Preventing Heartworm Infection in Dogs: 2025 Vet Guide by Dr Duncan Houston 🐶🛡️

Hello, I’m Dr Duncan Houston, BVSc. Heartworm disease remains a serious threat to dogs across North America, but with consistent prevention and smart strategies, it’s entirely avoidable. This guide outlines current best practices for protecting your pup in 2025.

1. 💊 Monthly Preventive Medications

All preventives work by killing immature heartworm larvae (L3/L4) acquired over the past month. Administer at the end of each monthly period after exposure.

Options include:

• Ivermectin-based: Heartgard®, Iverhart®, Tri-Heart—safe even if a dog accidentally has early infection; provides partial coverage for other worms via added pyrantel.
• Milbemycin-based: Sentinel®, Interceptor®—also targets round, hook, and whipworms; reach-back protection is slightly less than ivermectin. :contentReference[oaicite:3]{index=3}
• Selamectin: Revolution® (topical) – covers heartworm and ectoparasites; not used for treating active infections.
• Moxidectin: Advantage Multi®, Coraxis® (topical) – combined with flea/tick agents; Proheart6/12® (injectable) – delivers 6 or 12 months of continuous protection. Puppies must be ≥6 months old (Proheart6) or ≥12 months old (Proheart12). :contentReference[oaicite:4]{index=4}

2. 🧭 Annual Testing & Start‑Up Protocol

Before beginning or restarting prevention, test for heartworm via antigen and microfilaria screening—this confirms your dog is negative and safe to begin med use. :contentReference[oaicite:5]{index=5}

Test annually even if on prevention; early detection speeds up treatment and improves outcomes. :contentReference[oaicite:6]{index=6}

3. ⏳ Seasonal Timing Made Simple

Heartworm transmission requires a warm period (>57°F) totaling approximately 234 degree-days (Heartworm Development Units) in 30 days. :contentReference[oaicite:7]{index=7}

Rather than calculating this manually, the American Heartworm Society (AHS) recommends year-round prevention—especially given unpredictable weather and mosquito presence indoors. :contentReference[oaicite:8]{index=8}

4. 🦟 Double Defense: Add Mosquito Repellents

New recommendations from AHS & CAPC include pairing preventives with an EPA-approved mosquito repellent (permethrin-based) to “double defend” against bites and drug-resistant heartworm strains. :contentReference[oaicite:9]{index=9}

This significantly reduces infection risk and helps combat resistant heartworms in high-prevalence areas. Topical products include Vectra 3D® or K9 Advantix® II—safe when used as directed. :contentReference[oaicite:10]{index=10}

5. 🌍 Environmental Control

• Limit standing water & treat breeding areas.
• Use yard sprays with mosquito repellents (per EPA-registered products).
• Avoid peak mosquito times—dawn/dusk walks, screened windows, indoor fans.

6. 🐾 Reach‑Back: Missed Doses

If you accidentally skip a preventer for several months, restarting is still effective—but reach-back effectiveness is highest with ivermectin-based products (~95% reduction in infections), lower with milbemycin (~41%). :contentReference[oaicite:11]{index=11}

7. 🏥 Managing Resistance in the Southeast

Resistant heartworm strains have been documented in the Mississippi Delta region. Here, strict compliance with prevention, testing, and repellents is mandatory. Avoid “slow-kill” strategies using only macrocyclic lactones. :contentReference[oaicite:12]{index=12}

8. ✅ Final Takeaways

• ✔️ Year‑round monthly preventives are the foundation
• ✔️ Add mosquito repellents (“Double Defense”) for extra safety
• ✔️ Test before starting and annually after
• ✔️ Stay consistent—even short lapses can risk infection
• ✔️ Environmental steps—drains, screens, avoidance—help reduce exposure

📲 Need Help? Ask A Vet Has You Covered

Not sure which medication fits your pup’s age, lifestyle, or local environment? Use the Ask A Vet app for real-time guidance tailored to your dog’s needs—veterinary care, anytime, anywhere. 🐶❤️