized is that testing is difficult. Blood tests are available but can give a false negative. Echocardiography can sometimes find adult worms in the heart but in many cases adult worms are not present and therefore cannot be found. It is thought that many cats that we believe have asthma may actually have Heartworm Associated Respiratory Disease.

There is no safe or approved treatment of heartworms in cats. Once a heartworm infection develops there is little that we can do except try and manage the symptoms. The adult heartworm may survive in the cat for 1-3 years.

The best treatment for feline heartworm disease or Heartworm Associated Respiratory Disease is to prevent it from happening in the first place. Studies have shown that in some areas 16% of cats have been exposed to and may be infected with heartworm. This is frightening when compared to more commonly known cat diseases such as Feline Leukemia Virus or Feline Immunodeficiency Virus which each affect only 1-3% of cats nationally.

Indoor cats are not protected without a preventative. 27% of heartworm infected cats are strictly indoor only.

Fortunately, there are several products available which are very effective at preventing heartworm infection in cats. All of the available preventatives kill the larval forms which helps prevent the development of HARD. All of these products are monthly products (similar to what is available for dogs) and should be given on a year-round basis.

Feline Heartworm Preventatives

  • Revolution
    Active Ingredient: Selemectin
    Route of Administration: Topical
  • Advantage Multi  
    Active Ingredient: Moxidectin/imidacloprid
    Route of Administration: Topical
  • Interceptor
    Active Ingredient: Milbemycin
    Route of Administration: Oral
  • Heartgard
    Active Ingredient: Ivermectin
    Route of Administration: Oral

Please feel free to talk to the doctors and staff at Anchor Animal Hospital about feline heartworm disease and prevention. This is a serious disease that can cause life-threatening problems and has been under recognized in the past. Simple monthly preventatives can help save your cat’s life.

Source material:

  1. Dirofilaria immitis in cats: Anatomy of a disease, Compendium, July 2008, pp 382-389.
  2. Dirofilaria immitis in cats: Diagnosis and management, Compendium, July 2008, pp 393-400.
  3. A round table discussion: Feline heartworm disease, Supplement to Compendium, August 2008
  4. Materials provided by Pfizer Animal Health