Heartworm Prevention—Why Is It Important?

Most pet owners have heard about heartworm medications for their cats and dogs, but many do not know what how severe heartworm disease can be and how important preventative medication really is. Heartworm disease doesn’t discriminate either. It can affect both dogs and cats. While it is more prevalent in certain areas of the United States, heartworm can affect all pets in all areas of the country.

So how does a pet get exposed to heartworm disease? Heartworm disease is caused by a parasite called Dirofilaria immitis. The parasite is transmitted to cats and dogs via mosquitos. It is not directly transmitted from one cat or dog to another pet. When a mosquito bites an infected cat or dog, parasites called microfilariae enter the mosquito. The microfilariae mature within the mosquito and are then transmitted to another cat or dog when the mosquito bites that pet.  When the parasite is transmitted to a new pet in this way, the parasite can mature into worms and migrate to the cat or dog’s heart.

After the parasite migrates into the heart, the worms can mate and produce new microfilariae. Microfilariae circulate in the bloodstream and are measured by a heartworm test. However, the microfilariae are not detectable until the pet has been infected for 6-8 months. During this 6-8 month period, the worms get larger and grow inside the major vessels and heart. The worms can eventually grow so large that they can block blood flow to the heart and lungs, causing heart failure and death.

In addition to being serious as a disease itself, heartworm disease is difficult to treat and complications can be associated with treatment of mature worms. In some cases, trying to treat heartworm disease can cause death of a cat or dog. For that reason, regular administration of an effective heartworm prevention product can save your pet’s life. It is important to note that it is possible even for pets that stay indoors to be bitten by mosquitos, so all pets should be on heartworm preventatives.

There are several options for heartworm prevention products, including chewable tablets, pills, injections, and topical medications. It is important to visit your veterinarian to discuss your cat or dog’s lifestyle and any healthy considerations in addition to have a heartworm test done before your pet receives heartworm preventatives. Because administering heartworm prevention products to an already infected pet can cause health problems, testing is an important part of a complete heartworm prevention program. Testing ensures that your pet is free of adult heartworms, and ready to start or continue on heartworm prevention medication. Heartworm testing should be done yearly, even for animals on regular heartworm prevention.

While heartworm disease, in its early stages, can be asymptomatic, signs of heartworm disease in dogs include:

  • Coughing
  • Exercise intolerance or fatigue after exercise
  • Reluctance to exercise
  • Decreased appetite
  • Weight loss
  • Heart failure
  • Accumulation of fluid in the abdomen
  • Cardiovascular collapse
  • Death

Cats show more subtle signs of heartworm disease, as they do not typically have reproducing worms in their hearts. However, cats can still get heartworm disease with severe complications, and should be on heartworm preventative medication.  Signs of heartworm disease in cats include:

  • Coughing
  • Asthma-like signs
  • Periodic vomiting
  • Lack of appetite
  • Weight loss
  • Sudden collapse
  • Sudden death

Despite being a preventable disease, only about 50% of dogs and even fewer cats in areas where heartworm is prevalent are actually on preventative medication, and not all animals on preventative medication receive it monthly or as otherwise instructed. Discuss prevention with your veterinarian, as some preventatives work better for some cats and dogs versus others. For example, a dog that swims on a regular basis may not benefit as much from topical heartworm prevention due to the fact that it can wash off, and dogs who swim or are bathed frequently may benefit more from oral or injectable preventatives. It is more difficult, on the other hand, to administer oral medication to a cat, making topical medications popular for our feline companions.


Remember that only your veterinarian can perform testing to properly identify, diagnose, and prescribe the best treatment plan to manage and prevent heartworm disease in your dog or cat.

Tips to Take Away

  • 01

Heartworm disease can affect both dogs and cats

  • 02

The parasite is transmitted to cats and dogs via mosquitos

  • 03

It is important to note that it is possible even for pets that stay indoors to be bitten by mosquitos, so all pets should be on heartworm preventatives