Fleas, Ticks and Heartworm Are Already Emerging as Temperatures Rise Across the US

As temperatures begin to warm across much of the United States, veterinarians say fleas, ticks and mosquitoes that spread disease to pets are already becoming active.

Veterinary experts at Thrive Pet Healthcare are reminding pet owners that year-round parasite prevention is essential to protect dogs and cats from infections that can cause serious illness.

“Parasites are increasingly becoming a year-round issue,” said Dr. Gerard Ferrick, medical director at Thrive Pet Healthcare in Natick, Massachusetts. “Warmer temperatures are extending parasite activity and expanding where they can live. That’s why pets should remain on veterinarian-approved flea, tick and heartworm prevention throughout the year. Consistent protection helps reduce the risk to both pets and people.”

Veterinarians across Thrive Pet Healthcare’s national network say parasite activity varies by region but remains a concern in every part of the country.

In the Upper Midwest, veterinarians say ticks can appear earlier than many pet owners expect.

“Ticks can become active whenever temperatures rise above about 35°F, so early spring in the Chicago area is often when we start seeing them on pets,” said Dr. Karen Woodard, medical director at Thrive Pet Healthcare in Elmhurst, Illinois. “As the weather warms, flea activity increases as well. Because fleas and ticks can spread diseases that affect both pets and people, consistent parasite prevention is important throughout the year.”

In the Northeast, tick-borne diseases such as Lyme disease remain a persistent concern.

“In the Northeast, ticks remain a significant concern. Approximately 8-12% of dogs in our region test positive for tick-borne disease during routine screening,” said Dr. Ferrick. “Ticks can be active year-round depending on temperature fluctuations, and thrive in wooded and grassy areas. Pets that are not on a preventive regimen can be exposed quickly.”

In the South, veterinarians frequently diagnose heartworm disease, which is spread by mosquitoes.

“In Texas, heartworms are a year-round risk because mosquitoes don’t take a season off,” said Dr. Karri McCreary, medical director at Thrive Pet Healthcare in McKinney, Texas. “Keeping pets on monthly prevention and testing annually is the most effective way to protect them from this dangerous disease.”

Even in regions where parasite risk may seem lower, veterinarians say pets can still be exposed.

“In Colorado, we often have crisp mornings or patches of snow on the ground well into spring, giving pet owners the impression that parasites aren’t active yet,” said Dr. Emily Bégin, medical director at Hampden Family Pet Hospital in Englewood, Colorado, a Thrive Pet Healthcare partner. “But warmer afternoons and temperatures above 50°F can trigger mosquitoes and other parasites to become active, making year-round preventive care important. Parasites don’t wait for warm weather, and they certainly don’t disappear just because there’s snow.”

Veterinarians also caution that parasites can affect pets that rarely go outside.

“Parasites can affect cats even if they spend most of their time indoors, making year-round protection that much more important,” said Dr. Jennifer Taketani, medical director at Corona Del Mar Animal Hospital, a Thrive Pet Healthcare partner, in Corona Del Mar, California. “Fleas and other parasites can enter homes on clothing, other pets, or through small openings around doors and windows. Consistent prevention is far safer and more economical than treating infestations or infections after they occur.”

Parasites that veterinarians are already seeing this spring

Across Thrive Pet Healthcare hospitals, veterinarians report early cases, including:

• Dogs exposed to ticks during outdoor walks
• Flea infestations affecting both dogs and indoor cats
• Heartworm infections detected during routine testing

Veterinarians say these cases highlight the importance of routine wellness visits and parasite prevention.

“Many parasitic infections develop quietly before pets show any symptoms,” said Dr. Bégin. “Conditions like heartworm disease, intestinal parasites, and tick‑borne illnesses can cause serious damage before they’re detected. With preventive care and routine screening, we can interrupt those disease cycles early, protect vital organ systems, and avoid the serious and sometimes irreversible health complications that parasites can cause.”