Is your pet protected from heartworm disease?

April is Heartworm Prevention month, and it’s an opportunity to raise awareness about the risks of heartworm disease in pets. Heartworm disease is a potentially fatal disease caused by heartworms (Dirofilaria immitis), which are transmitted by mosquitoes. The foot-long heartworms live in the heart, lungs, and associated blood vessels of the affected pet. The presence of heartworms in your pet causes severe lung disease, heart failure, and damage to other organs in the body.

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

Our board-certified cardiologists at Boundary Bay Veterinary Specialty Hospital have the training and experience to help diagnose and treat heartworms, including surgically removing heartworms.

Who can get Heartworm Disease?

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 is heartworm transmitted?

The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected animal produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days.

Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to develop into sexually mature adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.


Is Heartworm Disease different in dogs and cats?

Dogs: The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate, and produce offspring. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, heartworm prevention for dogs is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible. Learn more about heartworm medicine for dogs.

Cats: Heartworm disease in cats is very different from heartworm disease in dogs. The cat is not a typical host for heartworms, and most worms in cats do not survive to the adult stage. Cats with adult heartworms typically have just one to three worms, and many cats affected by heartworms have no adult worms. While this means heartworm disease often goes undiagnosed in cats, it’s important to understand that even immature worms cause real damage in the form of a condition known as heartworm associated respiratory disease (HARD). Moreover, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.

How can I tell if my pet has heartworm infection or disease?

Dogs: If your dog has been recently or mildly infected with heartworms, he/she may show no signs of illness until the adult worms have developed in the lungs and signs of heartworm disease are observed. As the disease progresses, your dog may cough, become lethargic, lose his/her appetite, or have difficulty breathing. You may notice that your dog seems to tire rapidly after only moderate exercise.

Microfilaria (baby heartworms) in a blood sample from a heartworm positive patient.
Microfilaria (baby heartworms) in a blood sample from a heartworm positive patient.

Blood tests are performed by your veterinarian to detect the presence of adult heartworm infection (> 6-month-old infections) in your dog. Antigen tests detect the presence of adult female heartworms, and antibody tests determine if your pet has been exposed to heartworms. The antigen test is most commonly performed and is very accurate in dogs. Further tests, such as chest radiographs (x-rays), a blood profile and an echocardiogram (an ultrasound of the heart), may be necessary to confirm the diagnosis, to evaluate the severity of the disease, and to determine the best treatment plan for your dog.

Cats: Signs of possible heartworm disease in cats include coughing, respiratory distress, and vomiting. In some cases, a cat may suddenly die from heartworms.T

he diagnosis of heartworm infection in cats is more difficult than it is with dogs. A series of different tests may be needed to help determine the likelihood of heartworm infection as the cause of your cat’s illness and, even then, the results may not be conclusive. In general, both antigen and antibody tests are recommended for cats to give the best chances of detecting the presence of heartworms.



When should my dog or cat be tested for heartworm?

The American Heartworm Society recommends testing pets every 12 months for heartworm and giving your pet a heartworm preventive 12 months a year.

Testing of cats is recommended before placing them on heartworm preventive, and afterward per your veterinarian’s advice. This includes pets that never go outside, or only go outside occasionally.

Can heartworm disease be prevented?

Heartworm infection is almost 100% preventable in dogs and cats. There are several FDA-approved heartworm preventives available in a variety of formulations. Your veterinarian can recommend the best method of prevention based upon your pet’s risk factors and lifestyle. Of course, you must remember to give your pet the preventive for it to work!

The preventives do not kill adult heartworms and will not eliminate heartworm infection or prevent signs of heartworm disease if adults are present in the pet’s body. Therefore, a blood test for existing heartworm infection is recommended before beginning a prevention program to assess the pet’s current heartworm status. Because it is more difficult to detect heartworms in cats, additional testing may be necessary to make sure the cat is not infected.

How can my pet be treated if they have heartworm disease?

Heartworm is a progressive, life-threatening disease. The earlier it is detected and treated, the better the chances that your pet will recover and have less complications.

In BC, where heartworm is less common, infected dogs are often referred to a cardiologist or internal medicine specialist. In places where heartworm is common, treatment is often done by general practice veterinarians.

What will the cardiologist do?

When your pet sees a cardiologist, they will do a scan called an echocardiogram (ultrasound of the heart) to determine if there are any heartworms within the heart itself. If this occurs, it can cause a life-threatening condition called caval syndrome. The only treatment at this stage is surgical removal of the worms.

If the worms have not yet invaded the heart, your pet will be treated with a medical protocol laid out by the American Heartworm Society. There are several stages to treatment: first, we weaken the worms with oral antibiotics, then kill them with three separate injections. During the whole procedure (at least five months), your pet will be under extremely strict rest. If they move or run around too much, there is a risk of a dead worm blocking off a blood vessel causing severe complications. The protocol takes a long time, but is worth it – over 98% of dogs will be heartworm negative after going through the protocol, and the cardiology team will be with you every step of the way!

Dogs: As with most medical problems, it is much better to prevent heartworm infection than to treat it. However, if your dog does become infected with heartworms, treatment is available. There is substantial risk involved in treating a dog for heartworms. However, serious complications are much less likely in dogs that are in good health and when you carefully follow your veterinarian’s instructions.

The goal of heartworm treatment is to kill the adult worms and microfilariae present in your dog, as safely as possible. However, when a dog is treated it is important to consider that heartworms are dying inside the dog’s body. While your dog is treated, it will require complete rest throughout hospitalization and for some time following the last treatment. Additionally, other medications may be necessary to help control the body’s inflammatory reaction as the worms die and are broken down in the dog’s lungs.

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

Can heartworms be surgically removed?

Adult heartworms removed from a 12 lb dog.
Adult heartworms removed from a 12 lb dog.

Surgical removal of heartworms from dogs and cats is a high-risk procedure and is typically reserved for severe cases. However, in many cases surgical removal of heartworms may be necessary to afford the best opportunity for the pet’s survival.

A Heartworm Success Story at Boundary Bay


This is Behr, a 4-year-old toy poodle, who came to Boundary Bay Veterinary Specialty Hospital after being diagnosed with right-sided heart failure secondary to heartworm disease. Behr was urgently referred to our veterinary cardiology department to see cardiologist Dr. Mark Harmon, DVM, Dipl. ACVIM (Cardiology). Using a fluoroscope (a real-time x-ray look at the heart), Dr. Mark Harmon passed instruments into Behr’s heart and cleared his heart of two very large heartworms – one measuring about 24 cm (9.5 inches) and the other 22 cm (8.75 inches)!







Where can pet owners go for more information?

The American Heartworm Society website has many resources for vets and pet parents –

How do I find out more about BBVSH cardiology services?

Please visit our cardiology service page. Owners can also request a referral.



American Veterinary Medical Association

American Heartworm Society



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