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Heartworm: A Not So Loving Worm

Updated: Apr 20, 2023

Vet Cat Dog Camden County Veterinary Animal

These are just a few common questions that are posed to us in the exam room. This article will enlighten you about the transmission and life cycle, symptoms, prevention, and the difference between cats and dogs in regard to heartworm.

What is heartworm?

Heartworm (scientific name: Dirofilaria immitis) is a worm that has the ability to grow over twelve inches long while living within the heart and pulmonary (lung) arteries.

Transmission and life cycle:

Mosquitos plays an essential role in the heartworm life cycle. Adult female heartworms live and grow within the heart and pulmonary (lung) vessels. The adults living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate throughout the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which then develop and mature into “infective stage” larvae over a period of 10 to 14 days.

Then the infected mosquito bites another dog or cat, and the infective larvae are deposited onto the surface of the pet’s skin entering through the mosquito’s bite wound. Once inside a pet, it takes approximately 6 months for the larvae to mature into adult heartworms.

Did you know that the adult heartworm could live for 5 to 7 years in dogs and up to 2 or 3 years in cats? Due to the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.

Symptoms of Heartworm disease: Common symptoms of heartworm disease in dogs include:

  • No symptoms

  • Mild persistent cough

  • Reluctance to exercise or fatigue after exercising

  • Decreased appetite

  • Weight loss

  • Heart failure

  • Distended/swollen abdomen

  • Cardiovascular collapse

Common symptoms of heartworm disease in cats include:

  • No symptoms

  • Coughing

  • Asthma-like attacks

  • Occasional vomiting

  • Decreased appetite

  • Weight loss

  • Difficulty walking

  • Fainting or seizures

  • Distended/swollen abdomen

  • Sudden collapse or death


In response to the question, “Why do I have to test my pet annually if I give heartworm preventatives year-round?”, we have provided you with a few benefits for doing so.

Preventative companies will often guarantee their respective products and even help cover medical bills should your pet become infected. However, certain criteria must be met for this to happen, which usually consists of annual testing, purchasing your products at a veterinary hospital, and year-round usage of heartworm prevention.

Additionally, the American Heartworm Society, which sets guidelines for veterinarians and veterinary hospitals, is a strong advocate of annual testing to help decrease the frequency in which the disease is seen.

While trying to be diligent in giving your pet monthly heartworm preventative, sometimes the medication is just not absorbed properly. Stressful events in your pet’s life, such as an illness or vomiting after administering the preventative can affect the absorption of the product within the gastrointestinal tract.

In addition to the above reasons, the veterinary community is starting to see resistance to some preventative medications. Resistance of heartworm to these preventatives is often only detected by testing annually.

While you may bring in a fecal sample to test for parasites, the heartworm test actually takes a few drops of blood. The testing for both cats and dogs is fairly similar. The few drops of blood that are collected are placed into a cartridge. Much like a human pregnancy test, the blood when mixed with the test reagent, scrolls across a window collecting at a certain point in the test. If the pet is infected, a “positive” dot will show up along with a “positive” control dot for comparison. If a pet is not infected, the only dot that will be visible is the control dot.


We cannot stop by answering just one of your questions. Two other questions that arise often at an annual exam are: “Why should I give monthly heartworm preventatives to my pet year-round?” and “My pet doesn’t go outside; so why do I need to medicate?”

One reason that monthly prevention is advised is due to the lifecycle of the heartworm. The average lifecycle is 45 days. Therefore, giving a monthly preventative ensures that you are always medicating within that standard time and accounting for those worms that have a shorter lifecycle.

Oftentimes heartworm prevention is discontinued during the winter months due to mosquitos dying. While mosquitos can only live above 57°F, they preferentially seek out warm places to survive such as your house. So, the only way to completely protect your pet is to provide year-round prevention.

We like to believe that our indoor-only pets are truly indoor-only pets, but mosquitos like to sneak in while the door is open, or bite your pets while they are outside doing their business. Treating your indoor-only pet with monthly preventatives is the only way to ensure that heartworm disease stays out of your home.

Some common preventatives that are available through Pennsauken Animal Hospital:

  • Triheart® – a flavored chewable that most dogs will take like a snack. This is distributed for dogs only.

  • Interceptor Plus® – a flavored chewable tablet. This is distributed for dogs only.

  • Revolution® – a topical application placed on their skin between the pet’s shoulders. This topical preventative also has the bonus of helping with fleas and ticks. This is distributed for dogs and cats.

Difference between dogs and cats:

Most owners are aware that they should be treating their dogs for heartworm disease, but fewer pet owners know that they should also be treating their cats. Cats, while more commonly indoors only and oftentimes exposed less, are still susceptible to heartworm. Heartworm is far more debilitating in cats than it is in dogs. While it may take 15-20 heartworms to be fatal in a dog, it only takes 1-2 in cat! This is based on their body size and the respective size of their hearts and blood vessels. Testing and year-round prevention are also recommended for your furry felines!

For more information please visit the American Heartworm Society website at:

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