What is Heartworm Disease?

Heartworm disease is caused by Dirofilaria immitis, a parasitic roundworm that is transmitted through mosquito bites. Adult heartworms live in the heart, pulmonary arteries, and lungs of infected animals, where they can grow up to 30 cm long and live for 5–7 years in dogs. The worms cause progressive damage to the heart, lungs, and blood vessels, leading to heart failure and death if untreated.

Dogs are the natural definitive host for heartworms, meaning the parasites complete their full lifecycle in dogs. Cats are atypical hosts — fewer worms survive to adulthood, but even a single worm can cause serious, life-threatening disease.

Lifecycle

  1. A mosquito bites an infected animal and ingests microscopic larvae (microfilariae)
  2. Larvae develop within the mosquito over 10–14 days
  3. The mosquito bites another animal, depositing infective larvae into the skin
  4. Larvae migrate through tissues over several months, eventually reaching the heart and pulmonary arteries
  5. Worms mature to adults in approximately 6 months
  6. Adult worms produce microfilariae, completing the cycle

Symptoms

In Dogs

  • Early stages: Often no symptoms
  • Mild persistent cough
  • Exercise intolerance and fatigue
  • Decreased appetite and weight loss
  • Swollen abdomen (fluid accumulation from heart failure)
  • Caval syndrome: Sudden cardiovascular collapse from heavy worm burden — a life-threatening emergency

In Cats

  • Coughing and asthma-like episodes (HARD: Heartworm Associated Respiratory Disease)
  • Vomiting (not related to eating)
  • Weight loss
  • Lethargy
  • Sudden death (sometimes the only sign)

Diagnosis

Dogs

  • Antigen test: Blood test detecting proteins from adult female heartworms; the standard screening tool
  • Microfilaria test: Detects baby heartworms in the blood
  • X-rays and echocardiography: To assess severity of heart and lung damage

Cats

  • Diagnosis is more difficult in cats
  • Both antigen and antibody tests may be needed
  • X-rays and echocardiography help confirm diagnosis
  • Many cases are only diagnosed post-mortem

Prevalence in Canada

Heartworm is present in Canada, particularly in:

  • Southern Ontario
  • Southern Quebec
  • Manitoba
  • Okanagan Valley (British Columbia)

Climate change is expanding the range of mosquitoes, increasing heartworm risk in previously unaffected areas. The Canadian Parasitology Expert Panel recommends prevention for all dogs in endemic areas.

Prevention

Prevention is far easier, safer, and more affordable than treatment:

  • Monthly oral preventatives: Ivermectin-based (Heartgard), milbemycin (Interceptor, Sentinel)
  • Monthly topical preventatives: Selamectin (Revolution), moxidectin (Advantage Multi)
  • Injectable preventative: ProHeart (moxidectin), lasting 6–12 months
  • Prevention should be given year-round or at minimum from June through November in Canada

Annual Testing

Even dogs on prevention should be tested annually because:

  • No preventative is 100% effective
  • A missed or late dose can allow infection
  • Early detection allows prompt treatment

Treatment

Dogs

Treatment is available but risky and expensive:

  • Melarsomine (Immiticide): Injected into the back muscles to kill adult worms
  • Strict exercise restriction for months (activity can cause dead worms to break loose and cause embolisms)
  • Supportive care: Steroids, antibiotics (doxycycline to target Wolbachia bacteria that heartworms depend on)
  • Treatment can take 6–12 months total

Cats

There is no approved treatment for heartworm in cats. Management focuses on:

  • Supportive care for symptoms
  • Monitoring and waiting for worms to die naturally (lifespan in cats: 2–3 years)
  • Emergency treatment if crisis occurs
  • This is why prevention is absolutely critical for cats
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