What is heartworm disease in dogs and cats?

By Tinamaria Colaizzi

Heartworm disease is a serious, preventable illness affecting domestic pets and wild animals all around the world. Heartworms are worms that can be up to a foot-long and can cause lasting damage in an animal’s heart and lungs, severely impacting their quality of life. Companion animals (including dogs, cats, and certain pocket pets) are most at risk for developing a heartworm infection. According to a 2022 study from the Companion Animal Parasite Council (CAPC), 1 in 200 dogs tested positive for heartworm throughout Canada in 2022. Additionally, the American Heartworm Society (AHS) reports that more than a million pets in the United States alone have heartworms.

What causes heartworm?

Dirofilaria immitis, a nematode parasite, causes heartworm disease, and is only transmitted to animals through mosquito bites. If you’ve ever wondered, “Can dogs give other dogs heartworms?”, rest assured that the disease is not “contagious”. It is only transmissible through bites from infected mosquitoes.

Mosquitoes ingest the microfilariae (immature worms) when they bite an infected animal. Later, when they bite another animal, they transmit the disease by depositing infective larvae under the skin of the new host. The larvae will migrate as they mature into adult heartworms living in the heart, lungs, or other blood vessels. The following infographic, provided by the AHS, shows the heartworm lifecycle in an infected dog. A heartworm test for dogs would detect an antigen produced by adult female heartworms. It can take at least six months for the heartworms to mature and produce enough antigens to be detected by the most commonly used heartworm tests.

Where is heartworm found?

Geographically, heartworm can be found on nearly every continent, and is widespread throughout North America. The most recent statistics show a higher number of cases throughout the Atlantic coast and southern regions of the United States. In fact, preventative treatments tend to be less effective in areas surrounding the Mississippi River, due to the spread of drug-resistant heartworms in that region. On the other hand, rates of heartworm spread tends to be lower in areas with colder climates, including various regions of Canada. While Canada has lower rates of heartworm in pets, it still occurs. Specifically, one study found that the highest number of test-positive canines were located in southern Ontario, southern Manitoba, and southern Quebec.

Signs of heartworms in dogs

While the scope and timing of heartworm symptoms in dogs may vary, the Alberta Veterinary Medical Association (AVMA) reports that coughing, respiratory issues, lethargy, and loss of appetite are the main symptoms related to heartworms in dogs.

According to the Food and Drug Administration (FDA), there are four stages of heartworm disease in dogs, each increasing in severity. These stages mostly refer to worm burden and size, which can relate to the degree of blood flow obstruction.

Stage 1 –The first signs of heartworm in dogs may not even be noticeable, as symptoms can range from non-existent to mild.

Stage 2 – Symptoms can range from mild to moderate, including an occasional cough or lethargy.

Stage 3 – Symptoms may be more noticeable at this stage, and can include a persistent cough, trouble breathing, and a change in appearance.

Stage 4 – Symptoms are the most severe during the last stage, and heartworms are likely causing a blockage near the heart. Immediate attention is needed.

In serious cases and those specified as “Stage 4”, dogs are at risk for developing caval syndrome, which can include symptoms such as sudden breathing difficulties, pale gums, and dark urine. Keep in mind that mild heartworm infections with a single, or small worm, may not show symptoms. In fact, in areas where heartworm is less prevalent, most infected dogs may have just one or two heartworms, and can be asymptomatic for years. Additionally, most infections are detected using various blood chemistry or microscopic means. Chest x-rays alone can’t always detect heartworm infections, as radiographic changes only become visible in advanced stages.

Symptoms of heartworm in cats

Cats are not natural hosts for heartworm infections, which means that the disease and its symptoms are quite different to what is seen in dogs. For instance, the adult worms affecting cats tend to be shorter and hardly ever produce microfilariae.

According to one study, heartworm symptoms in cats can range from respiratory issues (such as coughing and asthmatic attacks) to sudden death. For the most part, heartworm symptoms in cats are often general and are unlikely to show specific signs of heart failure.

Despite a low prevalence of heartworm cases in cats, severe cases can still occur based on just a scarce number of worms. In fact, the AHS cites that cats affected by heartworms may not have any adult worms at all. Even so, underdeveloped worms can still cause damage to a cat’s heart, most notably through heartworm-associated respiratory disease (HARD). Symptoms of HARD may easily be confused for general asthmatic symptoms or feline bronchitis and can often lead to lasting respiratory damage.

Testing and diagnosing dogs and cats for heartworm

Given its progressive nature, detecting heartworm infection as early as possible can improve your pet’s ability to recover. The CAPC recommends that dogs undergo an exam for heartworm disease at least once a year.

There are a range of heartworm tests for dogs used to confirm and detect heartworms, including antigen, antibody, and blood tests. Most often, heartworm proteins in your pet’s blood sample will verify the presence of heartworms. If the results are positive, your veterinarian may require additional tests to confirm the diagnosis. In some cases, radiographs or echocardiograms can be used to analyze the severity of heartworm infections.

Because of their natural immune response, heartworm testing tends to be less sensitive for our feline companions. In fact, heartworm in cats may often remain undiagnosed. As a comprehensive diagnostic test to detect heartworm in cats is not currently available, multiple tests (including thoracic radiography and serum antigen tests) must be used to diagnose feline heartworm disease. In addition to these measures, heartworm antibody tests have also been approved for use in cats.

Heartworm treatment

Heartworm treatment is available through your veterinarian and should always be used under their guidance and direction. It typically involves a year-long process of medicating and monitoring. Currently, Health Canada-approved treatments include one or two doses of melarsomine, combined with monthly treatments with a macrocyclic lactone drug. This therapy should not be confused with the various types of medications used to prevent heartworm disease. In severe cases, surgical removal of heartworms may be necessary.

In the unfortunate event that your pet contracts heartworm, has been prescribed new medicine or starts a new treatment plan, you can stay organized and keep track of this critical information in one place with 24PetMedAlert^®. This service is part of our Lifetime Protection Membership and Lifetime Protection Membership Plus, and allows you to store your pets’ entire medical history in the same place. And, if your companion ever goes missing, important health and behavioral information will be relayed to the finder or facility. Learn more here.

If you’re wondering what to expect after heartworm treatment, the AVMA notes that your dog will likely require a lot of rest. Your veterinarian may also prescribe anti-inflammatory medication as your dog’s body fights off the infection. In general, exertion should be restricted throughout the heartworm treatment phase, since physical activity can increase damage in your companion’s heart and lungs.

Currently, an effective treatment has not been found for heartworm in cats. However, your veterinarian may prescribe anti-inflammatory medication to manage symptoms, or in some cases, a surgical treatment.

Heartworm prevention

Although your pet may appear healthy and without symptoms, they could be infected with microfilariae or still be in the beginning stage of heartworm disease. When it comes to heartworm infections, prevention is key.

You may already be familiar with different forms of heartworm prevention for dogs, such as chewable medications or topical treatments. Keep in mind that these preventative medications do not kill adult heartworms, nor do they fight off an active heartworm infection.

The Canadian Veterinary Medical Association (CVMA) suggests administering monthly preventive heartworm medicine, approved by Health Canada, either orally or topically. Standard protocol for annual heartworm exams and prevention recommendations can vary depending on the age of your dog, as well as your location and climate. Check with your veterinarian to find out what’s best for your pet. In most cases, puppies 6 months of age and younger can begin heartworm prevention medication without undergoing a heartworm examination. In fact, the Canadian Kennel Club (CKC) recommends parasite control in puppies as early as 8 weeks. However, dogs older than 7 months must be tested for heartworms before being placed on a preventive medication. As always, consult with your veterinarian regarding best practices for your pet.

Location also plays a part in heartworm prevention. Whether you are planning to relocate or travel to new areas with your dog or cat, checking in with your veterinarian is a great way to understand the risks posed to your companion.

If you can’t make it to see your vet in person, you can get your heartworm-related questions answered via phone, email, or chat with our 24/7 Vet Helpline, included in our Lifetime Protection Membership. If an in-person vet visit is warranted, one of the licensed veterinary professionals will let you know, saving you time, money, and stress.

The information provided and contained herein are the opinions of Pethealth Services Inc. which are based on external publication. The content is not intended to be a substitute for professional veterinary advice. Pethealth Services Inc. assumes no responsibility or liability for any loss, claims or damages arising out of the within content