Heartworm Prevention: Protecting Dogs and Cats Year-Round

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm transmitted through mosquito bites. Adult worms live in the heart, lungs, and associated blood vessels, growing up to 30 cm long. A single dog can harbour over 250 worms, causing severe lung disease, heart failure, and organ damage.

In cats, even 1–2 worms can be fatal because their hearts and blood vessels are much smaller. Cats are "atypical hosts" — the worms don't thrive as well, but the immune response they trigger can be devastating.

"Heartworm treatment in dogs is expensive, painful, and carries real risks. Prevention costs a fraction of treatment and is nearly 100% effective. There's no good reason to skip it." — Dr. Sarah Chen, DVM

Heartworm is one of several dangerous parasites covered in our complete parasites guide.

Understanding the lifecycle explains why monthly prevention works:

• Step 1: A mosquito bites an infected animal and picks up microscopic larvae (microfilariae)
• Step 2: Inside the mosquito, larvae develop into infective stage over 10–14 days
• Step 3: The mosquito bites your pet, depositing larvae on the skin. They enter through the bite wound
• Step 4: Larvae migrate through tissue over 2–3 months, eventually reaching the heart and lungs
• Step 5: Worms mature over 6–7 months and begin producing microfilariae, completing the cycle

Key insight: Monthly preventatives kill larvae deposited in the previous 30 days — before they reach the heart. This is why consistent, on-time dosing is critical. Missing even one month creates a window for larvae to develop beyond the point where preventatives can kill them.

Heartworm cannot spread directly between pets — it always requires a mosquito as intermediary.

In Dogs

Early infection often shows no symptoms. As worm burden increases:

• Persistent soft cough
• Reluctance to exercise or tiring quickly on walks
• Reduced appetite and weight loss
• Swollen abdomen (fluid accumulation from heart failure)
• Caval syndrome — sudden collapse, dark bloody urine, laboured breathing (emergency)

In Cats

Symptoms can mimic asthma or be completely absent until sudden death:

• Coughing or wheezing (often misdiagnosed as feline asthma)
• Vomiting (not related to eating)
• Difficulty breathing
• Weight loss and decreased appetite
• Sudden collapse or death — sometimes the first and only sign

If your pet shows any of these signs, our pain recognition guide can help you assess urgency.

All heartworm preventatives require a prescription. Your vet will recommend the best option based on your pet's species, size, and lifestyle.

Monthly Oral Tablets (Dogs)

• Most popular option — chewable, flavoured tablets
• Many also prevent intestinal worms (roundworm, hookworm)
• Must be given on the same date each month

Monthly Topical (Dogs & Cats)

• Applied to the skin between shoulder blades
• Good for pets who won't take oral medication
• Some formulations also cover fleas and ear mites

Injectable (Dogs Only)

• A single injection lasts 6 or 12 months
• Eliminates the risk of missed monthly doses
• Administered by your vet

Heartworm prevention often pairs with flea and tick prevention — some products cover all three in a single dose.

Some owners only give heartworm prevention during "mosquito season," but vets strongly recommend year-round protection:

• Climate unpredictability — warm spells in winter can bring mosquitoes earlier than expected
• Indoor mosquitoes — they breed indoors in standing water (plant saucers, pet bowls)
• Travel exposure — a single holiday in a warmer region can expose your pet
• Compliance gaps — stopping and restarting increases the chance of missing a dose
• Testing requirement — if you stop prevention, your vet must test before restarting (to avoid dangerous reactions)

Testing: Dogs should be tested annually, even on prevention. Tests detect adult worms in the bloodstream and catch rare breakthrough infections early. Cats are harder to test accurately — prevention is the primary strategy.