At the Animal Clinic of Billings, our veterinarians and veterinary technicians have been educating our clients on heartworm prevention in cats since 1981.

We believe the importance of heartworm prevention in cats can’t be overstated. Once a cat has contracted heartworms, the condition cannot be cured.

Therefore, heartworm preventive measures need to be followed in order to keep your beloved feline friend healthy, happy, and safe.


Cat Heartworms, or Dirofilaria immitis, is a blood-borne parasitic nematode (roundworm), that veterinarians refer to as cat heartworms. Heartworms are contracted in cats through mosquitos carrying heartworm larvae. Heartworms severity depends on the number of worms present in the cat’s body, the incubation time, and how the infected cat responds.

While heartworms in cats are less prevalent than in dogs, heartworm disease is dangerous and is growing in America. The risk of heartworm is equal for indoor and outdoor cats. If you don’t use preventive medication, the risk of contracting cat heartworm disease increases exponentially, and preventive cat heartworm medication can protect your cat.


The most challenging aspect of diagnosing heartworms in cats is there is no definitive clinical sign indicating the presence of cat heartworm disease. This does not mean cat heartworm disease isn’t present. Vomiting and coughing are, which are the two most common symptoms, but there may be other symptoms, include:

  • Labored breathing
  • Convulsions
  • Blindness
  • Lethargy
  • Fluid in the lungs
  • Anorexia/Weight Loss
  • Sudden Death

Sometimes, an apparently healthy cat is found dead, from unexpected respiratory failure that is diagnosed during a postmortem exam.


Nearly 30 species of mosquitoes transmit heartworm to cats. Mosquitoes ingest immature heartworm larvae,(microfilaria) by feeding on infected cats or dogs. The microfilariae continue to develop for between 10 and 30 days in the mosquito’s gut and then enter parts of its mouth.When an infected mosquito bites a cat, the heartworm larvae are injected into the cat. Then, the larvae migrate and mature for several months, eventually landing on the right side of the heart and in the pulmonary arteries. When this occurs, they mature into adult cat heartworms and can reproduce about six months from the time of the invasion. 

Eight months following the invasion, cat heartworm begins to create a new crop of microfilariae that lives in the cat’s blood for one month. At this stage, most cats show symptoms of cat heartworm, and the disease can very quickly become fatal.


Unfortunately, there is not a benchmark standard used across the veterinary industry for diagnosing heartworm for cats. Instead, our veterinarians employ a battery of lab tests to diagnose cat heartworm. veterinarian checking a cats heart beat

These tests include:

  • A urinalysis (testing a cat’s urine).
  • A heartworm antibody test determines whether a cat’s immune system is exposed to heartworms. This is a sensitive test and is often employed first.
  • A heartworm antigen test confirms the presence of adult female heartworms. 
  • The test is more specific than an antibody test, but not as accurate.

X-Rays allow veterinarians to view the size and shape of a cat’s heart. Many cats with cat heartworm contract enlarged pulmonary arteries, and suffer from obstructions in the arteries that lead to the lungs.

  • Ultrasounds allow veterinarians to view the internal structures of the heart and surrounding vessels and to assess the condition of the heart. However, for some cats with low levels of cat heartworms, this test does not always yield accurate results.
  • A white blood cell count is measured in cats that show symptoms of heartworms. Eosinophils are a white blood cell that occurs in higher numbers when heartworms are present, and may also indicate the presence of other parasites.



Unfortunately, there is not a viable heartworm medicine for cats that can fight off an active infestation. Therefore, if your cat receives a heartworm disease diagnosis, we cannot cure it. However, while we cannot defeat existing heartworm with medication, this isn’t necessarily an indication they will die. While sudden death is possible, it is uncommon.

If your cat is diagnosed with heartworm disease, he or she may still live a significant amount of time if the proper medical care and treatment is effectively administered by your veterinarian. This may include anti-inflammatory medicines similar to those used to treat asthma.


veterinarian with cat

The good news for cat owners and their furry friends is that heartworm prevention for cats does exist. Veterinarians strongly recommend all cats receive monthly heartworm preventive medications every month where mosquitoes are active in your area.

In Montana, our mosquito population can fluctuate greatly depending upon the amount of moisture we get. Nevertheless, there is never a shortage of places in and around the Billings area where many times spring and summer mosquito populations remain relatively unaffected by rainfall, for instance within the Yellowstone and Missouri River bottoms for example. Because of this, we highly recommend all cat owners adhere to a heartworm preventive medicine protocol for each mosquito month starting in April through the end of October.


The Animal Clinic of Billings recommends you schedule a meeting to discuss heartworm prevention for cats. If you see any symptoms indicative of cat heartworm disease, contact us immediately to schedule an appointment. Our veterinary team will give you and your feline friend with an effective cat heartworm prevention protocol right away.

Our highly trained and experienced team of veterinarians and veterinary technicians will help you keep your cat as happy, healthy, and free of heartworms as they can be.

Call the Animal Clinic of Billings to schedule your cats next wellness examination with us!

1414 10th St. West, Billings MT 59102

406-252-9499      REQUEST AN APPOINTMENT