Forest Fragmentation May Increase Lyme Disease Risk

"These results suggest that...habitat fragmentation can influence human health," say Felicia Keesing of Bard College in Annandale, New York; Brian Allan of Rutgers University in New Jersey; and Richard Ostfeld of the Institute of Ecosystem Studies in Millbrook, New York, in the February issue of Conservation Biology.

While fragments generally have fewer species than continuous habitat, some species actually do better in small patches. Notably, white-foot mice are more abundant in forest fragments in parts of the U.S., presumably because there are fewer predators and competitors left. White-footed mice are particularly abundant in fragments smaller than about five acres, and this could mean trouble for people living nearby because the mice are the main carriers of Lyme bacteria. In eastern and central North America, people catch Lyme disease via blacklegged ticks: first, larval ticks feed on infected mice, and then the infected larvae molt into nymphs that bite people.

Lyme disease is concentrated in areas where lots of people live near forest with blacklegged ticks and their hosts. Lyme disease is rising in the U.S. and is far more common than West Nile virus and other mosquito-borne diseases.

To see if forest fragmentation could increase the risk of Lyme disease, Keesing and her colleagues studied blacklegged tick nymphs in 14 forest patches ranging from 1.7 to about 18 acres in Dutchess County, New York, which recently has had the most people with Lyme disease in the U.S. The researchers determined the densities of both tick nymphs and infected tick nymphs.

The researchers found that smaller forest fragments had both more tick nymphs and more infected tick nymphs, which could mean more Lyme disease. Fragments that were smaller than three acres had an average of three times as many total nymphs than the larger fragments did (0.1 vs. 0.03 nymphs per study plot) and seven times more infected nymphs (0.07 vs. 0.01 infected nymphs per study plot). As many as 80% of the nymphs were infected in the smallest patches, the highest rate the researchers have seen.

"Our results suggest that efforts to reduce the risk of Lyme disease should be directed toward decreasing fragmentation of the deciduous forests of the northeastern United States into small patches, particularly in areas with a high incidence of Lyme disease," say Keesing and her colleagues. "The creation of forest fragments of less than five acres should especially be avoided."