What is heartworm?

Heartworm in cats and dogs is caused by Dirofilaria immitis, a long, thread-like parasite that primarily lives in the pulmonary arteries and sometimes the heart. It is found in tropical and subtropical climates, including southern Quebec.

While dogs are the main host (allowing the parasite to reach maturity), other wild and domestic canids—like foxes, coyotes, and wolves—can also be infected and act as reservoirs. Additionally, cats, ferrets, raccoons, bears, horses, and even humans may be affected.

How is heartworm in cats and dogs is transmitted?

Heartworm is transmitted by the bite of an infected mosquito. Here’s how the cycle works:

• A mosquito bites an infected dog and ingests microfilariae (immature larvae).
• These larvae develop into infective forms inside the mosquito.
• The next time the mosquito bites an animal, it injects the larvae, starting a new infection.

Heartworm life cycle in dogs

In dogs:

• Adult female worms produce microfilariae that circulate in the blood.
• Mosquitoes pick up these microfilariae, which then develop and are passed on to other animals.
• Once inside a new host, the larvae mature over several months and migrate to the pulmonary arteries.

Heartworm life cycle in cats

In cats:

• The immune system often kills the larvae before they mature.
• Few microfilariae are produced.
• Despite low worm numbers, heartworm can still cause severe illness, including sudden death.

Clinical signs of heartworm

In dogs, common symptoms include:

• Persistent coughing
• Fatigue and reduced activity
• Weight loss
• Breathing difficulties

In cats, symptoms often mimic feline asthma:

• Coughing episodes
• Vomiting
• Respiratory distress
• In some cases: sudden death

How is heartworm diagnosed?

Diagnosis is easier in dogs than in cats.

• Antigen and antibody blood tests detect adult worms in dogs.
• In cats, these tests are less reliable. Vets may need X-rays or ultrasounds to detect signs of infection.

Treatment and prevention

Currently, no approved treatment exists for heartworm in cats. This makes prevention absolutely essential.

• Preventive treatments are available as chewable tablets or topical solutions.
• They should be given monthly from June to November in high-risk areas like Quebec.

Why is annual testing important?

Even pets on prevention should be tested once a year because:

• Missed doses, incorrect administration, or parasite resistance can lead to infection.
• Early detection allows for quicker medical action.

Heartworm in humans

While rare, human cases of heartworm have been reported since 1991.

• Symptoms may include chest pain, coughing, and fever.
• No reliable test exists for humans, so the best prevention is to protect pets, breaking the parasite’s life cycle.

Conclusion

Heartworm is a real threat to both cats and dogs. But the good news is that it’s preventable. Speak to your veterinarian about yearly screening and a proper prevention plan to keep your pet safe from this dangerous parasite.