Lyme disease incidence is rising in the United States and is in fact far more common than West Nile virus and other insect-borne diseases.  Forest fragmentation could explain the increase.

Credit: Nicolle Rager, National Science Foundation

Areas of patchy woods, which are very common in cities and suburban and rural areas, may have higher populations of Lyme-disease carrying ticks than forest fragments, which generally have fewer species than continuous habitat. This is because some species thrive in smaller places. 

White-footed mice, for example, are more abundant in forest fragments in some parts of the country, likely because fewer predators and competitors remain there. These mice are particularly abundant in patches smaller than about five acres, which could spell trouble for people living nearby: the mice are the main carriers of Lyme disease-causing bacteria. In the Eastern and Central United States, Lyme disease is contracted via blacklegged ticks that feed on infected mice, and then transmit the bacteria when the ticks bite people. As a result, says biologist Felicia Keesing of Bard College in Annandale, New York, Lyme disease is concentrated in areas where people live near forests with blacklegged ticks.

Keesing and other scientists found that smaller forest fragments had more infected ticks, which could translate to more Lyme disease. Forest patches that were smaller than three acres had an average of three times as many ticks as did larger fragments, and seven times more infected ticks. As many as 80 percent of the ticks in the smallest patches were infected, the highest rate the scientists have seen.

"Our results suggest that efforts to reduce the risk of Lyme disease should be directed toward decreasing fragmentation of deciduous forests of the Northeastern United States, particularly in areas with a high incidence of Lyme disease," said Keesing. "The creation of forest fragments smaller than five acres should especially be avoided."

For more information, visit the Center for Disease Control's Web page on Lyme Disease http://www.cdc.gov/ncidod/dvbid/lyme/