Heartworm Prevention 101: What Every Dog Owner Needs to Know

Heartworm disease is serious, and one of the most preventable conditions in dogs. The good news: with the right medication and an annual vet check, you can keep your dog protected. Here's a straightforward guide to how heartworms work and what your options are, reviewed by your Petco Chief Veterinarians.

What Are Heartworms?

Heartworms (Dirofilaria immitis) are parasitic roundworms spread through the bite of an infected mosquito. They are not contagious between dogs or from dogs to people — mosquitoes are the only transmission route. However, animals are affected by mosquitoes when they travel across communities and backyards and infect them. Heartworms have been diagnosed in all 50 U.S. states, according to the American Heartworm Society.

After a bite, larvae gradually mature over 6–7 months into adult worms that can live in the heart and lungs. Because this process is slow and early infection rarely causes obvious symptoms, regular testing and consistent prevention are the most effective tools a pet parent has.

How do I know if my Pet has Heartworm Infection?

Early heartworm infection is often symptom-free, which is one reason annual testing matters even for dogs on prevention. If infection progresses without detection, signs can include:

• A mild, occasional cough
• Tiring more easily during walks or play
• Reduced appetite or gradual weight loss
• A slightly rounded belly in more advanced cases

If you notice any of these, your Petco veterinarian can run a quick blood test to check. The earlier any issue is caught, the more straightforward the path forward.

Good to know: Heartworms have been detected in all 50 U.S. states, according to the American Heartworm Society — so prevention is worthwhile regardless of where you live or whether your dog spends time outdoors.

What are Heartworm Prevention Options?

All heartworm preventatives require a prescription from your veterinarian. Your Petco veterinarian will recommend the best fit based on your dog's weight, lifestyle, and any other parasite concerns in your area.

Year-Round vs. Seasonal Prevention: What do Petco Veterinarians Recommend?

The American Heartworm Society recommends year-round prevention for all dogs. The reason is practical: these medications work by clearing any larvae your dog may have picked up in the past 30 days. Staying consistent — rather than pausing in cooler months — is the simplest way to keep that protection seamless. your Petco veterinarian can help you find an option that fits easily into your routine.

Why Does Annual Testing Still Matter?

Even dogs on prevention benefit from an annual heartworm test. It's a quick, simple blood draw at your Vetco clinic or Vetco Total Care hospital and gives you peace of mind. If there's ever a missed dose or an unusual circumstance, catching anything early makes a meaningful difference in how smoothly it can be addressed.

What If My Dog Tests Positive?

A positive test is understandably stressful news, but it's important to know that heartworm disease is treatable. Your Petco veterinarian will walk you through a step-by-step treatment plan and be with you throughout the process. Treatment does require rest and recovery time for your dog, so the emphasis on prevention is really about keeping things simple — for your dog and for you.

How Can I Fill my Dog's Heartworm Prescription at Petco?

Your Petco veterinarian can prescribe heartworm prevention at your appointment. Fill it in-store or online through the Petco Pharmacy with Autoship, so staying on schedule is one less thing to think about. Our Vetco Total Care hospital and Vetco Vaccination Clinic prescriptions automatically transfer to your Petco.com profile, for easy fulfillment by our partner Vetsource. If you’re not a Petco Vet customer, it’s still easy to order online. Simply select contact my vet at checkout or mail in your prescription for validation.