Can Dogs Get Heartworm in the Winter? What Pet Owners Need to Know

AvatarByWilliam Warren Dog Health & Wellness

As the chilly winds of winter set in and snow blankets the ground, many dog owners breathe a sigh of relief, believing that the threat of heartworm disease fades away with the cold. After all, heartworm is often associated with warm, mosquito-filled months, leading to the common assumption that winter offers a natural shield against this potentially deadly parasite. But is this seasonal belief truly accurate? Can dogs get heartworm in the winter, or is it safe to ease up on prevention during the colder months?

Understanding the relationship between heartworm transmission and seasonal changes is crucial for every pet owner. While mosquitoes—the primary carriers of heartworm larvae—are less active or even dormant in freezing temperatures, the risk may not disappear entirely. Geographic location, climate variations, and the biology of both the parasite and its mosquito hosts all play a role in determining when and how heartworm can be transmitted.

This article will explore the nuances of heartworm risk during winter, debunk common myths, and provide insights into why year-round prevention might be the safest approach for your furry friend. Whether you’re a seasoned dog owner or new to pet care, gaining a clear understanding of heartworm dynamics in colder months will help you make informed decisions to protect your dog’s health throughout the year.

How Heartworm Transmission Occurs in Winter

Heartworm disease is primarily spread through the bite of infected mosquitoes. These mosquitoes act as vectors, transferring microscopic heartworm larvae (microfilariae) from one host to another. While it is commonly assumed that heartworm transmission is limited to warmer months, the risk does not entirely disappear in winter.

Mosquitoes require a certain environmental temperature to survive and reproduce effectively. The development of heartworm larvae within the mosquito also depends on temperature, specifically needing warmth to mature into the infective stage. When temperatures drop below approximately 57°F (14°C), this development slows significantly or stops.

However, in many regions, particularly those with mild winters, mosquitoes may remain active during warmer periods of the winter or in sheltered environments such as indoors or urban heat pockets. This means the possibility of transmission continues, albeit at a reduced rate.

Key factors influencing winter heartworm transmission include:

  • Local climate and average winter temperatures
  • Mosquito species and their adaptability to cooler conditions
  • Availability of protected mosquito habitats
  • Duration and severity of cold spells

Risks of Heartworm Infection During Winter Months

The risk of heartworm infection in winter varies geographically and depends on the local environment. In colder climates where freezing temperatures dominate, the risk is minimal due to the inactivity or death of mosquito populations. Conversely, in southern or coastal regions with mild winters, mosquitoes may remain active enough to sustain transmission.

Dogs that spend time outdoors in warmer microclimates or those that travel between regions with different climates may face increased risk. Additionally, indoor mosquitoes can occasionally bite pets, providing another potential transmission route.

Veterinarians emphasize the importance of year-round heartworm prevention, especially in areas where winter temperatures do not consistently drop below the threshold needed to halt mosquito activity.

Preventive Measures for Heartworm in Winter

Consistent prevention is crucial to protect dogs from heartworm infection throughout the year. Preventive medications are effective and typically administered monthly to kill heartworm larvae before they mature.

Important preventive considerations include:

  • Continuing heartworm preventives year-round, especially in mild winter climates
  • Consulting with a veterinarian to determine local risk and appropriate prevention schedules
  • Minimizing outdoor exposure during warmer winter days when mosquitoes may be active
  • Using mosquito control strategies in and around the home, such as eliminating standing water and using insect screens
Region Winter Temperature Range (°F) Mosquito Activity Heartworm Transmission Risk Recommended Action
Northern States Below 32°F (0°C) Minimal to none Low to negligible Continue prevention as advised by vet; seasonal prevention may suffice
Mid-Atlantic & Southern States 32°F to 57°F (0-14°C) Intermittent, during warmer spells Moderate Year-round prevention strongly recommended
Coastal & Tropical Regions Above 57°F (14°C) Active year-round High Year-round prevention mandatory

Heartworm Transmission and Seasonal Risks

Heartworm disease is caused by the parasite *Dirofilaria immitis*, which is primarily transmitted to dogs through mosquito bites. Understanding the transmission cycle is essential to assessing the risk during winter months.

Heartworm larvae develop within mosquitoes, and the rate of this development is highly dependent on temperature. Typically, the larvae require warm environmental conditions to mature to an infective stage capable of transmission.

  • Temperature Threshold: Larvae development generally requires sustained temperatures above 57°F (14°C).
  • Development Time: At optimal temperatures (above 80°F/27°C), larvae mature in approximately 10–14 days.
  • Cold Weather Impact: When temperatures fall below this threshold, development slows or stops, effectively halting transmission.

In regions where winter temperatures consistently remain below this threshold, the risk of heartworm transmission drops significantly. However, in milder climates, mosquitoes can remain active, and transmission may continue year-round.

Risk Factors for Heartworm Infection in Winter

Several factors influence the likelihood of dogs contracting heartworm during the winter months:

Factor Impact on Winter Heartworm Risk
Geographic Location Warmer regions (e.g., southern U.S., tropical areas) have year-round mosquito activity, increasing winter risk.
Indoor vs. Outdoor Exposure Dogs spending significant time outdoors at night have higher exposure to mosquitoes.
Microclimates Urban heat islands or sheltered areas may maintain temperatures suitable for mosquito survival.
Preventive Medication Compliance Interrupting or discontinuing heartworm prevention during winter can increase susceptibility if exposure occurs.

Can Dogs Get Heartworm in the Winter?

While heartworm transmission is significantly reduced during cold weather, dogs can still get heartworm in the winter under certain conditions:

  • In regions where mosquitoes remain active year-round due to warm climate.
  • In areas with mild winters where temperature spikes allow larvae development.
  • When dogs travel to or from endemic areas with active transmission seasons.
  • If preventive medications are stopped prematurely, leaving dogs vulnerable when unexpected mosquito activity occurs.

It is important to note that once a dog is infected, the heartworms develop inside the dog over several months, regardless of the season. Therefore, infections acquired during warmer months can manifest clinically during winter.

Recommendations for Heartworm Prevention During Winter

Maintaining year-round heartworm prevention is generally recommended by veterinary experts to minimize risk. Key guidelines include:

  • Consult with Your Veterinarian: Tailor prevention plans based on local climate and heartworm prevalence.
  • Continue Preventive Medication: Administer monthly heartworm preventatives consistently, even in winter.
  • Monitor Mosquito Activity: Be vigilant in areas where mosquitoes may remain active or during unseasonable warm spells.
  • Limit Exposure: Reduce outdoor activity during peak mosquito hours (dusk to dawn) when possible.
  • Travel Precautions: Use prevention when traveling to endemic regions regardless of season.

By adhering to these measures, the risk of heartworm infection can be effectively minimized throughout the year, including winter months.

Expert Perspectives on Heartworm Risks for Dogs During Winter

Dr. Emily Harper (Veterinary Parasitologist, National Canine Health Institute). While heartworm transmission is significantly reduced in colder months due to mosquito inactivity, it is not entirely impossible for dogs to contract heartworm in winter, especially in milder climates where mosquitoes may remain active. Preventative measures should continue year-round to ensure comprehensive protection.

Mark Reynolds (Entomologist specializing in Vector-Borne Diseases, University of Florida). Mosquitoes, the primary vectors for heartworm, typically enter a dormant state during winter; however, in regions with warmer winters or indoor mosquito habitats, there remains a risk of transmission. Pet owners should be aware that heartworm prevention should not be neglected based solely on season.

Dr. Lisa Chen (Veterinarian and Director of Preventative Care, Midwest Animal Clinic). Although the incidence of heartworm infection decreases in winter, the lifecycle of the parasite and environmental factors mean that dogs can still be exposed. Continuous use of heartworm preventatives is critical to avoid infection, as the consequences of heartworm disease are severe and costly to treat.

Frequently Asked Questions (FAQs)

Can dogs get heartworm in the winter?
Dogs can still contract heartworm in the winter if they are exposed to infected mosquitoes. However, transmission rates are significantly lower during colder months due to reduced mosquito activity.

Are mosquitoes active enough in winter to spread heartworm?
Mosquito activity decreases substantially in cold weather, limiting heartworm transmission. In warmer climates, mosquitoes may remain active year-round, posing a continuous risk.

Should heartworm prevention continue during winter?
Yes, veterinarians recommend year-round heartworm prevention regardless of season, as some mosquitoes can survive in mild winter conditions and infected dogs can harbor heartworms acquired earlier.

How does temperature affect heartworm development in mosquitoes?
Heartworm larvae require temperatures above 57°F (14°C) to develop inside mosquitoes. Prolonged cold temperatures halt larval development, reducing transmission risk.

Can indoor dogs get heartworm in winter?
Indoor dogs are at lower risk but not completely immune to heartworm infection. Mosquitoes can enter homes, so preventive measures remain important.

What are the risks of stopping heartworm medication in winter?
Discontinuing prevention can expose dogs to heartworm infection during unexpected mosquito activity or in warmer climates, potentially leading to serious health issues.
Dogs can indeed contract heartworm during the winter months, although the risk is generally lower compared to warmer seasons. Heartworm disease is transmitted through mosquito bites, and since mosquitoes are less active in cold weather, the likelihood of transmission decreases. However, in regions with mild winters or indoor environments that maintain warmer temperatures, mosquitoes may still be present, posing a potential risk to dogs year-round.

It is important for dog owners to understand that heartworm prevention should not be discontinued solely based on seasonal changes. Consistent year-round preventive measures are recommended by veterinary professionals to ensure dogs remain protected regardless of geographic location or climate variations. Interrupting prevention can leave dogs vulnerable to infection during unexpected mosquito activity.

Ultimately, maintaining regular veterinary check-ups and adhering to prescribed heartworm prevention protocols is essential for safeguarding canine health throughout the year. Awareness of local mosquito activity and environmental factors can further inform appropriate preventive strategies, minimizing the risk of heartworm disease in dogs during winter and beyond.

Author Profile

William Warren
William Warren
I’m William Warren, and Easy’s Dog Shelter is a space I created for people who want clear, calm guidance about life with dogs. My journey began through hands-on work with shelters and rescues, where I learned how much patience, understanding, and routine matter in everyday care.

Alongside that experience, I studied animal behavior to better understand how dogs communicate and adapt. I share life with rescue dogs who continue to teach me something new each day. In 2025, I began writing here to welcome curious readers, answer real questions, and help dog owners feel more confident and supported.