A single mosquito bite can deposit microscopic larvae that mature into foot-long worms living in heart and pulmonary arteries. Heartworm disease progresses silently months, then cough, exercise intolerance, heart failure appear. Monthly prevention kills incoming larvae before adulthood — cheaper and kinder than arsenic-style treatment adult infection requires.
Key points at a glance
- Mosquitoes transmit — indoor dogs not immune.
- Preventives work backward — kill recent infection, not existing adults — test before starting if lapses.
- Skip months in warm climates — high exposure risk.
- Cats get heartworm too — different protocol — this article dog focus.
- Treatment for adult worms — months rest, pain, expense — prevention wins.
Life cycle simplified
- Infected dog has microfilariae in blood.
- Mosquito picks up larvae feeding.
- Mosquito bites your dog — injects larvae.
- Larvae migrate — mature in 6–7 months to adult worms.
- Adults reproduce — cycle continues.
Geography expanded — all 50 US states report cases — travel and relocated dogs spread reservoir.
Prevention options
| Type | Examples | Notes |
|---|---|---|
| Monthly oral | Ivermectin-family chew | Common — tasty — remember date |
| Monthly topical | Moxidectin spot-on | Also fleas some formulas |
| Injectable 6–12 mo | ProHeart | Clinic administered |
All require prescription — vet matches product to breed MDR1 status (some collies sensitive ivermectin — gene test).
Why monthly matters
Larvae arrive constantly in mosquito season. One missed month leaves window for L3/L4 stages to survive if product does not retroactive kill adults.
Year-round recommended most US regions — mosquito indoor winter happens; consistent habit beats calendar guess.
Testing before prevention restart
Adult heartworms present — giving preventive without test can rarely trigger reaction — annual antigen test standard.
Puppies — start age per label, test at 7 months first time.
Early vs late disease signs
Early: often none — mild cough dismissed.
Moderate: exercise fatigue, weight loss.
Severe: pot belly fluid, collapse, dark urine — caval syndrome emergency.
Chest X-ray and antigen blood diagnose.
Treatment vs prevention cost
Prevention — dollars monthly.
Adult treatment — thousands, strict crate rest months, complication risk — not "just pills."
The bottom line
Heartworm disease in dogs is mosquito-spread, silent, then deadly — monthly vet-prescribed prevention kills larvae before worms grow. Test on schedule, do not skip warm-month doses because your dog "stays inside," and treat lapses with a vet visit before restarting chews. Prevention is boring insurance; adult heartworm treatment is the bill you want never to open.
This article is for general education only and does not replace advice from a licensed veterinarian.