Dog Heartworm Prevention: What Every Owner Must Know

What Is Heartworm?

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm transmitted through mosquito bites. Adult worms can grow up to 30cm long and live in the heart, lungs, and blood vessels of infected dogs.

Left untreated, heartworm is fatal. Treatment is expensive (1000-3000 dollars), painful for the dog, and carries serious risks. Prevention costs 5-15 dollars per month.

How It Spreads

When a mosquito bites an infected animal, it picks up baby heartworms. These develop into infectious larvae inside the mosquito over 2 weeks. When the mosquito bites your dog, the larvae enter the skin and migrate to the heart over 6-7 months.

Prevention Options

• Monthly oral tablets: Ivermectin-based (Heartgard), milbemycin (Interceptor). Given once a month with food.
• Monthly topical treatments: Applied to the skin between the shoulder blades. Good for dogs that will not take pills.
• 6-month injection: ProHeart 6 or ProHeart 12. One injection protects for 6 or 12 months. Ideal for owners who forget monthly doses.

Why You Cannot Skip a Dose

Heartworm preventatives do not kill adult worms — they eliminate the larvae from the previous month. If you skip a dose, any larvae that entered your dog during that gap will survive and develop. It takes 6-7 months before they are detectable on a test.

Annual Testing Is Still Required

Even on prevention, your vet should test annually. You might have missed a dose, some parasites are developing resistance to ivermectin, and early detection means easier treatment.

Symptoms to Watch For

• Persistent cough (the number one sign)
• Decreased appetite and weight loss
• Reluctance to exercise
• Swollen belly from fluid buildup

The Bottom Line

Heartworm prevention is one of the simplest, most cost-effective things you can do for your dog. Set a phone reminder, use an automatic delivery service, or switch to the 6-month injection.