April marks Heartworm Prevention Month, a great time to remind pet owners to take proactive steps in safeguarding their furry companions from this potentially life-threatening condition.  

Heartworm disease is caused by heartworms (Dirofilaria immitis), which are transmitted by mosquitoes. The foot-long heartworms live in the heart, lungs, and associated blood vessels, which can cause severe lung disease, heart failure, and damage to other organs in the body. 

At Boundary Bay Veterinary Specialty Hospital (BBVSH), our board-certified cardiologists are equipped to diagnose, monitor, and help manage advanced cases of heartworm disease, including the use of diagnostic imaging, cardiac evaluations, and surgical intervention, as part of a collaborative care plan with your family veterinarian. 

How common is Heartworm Disease? 

Heartworm prevalence is low in Canada, with transmission occurring seasonally (in warmer months) in regions of southern British Columbia, Manitoba, Ontario, Quebec, and New Brunswick.  

Heartworm disease affects dogs, cats, and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions and—in rare instances—humans. Because wild species such as foxes and coyotes live in proximity to many urban areas, they are considered important carriers of the disease. 

How Heartworm Is Transmitted: Understanding the Life Cycle 

Heartworm disease is transmitted through the bite of an infected mosquito, and the life cycle of the parasite involves both the mosquito and the animal host. 

Here’s how it works: 

1. 1. Mosquito Bite: A mosquito bites an infected animal (usually a dog, coyote, or fox) and picks up microscopic baby worms called microfilariae. 

 

1. Development Inside the Mosquito: Over the next 10–14 days, the microfilariae mature into infective larvae inside the mosquito. 

1. Transmission to a New Host: When that mosquito bites another dog or cat, it deposits the infective larvae onto the skin, which then enter the body through the bite wound. 

1. 
   Maturation: Inside the pet, the larvae migrate through the tissues and eventually settle in the heart, lungs, and blood vessels. Within about six months, they mature into adult worms, reproduce, and continue the cycle. 

Adult heartworms can live 5–7 years in dogs and up to 2–3 years in cats, growing up to 12 inches long. In dogs, large numbers of worms can accumulate, while in cats, even one or two worms can cause serious complications. 

🦟 It only takes one mosquito bite to infect a pet. That’s why year-round prevention is essential—even for indoor animals.

Recognizing the Signs of Heartworm Disease in Dogs and Cats 

Heartworm disease can present very differently in dogs compared to cats, partly due to how the parasite develops in each species. Understanding these distinctions is key to early recognition and prompt veterinary intervention. 

In Dogs: The Natural Host 

Dogs are the primary, natural hosts for heartworms. The parasite is well-adapted to complete its full life cycle in a dog’s body—from larval stages introduced by a mosquito bite to fully mature adult worms residing in the heart, lungs, and surrounding blood vessels. Over time, the number of worms can increase, causing progressive damage. 

Common symptoms in dogs include: 

• Mild but persistent cough 

• Fatigue after moderate activity 

• Reluctance to exercise 

• Decreased appetite 

• Weight loss 

• Swollen abdomen due to fluid accumulation (in advanced stages) 

• Difficulty breathing 

• Signs of heart failure 

• In severe cases, caval syndrome may occur—a sudden blockage of blood flow within the heart, which is life-threatening and may require emergency surgery 

In Cats: Atypical Hosts with Subtle Signs 

Cats are considered an atypical host for heartworms. Most larvae do not survive to adulthood, but even immature worms can trigger a strong inflammatory response in the lungs and airways—referred to as Heartworm-Associated Respiratory Disease (HARD). 

Possible symptoms in cats include: 

• Coughing or wheezing 

• Labored or rapid breathing 

• Vomiting (not related to eating) 

• Loss of appetite 

• Weight loss 

• Lethargy 

• Sudden collapse or sudden death, especially if a worm dies and causes acute lung inflammation 

Because signs can mimic other common conditions like asthma or bronchitis, heartworm disease in cats is often difficult to diagnose without advanced testing. 

How Heartworm Disease is Diagnosed in Dogs and Cats 

The diagnostic process for heartworm disease varies between dogs and cats due to differences in how the disease progresses and how the animal’s body responds.

Diagnosis in Dogs 

Testing in dogs is more straightforward and typically involves: 

• Antigen Test: Detects proteins from adult female heartworms in the bloodstream 

• Microfilariae Test: Confirms the presence of baby worms (microfilariae) circulating in the blood 

• Additional Testing: Chest X-rays, echocardiograms, and bloodwork may be used to assess heart and lung damage 

Dogs should be tested annually, even if they are on regular prevention. This helps ensure that no doses were missed and that protection remains effective. 

Diagnosis in Cats 

Diagnosing heartworm in cats is more complex. Because they often carry fewer worms—or no adult worms at all—standard tests can yield false negatives. 

Veterinarians typically use a combination of: 

• Antigen Test: May detect adult worms, but less reliable in cats 

• Antibody Test: Identifies the cat’s immune response to larvae, indicating exposure 

• Imaging: Chest X-rays and echocardiograms to detect lung inflammation or visualize worms 

• Clinical Judgment: Diagnosis is often based on a combination of test results, history, and symptoms 

Because no single test is definitive, diagnosis in cats often requires a thoughtful, multi-step approach guided by your cardiologist and veterinarian. 

Heartworm Disease Is 100% Preventable 

Heartworm disease is entirely preventable with safe, veterinarian-prescribed medications. These preventives are typically administered monthly (as oral or topical products) or as an injectable for longer-lasting protection in dogs. 

Veterinarians recommend year-round prevention and annual heartworm testing, even for pets who consistently receive their medication. Testing ensures the prevention is working effectively and gives your veterinarian an opportunity to adjust care if your pet’s risk level changes. 

While products and dosage schedules differ for dogs and cats, the goal is the same: to stop heartworm disease before it starts. Preventive care should always be discussed with your family veterinarian as part of your pet’s wellness routine. 

Treatment Options for Heartworm Disease 

When to See a Cardiologist 

For pets with more advanced or complicated heartworm disease, your family veterinarian may refer you to a veterinary cardiologist. At Boundary Bay Veterinary Specialty Hospital, our board-certified cardiologists play a key role in diagnosing and managing severe cases—especially when there’s concern that worms may have reached the heart.

When your pet sees a cardiologist, the first step is typically an echocardiogram—an ultrasound of the heart. This allows the cardiologist to assess heart structure and function and determine if adult heartworms are present within the heart itself. If worms are detected in the heart, your pet may be experiencing a life-threatening condition called caval syndrome. In these cases, the only effective treatment is surgical removal of the worms.

If the heart is not yet involved, your pet can usually begin a medical treatment protocol based on guidelines from the American Heartworm Society. 

Treatment in Dogs: A Multi-Step Approach 

The medical protocol involves several carefully managed stages: 

1. Oral antibiotics are prescribed first to weaken the worms and eliminate Wolbachia, a symbiotic bacterium that contributes to inflammation. 

1. This is followed by three injections of a drug that kills adult heartworms. 

1. During the entire process—typically lasting at least five months—your dog must be kept under strict exercise restriction. Physical activity can increase the risk of serious complications, including dead worms blocking vital blood vessels. 

Though the protocol is lengthy, it’s highly effective: over 98% of dogs treated with this method will test heartworm-negative by the end. The cardiology team at BBVSH will remain involved throughout your pet’s care, ensuring that any complications are managed quickly and that follow-up testing is scheduled appropriately. 

Treatment in Cats: Supportive Care Only 

There is currently no effective medical treatment for heartworm infection or heartworm disease in cats. If your cat is diagnosed with heartworms, your veterinarian or cardiologist may recommend medications to reduce the inflammatory response and the resulting heartworm disease, or surgery to remove the heartworms. 

Because of the risk of sudden complications—and the lack of approved treatment—prevention remains the only reliable protection for cats. 

A Real-Life Example: Behr’s Life-Saving Surgery 

One of the most compelling examples of advanced heartworm care at Boundary Bay Veterinary Specialty Hospital is Behr, a 4-year-old toy poodle who was referred to the Cardiology Department after developing right-sided heart failure secondary to heartworm disease.

Behr’s condition had advanced to caval syndrome, a critical complication in which adult heartworms obstruct blood flow through the right side of the heart. This can result in collapse, difficulty breathing, and is often fatal without immediate intervention. 

Our cardiologist performed a minimally invasive surgical procedure using a fluoroscope—a real-time X-ray—to guide instruments into Behr’s heart. Two very large heartworms were successfully removed: one measuring 24 cm (9.5 inches) and the other 22 cm (8.75 inches). 

Thanks to the precision of the procedure and the support of the cardiology team, Behr made a remarkable recovery and was able to return home. 

Behr’s story is a powerful reminder of how serious—and urgent—heartworm disease can become. It also highlights the critical role of specialized veterinary care when advanced complications arise.