Seasonal Heartworm Prevention Guide (Monthly Dosing)

HeartPreventionMeongsiljang Veterinary Advisory Board

Should heartworm prevention only be given during mosquito season? We’ve compiled a monthly dosing schedule and seasonal precautions based on veterinary evidence.

Should Heartworm Prevention Be Given Only in Summer?

An illustration of a puppy alongside a 12-month medication calendar

Heartworm prevention is an essential part of care, involving monthly administration of oral or topical medications to block infection from mosquito bites. However, due to climate change, the mosquito season is getting longer, and mosquitoes can even be active indoors. Veterinary guidelines recommend year-round treatment. Administering the medication once a month on a consistent schedule is the most reliable way to prevent heartworm disease.

Test for Infection Before Starting Preventive Medication

Administering heartworm preventive medication to a pet that is already infected can cause serious problems. Therefore, veterinary textbooks recommend testing all pets for heartworm infection and confirming they are healthy before starting preventive treatment. If you are starting preventive medication for the first time or there has been a gap of more than one month in administration, be sure to have your veterinarian perform a blood test to check for infection first. The safest approach is to avoid starting medication without prior testing.

Spring–Summer (April–August): Peak Infection Risk Period

Mosquito activity increases significantly from spring through summer. As temperatures rise sufficiently, conditions within the mosquito’s body allow heartworm larvae to become infective. However, the risk of infection is not limited to seasons with high mosquito activity. According to veterinary literature, seasonal risk variations are not always consistent, and infections can occur in autumn and winter if the weather remains mild, making it difficult to designate any specific season as entirely safe.

April–May: Mosquitoes begin their activity. If you paused preventive medication during the winter, have your pet tested for infection and resume treatment before this period.

June–August: This is the peak season for mosquito activity. With increased outdoor exposure, never miss a dose of preventive medication. It is also advisable to check for mosquito bites after walks.

Autumn–Winter (September–February): Why You Should Not Skip Doses

As autumn arrives and mosquitoes become less visible, you might think it’s safe to stop preventive treatments. However, mosquitoes remain active through September and October, and larvae transmitted by bites during this period may still be developing inside your pet’s body. Since heartworm preventives work by killing larvae already present in the body, it’s essential to continue medication for at least one month after the mosquito season ends. Mosquitoes can also be active in winter, particularly in underground parking lots or near building entrances, which is why veterinary guidelines recommend year-round prevention.

An illustration of a dog that can be exposed to mosquitoes even indoors during autumn

Monthly Heartworm Prevention Schedule

This table provides a clear, month-by-month comparison of seasonal versus year-round medication schedules.

Month	Mosquito Activity	Infection Risk	Seasonal Medication	Year-Round Medication
January–February	Minimal	Low	No medication	Administer medication
March	Preparing to emerge	Low	Schedule testing	Administer medication
April–May	Activity begins	Moderate	Begin medication	Administer medication
June–August	Peak activity	High	Administer medication	Administer medication
September–October	Declining	Moderate	Administer medication	Administer medication
November–December	Minimal	Low	Final dose	Administer medication

Year-Round vs. Seasonal Administration Comparison

Compare the options to see which approach is best for your pet.

Category	Year-Round Administration (12 months)	Seasonal Administration (April–November)
Preventive Effectiveness	Most reliable	Risk of infection during off-season gaps
Medication Routine	Maintains a consistent routine	Requires careful management of start and stop dates
Infection Testing	Annual routine testing	Testing required before resuming medication each year
Veterinary Recommendation	Strongly recommended	Permitted depending on region and environment
Suitable For	All pets	Limited cases, such as high-altitude areas without mosquitoes

Practical Tips to Prevent Missed Doses

Set a recurring reminder for the same date each month: Use your phone’s calendar or a reminder app to schedule monthly alerts, which helps prevent missed doses. Pick an easy-to-remember date, such as “the 1st of every month is Heartworm Prevention Day.”

Keep a medication log: Record each dose date in a notebook or app so you can quickly see if any doses were missed.

Monitor weight changes: Preventive medication dosages are based on your pet’s weight. For growing puppies, check weight regularly and consult your veterinarian. If you’d like to learn more about the features of different preventive medications, see Types of Heartworm Preventives and Dosing Guide.

A smartphone displaying a medication reminder, alongside an illustration of a puppy

What to Do If There Is a Gap in Medication

If you’ve missed giving preventive medication for more than two weeks, do not resume treatment on your own—please visit your veterinarian. Since your pet may have been exposed to infection during the gap, it’s safest to restart medication only after testing and following your vet’s guidance. This is especially important if the lapse occurred during summer, when mosquito activity is high, so please schedule testing promptly.

Additional Considerations by Situation

Indoor pets: Even if your pet lives exclusively indoors, mosquitoes can still enter through doors or windows. Prevention is essential regardless of whether your pet goes outside.

Dogs (8 weeks and older): Most preventive medications can be administered starting at 8 weeks of age. Consult your veterinarian about the appropriate time to begin treatment before giving the first dose.

Senior pets: Older pets require more attentive care for their overall health. In addition to year-round heartworm prevention, make sure to schedule annual wellness exams.

Cats: Cats can also contract heartworm disease, but you should never use dog-specific medications on them. Always obtain a prescription for a cat-specific preventive. If you suspect your cat may already be infected, please review Heartworm Symptoms and Treatment Options first.

Illustrations showing preventive care for dogs, senior dogs, and cats in various situations

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

When should I start heartworm prevention for my pet?

It’s best to start preventive treatment in March or April, before mosquitoes become active. However, if your pet is already on year-round medication, there’s no need to set a specific start date. If there has been any gap in treatment, please have your pet tested for infection before beginning again.

Should I still give my pet heartworm medication during the winter?

Veterinary guidelines recommend year-round administration. This is because mosquitoes can remain active indoors during winter, and larvae acquired in the fall may still be developing within your pet’s body.

Do I need to give my pet their preventive medication on the exact same date each month?

Yes, it's important to maintain a 30-day interval. A difference of one or two days is fine, but if the interval becomes longer, the larvae that have entered the body may continue to develop beyond the effective range of the preventive medication.

How often should your pet be tested for heartworm?

Even if your pet is on year-round medication, we recommend an annual check-up. For seasonal medication, be sure to have your pet examined before restarting treatment each year.

Do pets that don’t go for walks still need preventive care?

Yes, mosquitoes can enter your home through the front door or windows. Even if your pet doesn’t go outside, there’s still a risk of infection, so prevention is important.

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References

[1] Handbook of Veterinary Pharmacology — III. Drugs for Heartworm Prevention and Therapy

[2] Textbook of Respiratory Disease in Dogs and Cats — Chapter 70: Heartworm Disease

[3] American Heartworm Society (2020) — Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs

[4] The Dog Care Handbook — Chapter 5: Parasites and Heartworm Prevention