Depending on where you live, you may think about fleas year-round or they may not cross your mind until you pack away your mittens as the mounds of snow start to melt.

But no matter where you are, fleas can be an issue for your pet all year long. Because even if your dog or cat doesn't have fleas living on them in the dead of winter, a flea infestation in the past might mean fleas or their eggs are hanging out in your nice warm house.

What determines flea season?

Outdoors, fleas are typically most active the warmer it gets. This map from the American Kennel Club shows peak flea activity across the United States.

flea map across the U.S. That's a lot of red: When fleas are most active throughout the U.S. (Photo: AKC)

"Fleas are activate by heat and humidity. That's why fleas are more prevalent where it's warm and/or humid like the South, East and pockets of the West and Midwest," Kwane Stewart, D.V.M. chief veterinarian officer with American Humane, tells MNN. "Where I am in California, because we had a very rainy season, we're expecting to see more fleas this year. If you're anticipating more moisture and more heat, you can expect to have more fleas."

If winter sticks around for a long time where you are, the fleas won't come calling for a while ... at least outside.

"If you have a really cold winter when you have many days in a row where you have freezing weather, that will kill fleas that aren’t on an animal," veterinarian Lori Bierbrier, D.V.M, a medical director at the ASPCA, tells MNN. "If there are fleas already on the animal, they'll survive because the animal is nice and warm."

"Warmer weather will present a busier flea season," Bierbrier says. "The fleas become more active, wanting to feed and to lay more eggs.

Although there's a lot of regional variation — like what kind of weather people had across the U.S. this winter and how rainy and warm it's been already this spring — Bierbrier has a prediction: "I have no doubt we will have lots of fleas this year."

Choosing a flea plan

dog and cat fleas Do you really want the dog flea, C. canis (left) and cat flea, C. felis (right) hanging out on your pets or in your home? (Photo: Parasite and Diseases Image Library, Australia)

Choosing the best flea plan can depend on where you live and your pet's lifestyle. If you have an indoor cat that never goes outside and is never exposed to other animals, your pet's flea exposure risk is likely very low, Bierbrier says. But if you have a dog that goes outside and spends time around other animals, he's more at risk of encountering fleas.

There are many very effective options to prevent fleas from making a home in your pet, the veterinarians say. Choose from topical treatments, chewable pills and newer generation collars that work much better than collars from years ago. Over-the-counter products like sprays and shampoos typically aren't as effective as those you buy from your vet.

Although often people in colder climates will use flea preventatives only in warmer months, you may want to consider a year-round plan.

"The advice now in really most of the U.S. is to use it year-round and not slack during the winter," Stewart says. "Recent research says to be successful you need to treat pet when fleas are dormant, dead or dying. The eggs are just waiting for the right moment to hatch and that could be the tail end of winter and beginning of spring."

If you're waiting to give your treatment and it warms up earlier than usual, the fleas can take advantage of your unprotected pet and dive on.

The same can happen if you stretch out treatment for an extra few days or few weeks, Stewart says.

"An adult flea spends its life on the pet and flea eggs roll off into the environment and wait for months sometimes to hatch and find the closest animal. If there's a slip in the coverage even for a few days, he can get bit."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.