How did mammal host diversity relate to Lyme disease incidence across US states?

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Since its discovery in 1975, Lyme disease has spread and increased across much of the central and eastern United States. Host diversity is thought to influence Lyme disease risk, and the direction of that relationship may vary with the spatial scale of observation.

Researchers modelled the effect of mammal host species richness on Lyme disease incidence from 1992 to 2011 across all US states with reported or established black-legged tick (Ixodes scapularis) populations. They tested two contrasting hypotheses: that host species richness had a positive relationship with Lyme disease incidence, or that it had a negative relationship. They also tested whether the strength of the diversity–disease-risk relationship increased over time as Lyme disease spread.

They observed a strong negative relationship between mammal host species richness and Lyme disease incidence, which became more negative over time. Lyme disease increased more rapidly over time in states with fewer host species than in states with more host species. The findings support the importance of mammal host richness for Lyme disease risk at large spatial scales, and of both spatial and temporal scales for the diversity–disease relationship.