When warm weather comes, encounters with biting insects increase as we head outdoors in increasing numbers. Ticks in particular are a concern in Oregon, because some carry Borrelia burgdorferi, the bacteria that causes Lyme disease.

Lyme disease is not a common infection in this area (there were 145 cases reported in Oregon between 1995 and 2003), but if not treated promptly, can have serious long-term health consequences. Some people believe that Lyme disease is vastly under-recognized and under-reported, while others believe it is overdiagnosed. It's sometimes called "the great imitator" because its symptoms mimic those of other diseases.

The western black-legged tick is the only species of tick in the western U.S. known to transmit Lyme disease. Oregon State University Extension Service scientists believe less than 5 percent of these ticks carry Borrelia burgdorferi.

(In other parts of the country, two other species of ticks also can transmit Lyme disease: the deer tick in the eastern part of the country and the eastern black-legged tick in the southeast.)

Immature western black-legged ticks usually feed on the blood of small animals, such as mice and rats, which can be infected with Lyme disease. Adult western black-legged ticks tend to feed on larger animals, such as deer, dogs and even humans.

In Oregon, these ticks are most active from spring through the early fall. When they need a meal of blood, they sit on bushes, shrubs, trees or tall grass and wait for an animal or person to pass by, then attach themselves.

Western black-legged ticks have black heads and legs. The rest of the body is a reddish color. Females are 4-5 mm long and males are 3-4 mm long. (There are 25.4 millimeters in an inch.) When ticks are engorged with blood, they swell up to a larger size.

To transmit the disease, a tick must be attached for at least 24-48 hours.

Lyme disease can be difficult to diagnose, because the signs and symptoms are not the same for every person.

In about 75 percent of cases, a painless, bull's-eye rash - a red ring with a clear center - appears at the site of the tick bite. This can happen within days of the bite, or weeks later. The rash may expand over several days, and can reach 12 inches across.

Also, soon after exposure to Lyme disease, flu-like symptoms can occur, with fever, chills, headache, body aches and fatigue.

The most worrisome symptoms of Lyme disease are neurological in nature, which can begin weeks, months or years after the initial infection. These problems can include: meningitis; numbness, weakness and movement problems of the arms and legs; paralysis of one side of the face; memory loss; mood changes; sleep difficulties and difficulty concentrating.

Other symptoms can include chronic inflammation of a knee or other joint called Lyme arthritis, irregular heart rhythms, and inflammation of the eye.

The standard treatment for a recent infection with Lyme disease is a 14- to 21-day course of oral antibiotics. For disease that has progressed beyond the early stages of infection, intravenous antibiotics are used.

The best way to avoid Lyme disease is to avoid tick bites, by covering your skin when you are outdoors in grassy or forested areas. Insect repellent on your skin and clothing can repel ticks. Most importantly, check yourself carefully for ticks after spending time in grassy or wooded areas, and remove them promptly.

If you find an attached tick, the best way to remove it is with tweezers. Grasp the head of the tick as close to the skin as possible and pull straight out. Wash thoroughly with soap and water. It's a good idea to place the tick in a container with rubbing alcohol, and label it with the date and place the tick came from, if known. This way, the tick can be identified later if needed.

Don't delay in seeking medical care if you develop symptoms after a tick bite, as prompt diagnosis and treatment of Lyme disease is important. There are several other infections carried by ticks, including relapsing fever, Colorado tick fever, Rocky Mountain spotted fever and tick paralysis, but these are unusual in the Pacific Northwest.

Kathryn B. Brown worked as a registered nurse and a nurse practitioner before coming to work for the East Oregonian. Her column appears here every other week. She can be reached at kbbrown@eastoregonian.com.

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