Evaluating seasonal tick-borne disease risk across a latitudinal and coastal gradient

Samantha Sambado, University of California, Santa Barbara, Cheryl J. Briggs, Ecology, Evolution & Marine Biology, University of California, Santa Barbara, Santa Barbara, CA and Andrew J. MacDonald, Earth Research Institute, Bren School of Environmental Science and Management, UC Santa Barbara, Santa Barbara, CA

Background/Question/Methods
Tick-borne disease risk is variable throughout the United States yet can be predicted based on ecological factors such as temperature, relative humidity and host community structure. These abiotic and biotic factors shape tick phenology (i.e. the relative timing of the emergence of the larval and nymphal tick life stages within a year) and can influence the efficiency of pathogen maintenance in a natural system. In particular, the sequential feeding of infected nymphs on uninfected hosts followed by the feeding of uninfected larvae on the newly infected host can facilitate persistence of the pathogen from one tick generation to the next. The duration and intensity of this interaction between vectors and hosts depends greatly on extrinsic factors such as regional seasonality and land use cover. Depending on the interaction, certain modes of transmission (horizontal, vertical, or co-feeding) may be more beneficial to pathogen maintenance and pathogen diversity within a natural community. 1) We collected empirical data from 11 sites that were sampled 5 times in one season to capture variation in tick phenology and pathogen maintenance. 2) We explored the variations of tick-host interactions across a latitudinal and coastal gradient to find potential mechanism for pathogen diversity and variation in tick-borne disease incidence.
Results/Conclusions
Using statistical and spatial models, we find that tick phenology varies across a North to South gradient that creates different mechanisms for pathogen maintenance. Although this pattern has been recognized before, we noticed a unique pattern that coastal sites with high relative humidity levels serve as microhabitats that promote different tick phenologies from their respective inland sites. During the summer months, two sites that are separated by 400 latitudinal miles, have more similar tick patterns than sites that are separated by 100 coastal miles due to the effect of the summer marine layer. The coastal effect on tick populations creates temporally different disease risk within a given region that needs to be highlighted by public health officials, especially in dense coastal communities in arid southern regions. Furthermore, variations in tick phenologies along a latitudinal gradient may benefit certain transmission modes and will shape variations in pathogen diversity within a natural community. With these observed trends, we predict how pathogen maintenance and pathogen community assemblage will shift with a changing climate. This framework can be applied to other vector-borne disease systems that are part of complex host-vector interactions and are responsive to multiple environmental predictors.