SAN FRANCISCO (CBS/AP) — San Francisco Bay Area’s trails and open space hold a higher risk of tick-borne disease than previously thought.

Scientists collected 622 ticks from 20 sites in recreational areas from Sonoma County in the north to Santa Cruz County in the south and found a higher percentage of young ticks were infected with bacteria that produces Lyme disease-like symptoms, according to a new study by Stanford University researchers.

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Because of their size, the smaller ticks are less likely to be discovered when they hitch themselves onto humans.

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Lyme disease can be difficult to diagnose. Early symptoms include fever, headache, fatigue and sometimes a rash that looks like a bull’s-eye centered on the tick bite. If left untreated, the infection can cause arthritis, joint pain, immune deficiencies and a persistent cognitive fog, the study says.

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