Effect of multi-year seasonal weather factors on Lyme disease incidence in New York State, 1991-2006

The influence of weather patterns on Lyme disease (LD) has not been studied extensively. We assessed the effect of two-year seasonal weather variables on incidence of LD in New York State (NYS) using an ecological study. We obtained LD incidence data (1991-2006) from the NYS Department of Health Communicable Disease Electronic Surveillance System. Weather data, including daily minimum temperature, precipitation, snowfall and snow-depth, were obtained from the US Historical Climatology Network and drought data (Palmer Hydrologic Drought Index (PHDI)) from the National Climatic Data Center. Climate thresholds based on importance for pathogen transmission as well as literature review were used to develop weather indicators. Cases were geocoded and assigned seasonal weather indicators at the climate division level. We calculated rate ratios (RR) and 95% confidence intervals for two-year seasonal weather and summer and fall LD association using Poisson Regression. There were 64770 LD cases that were geocoded and included in the analyses. Most cases occurred during summer with a secondary peak in the fall. Rate of summer and fall LD increased with same (RR 1.08 (95%CI 1.04, 1.12)) and prior (RR1.09 (95%CI 1.05,1.12)) year spring temperatures within optimum tick activity thresholds (Tmin: 40-45F), previous mild winter (Snow-depth<12 and Tmin>32F)(1.03(95%CI 1.01,1.06)), and prolonged spring (Tmin: 45-50F) in previous year (RR1.07 (95%CI 1.03,1.11)). High PHDI during the same (RR0.89 (95%CI 0.83, 0.95)) and previous (RR 0.87 (95%CI 0.81, 0.93)) summer was associated with a decreased risk of LD. Milder winters and early spring development may serve as indicators of increased LD incidence in NYS.