As the weather warms, nature lovers will begin to head out to trails in Cook County’s forest preserves. But hikers should be aware of a dangerous pest that has made a move into the area.

The Cook County Department of Public Heath has issued a warning that ticks in the southwest suburban forest preserves have tested positive for a bacteria that causes Lyme disease.

Kitty Loewy, spokesperson for the department, said this is the first time the bacteria has been identified in the county.

“This is not a high-risk situation,” Loewy said. “We are not telling people to not go out and visit the forest preserve. We are just saying, like you would with mosquitos for West Nile season, that you should use some precautions.”

Deer ticks, which often are only the size of a head or a pin or a sesame seed, can spread the bacteria after attaching onto the skin of animals and humans.

Although Cook County was tick-free 15 years ago, these insects have slowly migrated into the area on the backs of birds and deer, said Jeff Nelson, a North Park University biology professor who conducted a tick study last year for the Cook and DuPage county heath departments.

Out of the dozens of ticks tested in sites throughout the southwest preserves, Nelson said at least six tested positive for the bacteria. He said ticks in the DuPage County preserves also tested positive.

Nelson, however, declined to disclose the exact locations where the ticks were found, because he said the county does not want to discourage visitors to the preserves.

Speaking specifically about the disease, Nelson said ticks usually need to be attached to the skin for at least 24 hours for the bacteria to spread.

Early symptoms of Lyme disease include a “bulls-eye” looking rash, fever, swollen lymph nodes, headache, chills and pain in muscles or joints.

In later stages, Lyme disease can affect the musculoskeletal system, heart and nervous system, and in rare cases can be fatal. It can be treated with antibiotics.

Although tick season lasts into the fall, May and June are the insect’s most active months, Nelson said, adding that he will continue testing clusters of them for the bacteria through the spring.

The health department recommends that visitors to the forest preserves protect themselves against tick bites by staying in the middle of trails, using insect repellant containing the chemical DEET, wearing light-colored clothing to more easily spot the dark-colored ticks and checking for ticks after leaving wooded areas.

Loewy said there were two reported cases of Lyme disease in the county last year and two in 2004.

“Usually when we see a case of Lyme disease it’s because a person has traveled out of the area, such as up to Michigan where the ticks are much more common,” she said.