Tick-borne diseases in Minnesota: Increased incidence and projected change in geographic distribution

Background/Question/Methods

As humans and climate change alter the landscape, novel disease risk scenarios emerge.  Understanding the complexities of pathogen emergence and subsequent spread as shaped by landscape heterogeneity is crucial to understanding localized risks of emerging diseases, and mitigating those risks in a dynamic environment.  Vector-borne diseases have especially strong links to environmental conditions such as temperature and suitable vector habitat.  In recent years, the incidence of tick-vectored diseases, such as Lyme disease (LD), anaplasmosis, and babesiosis have all increased substantially in Minnesota.  We used human disease case data from 1996 to 2011 to determine how landscape and climatic factors modulated the spread of LD across Minnesota.  We used proportional hazard models to evaluate the relative impact of landcover types, climate variables and location factors on the risk of LD.  We mapped transects in N, NW, W, SW, and S directions from the initial detection point of LD in eastern Minnesota.  We used a spatial simulation model to determine whether the directionality of LD spread coincided with patterns of suitable tick habitat or other environmental factors.  Analyses were repeated at multiple scales to assess sensitivity to spatial error in disease case reporting.   

Results/Conclusions

Based on proportional hazard models, infection rates were strongly associated with forest cover and decreased in agricultural areas where habitat was unsuitable for ticks.  Rising temperatures in Minnesota have resulted in an increasing frequency of days above the threshold necessary for Ixodes scapularis (blacklegged tick) survival (degree days >3000).  Ixodes scapularis have spread to northern MN forests coinciding with increases in LD and other tick-borne diseases.  Spread of LD from a point of first detection in eastern Minnesota exceeded 10 km/yr in the first 10 years, but this rate decreased significantly in the second 10 years as a result of limits imposed by habitat and temperature.  Through risk mapping and spatial simulations we demonstrated that LD will likely increase most in northwest Minnesota where suitable tick habitat is plentiful and climate warming allows increased blacklegged tick survival.  Our research demonstrates the impact of climate change on emerging diseases with serious public health implications.  The spatial risk assessment provided here can be useful in public health planning and informative to disease risk mitigation efforts.