TICK SEASON is between May and September, right? That has been the conventional wisdom - until recently.

Two recently published studies found that deer, or black-legged, ticks - the ones that spread Lyme disease - are emerging earlier and expanding their range, thanks to warmer spring temperatures and milder fall weather. Based on data collected over 19 years, researchers concluded that milder weather allowed nymphs - immature ticks - to feed as much as three weeks earlier in spring. The change in activity may presage a spread of tick-borne diseases, one that's already being seen in some areas.

Tick populations have moved northward into Canada, making Lyme disease endemic in southern Ontario, says Susan E. Little, DVM, who teaches veterinary parasitology at Oklahoma State University and is president of the Companion Animal Parasite Council. Ticks also have moved up in altitude and are now found at higher elevations in Appalachia than in the past.

"It used to be that ticks weren't at 3,000 or 4,000 feet, and now they are," Dr. Little says. "That's a change that we think is due to warming trends."

Ticks are also moving farther south. From its origins in the Northeast, Lyme disease is now established down the Virginia coast and into North Carolina. In the Midwest, ticks are moving southward across Iowa, through the northern half of Illinois and most of Indiana, and into the lower peninsula of Michigan. One or more species of tick can now be found in every state, including Alaska.

Climate isn't the only culprit. Ticks piggyback on wildlife, such as white-tailed deer and coyotes, which spread them to new habitats; they can also be carried away by migratory birds. Habitat modification, such as increasing development in formerly rural areas, also contributes to the expansion of tick territory.

Even if ticks aren't active all 365 days of the year, they are active every month of the year. There will always be a few days warm enough for them to make an appearance.

A tick begins the disease transmission cycle when it inserts its sharp mouthpiece into a rodent or deer and feeds on its blood. In the process, the tick takes in bacteria, protozoa and viruses that it later passes on to cats, dogs and humans.

Yes, cats, too. They don't appear to get Lyme disease - yet - but ticks can transmit a deadly disease called cytauxzoonosis to cats, as well as ehrlichia and anaplasmosis.

What can you do to ward off the beastly bloodsuckers?

*  Start thinking about tick prevention before May.

* Talk to your veterinarian about the best tick preventive measures for your pet's lifestyle, as well as about the species of ticks found in your area. Products include oral preventives that kill both ticks and fleas on dogs, and long-lasting anti-tick collars for use on dogs and cats.

*  Keep grass short and foliage trimmed back to reduce tick habitat.

*  Discourage deer from grazing near your yard.

*  Indoor pets can be at risk, too. You can bring ticks in on your clothing or body - it's happened at my house - and the brown dog tick (Rhipicephalus sanguineus) prefers to live indoors.

Pets don't directly spread tick-borne diseases, but they can bring you into contact with ticks - and vice versa. If your cat goes outdoors or your dog hikes or hunts with you on a regular basis, tick prevention can help protect all of you from disease.

Pet Connection is produced by a team of pet-care experts headed by "The Dr. Oz Show" veterinarian Dr. Marty Becker and award-winning journalist Kim Campbell Thornton.