Spatial-Temporal Analysis of Lyme Disease Risk due to Climate Change in Southern Ontario from 2001 to 2014
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The number of Lyme disease cases in Ontario has increased in the last decade and is projected to continue to increase due to climate change. The northern limit of Lyme disease cases in Ontario has also been progressing northward from the United States. It is unknown whether the increase in Lyme disease cases is due to climate change, changes in host abundance, host and vector migration, or a combination of these factors. The objective of this research is to determine if the climate has changed to increase the geographic range of areas suitable for sustaining a reproductive tick population. Monthly air surface temperature values from the National Centers for Environmental Prediction’s North American Regional Reanalysis was used to yield positive degree days above 0 degrees Celsius from 1979 to 2013 and derive warming trends in Ontario. Next, the average of monthly mean day and monthly mean night time land surface temperature from MODIS were fed in to a basic reproductive tick population model to determine areas with climate suitable for sustaining tick populations. Principal components analysis was conducted to determine changing suitability patterns. The model outputs were compared against tick surveillance data supplied by Public Health Ontario from 2002 to 2012. Areas in Ontario were identified where Lyme disease risk has become suitable for sustaining a black-legged tick population due to climate change, particularly in 2005, and 2010 - 2012. This study shows that climate has allowed black-legged tick populations to sustainably move northward in the past decade. However, the vast majority of submitted ticks were collected by health practitioners in geographic locations that were already suitable for sustaining a reproductive tick population. This suggests that most Lyme disease cases in the past decade have been due to the tick population stabilizing in areas of suitability, or increased human-tick interaction in areas already suitable, rather than progression of ticks into new geographic areas due to climate change. This analysis supports climate change models that indicate Lyme disease risk will continue to move northward and also provides high resolution Lyme disease risk maps.