Heartworm disease is a life-threatening but preventable condition affecting more than 1 million U.S. pets, including dogs, cats, and ferrets. Learn how to protect your pet from this condition with our Palm Valley Veterinary Center guide to heartworm disease and prevention.

What are heartworms in pets?

Heartworms (Dirofilaria immitis) are parasitic worms that can grow up to 12 inches in length. Adult worms live inside a pet’s large lung vessels and heart, where they cause dangerous inflammatory reactions, chronic damage, and life-threatening blockages. Mature infections can trigger cardiovascular complications, including respiratory distress, severe weakness, and stroke. As adult heartworms mate and produce offspring, pets become disease reservoirs, allowing heartworms to spread to other unprotected pets.

How do pets get heartworm disease?

Heartworm disease is transmitted from animal to animal by infected mosquitoes. When a mosquito bites and feeds on a heartworm-positive pet, it also ingests microscopic heartworm larvae circulating in the infected pet’s blood. These immature heartworms—known as L1 or first stage larvae—undergo part of their life cycle inside the mosquito, and then are transferred to the next vulnerable pet through a bite. Once the infective larvae are under the pet’s skin, they molt to their next stage and begin a slow migration through the tissue to the circulatory system, and eventually toward the lungs and heart.

What are heartworm disease signs in pets?

Because heartworm disease has a slow maturation and migration process (i.e., around six months from infection to mature adults in the heart and lungs), visible signs don’t appear until the infection is in its late stages—making year-round prevention the only option for pets.

Heartworm disease signs also vary significantly by species. While dogs are considered a natural heartworm host (i.e., the parasite can carry out its life cycle from immature larvae to reproducing adult), cats are not. As a result, heartworm disease produces vague signs in cats and, in many cases, no signs at all. Ferrets are considered highly susceptible to infection, and, like cats, their small heart size makes a single adult worm life-threatening. 

Heartworm signs by species include:

  • Dogs:
    • Persistent cough
    • Exercise intolerance
    • Fatigue after brief activity
    • Decreased appetite
    • Weight loss despite pot-bellied appearance
    • Collapse
  • Cats: 
    • Asthma-like attacks
    • Coughing
    • Weight loss
    • Occasional vomiting
    • Decreased appetite
    • Fainting
    • Neurological signs, including loss of coordination or seizures
    • Sudden death
  • Ferrets:
    • Lethargy
    • Coughing or wheezing
    • Pale, grey, or blue gums 
    • Appetite and weight loss
    • Gasping
    • Panting
    • Open-mouth breathing 
    • Hind limb paralysis

Heartworm diagnosis and treatment in pets

If heartworm disease is suspected, your veterinarian will evaluate your pet’s blood for circulating microfilariae and the presence of adult female heartworms (i.e., antigen test). Additional imaging, such as chest X-rays and a cardiac ultrasound, may be recommended to evaluate heartworm-related changes and determine the stage of the disease. 

Although heartworm disease treatment is available for dogs, prevention is still the safest option. Canine heartworm treatment focuses on killing all adult and immature heartworms and safely eliminating them from the body. This process requires hospitalization to administer the injectable medication, followed by prolonged at-home cage rest to prevent complications as the worms die off and move out of circulation. 

Unfortunately, at this time no safe treatment is available for cats or ferrets. Pets may be managed with supportive care, including fluids, antibiotics, and medication to reduce the inflammatory response and minimize clinical signs, in hopes the pet will stabilize until the adult worm(s) dies naturally. Sadly, many heartworm positive cats and ferrets succumb to heartworm-related complications—making heartworm prevention the only safe and healthy choice in the fight against the disease.

Heartworm prevention in pets

Every pet should receive year-round veterinarian-prescribed heartworm prevention. Preventive products are available in a monthly tablet, chew, or topical formulation, as well as 6- and 12-month injections for dogs. Because mosquitoes are ubiquitous and 100% repellency is impossible, heartworm prevention works retroactively, meaning it eliminates circulating early stage microfilariae. In other words, heartworm prevention stops juvenile heartworms from reaching the heart and maturing into adults. For this reason and because of our tropical climate, heartworm prevention must be given year-round to ensure adequate protection.

Heartworm testing in pets

Palm Valley Veterinary Center recommends annual heartworm testing for all dogs, including those receiving year-round prevention. While prevention is your dog’s best chance for an infection-free life, mistakes do happen. This could include product failure, which is less common, or improper administration (e.g., dose is misapplied, pet does not swallow the dose or vomits after dosing, or the dose is forgotten altogether). Early disease detection with a heartworm test ensures your dog will receive fast and effective treatment before permanent damage occurs.

Annual testing is not typically required for cats and ferrets currently receiving preventives, but may be recommended before starting a prevention program. Talk to your veterinarian to determine what’s right for your pet.

Environmental strategies to reduce pet risk

In addition to year-round preventive use and heartworm testing, environmental management may help lower your pet’s exposure risk. Reduce mosquito encounters by removing standing water from your pet’s outdoor areas, installing pet-safe mosquito traps and repellents, and keeping pets indoors during peak mosquito hours (e.g., dawn and dusk). Avoid harmful sprays, aerosols, citronella, and DEET as these products cause lung, eye, and central nervous system damage in pets. 

Heartworm disease is an ever-present threat, but your pet doesn’t have to be its victim. If your pet isn’t currently protected by year-round heartworm prevention, contact Palm Valley Veterinary Center to schedule an examination and a heartworm test.