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Last updated January 1, 2014

Suburban Sprawl Could Increase Lyme Disease Infections

Myrna E. Watanabe, The Scientist, 17(5):31, March 10, 2003

Ixodes tick, courtesy of Scott Bauer
Ixodes tick. Photo by Scott Bauer
A recent study suggests that suburban sprawl leading to forest fragmentation could also increase the prevalence of ixodid ticks infected with Borrelia burgdorferi, the spirochaete bacterium that causes Lyme disease (K. LoGiudice et al., "The ecology of infectious disease: Effects of host diversity and community composition on Lyme disease risk," Proc Nat Acad Sci, 100:567-71, Jan. 21, 2003). Ixodid ticks become infected as nymphs by taking a blood meal from an infected animal, especially from the white-footed mouse, Peromyscus leucopus.

To devise a model for the presence of Borrelia-infected ticks, the researchers sampled bird and mammalian hosts, counting how many blood-engorged tick larvae fell off each animal. They then determined how many larvae were infected.

Using these and other data, researchers constructed a mathematical model of the effects of biodiversity on tick infestations. The model suggests that in habitats where there are more potential species of tick hosts, tick larvae are less likely to become infected because some of these hosts have low-reservoir competence; that is, even if bitten by an infected tick, the species either is unlikely to become infected or unable to pass on the infection.

"Forest fragmentation," the authors noted, "decreases mammalian biodiversity and results in areas of very high mouse density." Kathleen LoGiudice of Union College in Schenectady, NY, says "sprawling development ... may be increasing our exposure to Lyme and similar vector-borne diseases. Biodiversity can have some very real implications for our health and quality of life."

The ecology of infectious disease: effects of host diversity and community composition on Lyme disease risk.
LoGiudice K, Ostfeld RS, Schmidt KA, Keesing F. Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA.

The extent to which the biodiversity and community composition of ecosystems affect their functions is an issue that grows ever more compelling as human impacts on ecosystems increase. We present evidence that supports a novel function of vertebrate biodiversity, the buffering of human risk of exposure to Lyme-disease-bearing ticks.

We tested the Dilution Effect model, which predicts that high species diversity in the community of tick hosts reduces vector infection prevalence by diluting the effects of the most competent disease reservoir, the ubiquitous white-footed mouse (Peromyscus leucopus). As habitats are degraded by fragmentation or other anthropogenic forces, some members of the host community disappear.

Thus, species-poor communities tend to have mice, but few other hosts, whereas species-rich communities have mice, plus many other potential hosts. We demonstrate that the most common nonmouse hosts are relatively poor reservoirs for the Lyme spirochete and should reduce the prevalence of the disease by feeding, but rarely infecting, ticks.

By accounting for nearly every host species' contribution to the number of larval ticks fed and infected, we show that as new host species are added to a depauperate community, the nymphal infection prevalence, a key risk factor, declines.

We identify important "dilution hosts" (e.g., squirrels), characterized by high tick burdens, low reservoir competence, and high population density, as well as "rescue hosts" (e.g., shrews), which are capable of maintaining high disease risk when mouse density is low. Our study suggests that the preservation of vertebrate biodiversity and community composition can reduce the incidence of Lyme disease.

Full text: Proc Natl Acad Sci U S A. 2003 Jan 21;100(2):567-71. Epub 2003 Jan 13




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