Projected climate and land use change alter western blacklegged tick phenology, seasonal host‐seeking suitability and human encounter risk in California

Abstract

Global environmental change is having profound effects on the ecology of infectious disease systems, which are widely anticipated to become more pronounced under future climate and land use change. Arthropod vectors of disease are particularly sensitive to changes in abiotic conditions such as temperature and moisture availability. Recent research has focused on shifting environmental suitability for, and geographic distribution of, vector species under projected climate change scenarios. However, shifts in seasonal activity patterns, or phenology, may also have dramatic consequences for human exposure risk, local vector abundance and pathogen transmission dynamics. Moreover, changes in land use are likely to alter human–vector contact rates in ways that models of changing climate suitability are unlikely to capture. Here we used climate and land use projections for California coupled with seasonal species distribution models to explore the response of the western blacklegged tick (Ixodes pacificus), the primary Lyme disease vector in western North America, to projected climate and land use change. Specifically, we investigated how environmental suitability for tick host‐seeking changes seasonally, how the magnitude and direction of changing seasonal suitability differs regionally across California, and how land use change shifts human tick‐encounter risk across the state. We found vector responses to changing climate and land use vary regionally within California under different future scenarios. Under a hotter, drier scenario and more extreme land use change, the duration and extent of seasonal host‐seeking activity increases in northern California, but declines in the south. In contrast, under a hotter, wetter scenario seasonal host‐seeking declines in northern California, but increases in the south. Notably, regardless of future scenario, projected increases in developed land adjacent to current human population centers substantially increase potential human–vector encounter risk across the state. These results highlight regional variability and potential nonlinearity in the response of disease vectors to environmental change.

Impact of prior and projected climate change on US Lyme disease incidence

“…That is, we assume that the relationship between cumulative temperature, for example, and Lyme disease incidence in a given region will remain the same even as cumulative temperatures exceed prior values. This could generate inaccurate projections for regions near current tick upper thermal limits such as the Southeast and Southwest as further warming and drought here may reduce tick survival and host‐seeking suitability (Berger et al., 2014; MacDonald et al., 2020; Randolph & Storey, 1999; Schulze et al., 2001; Vail & Smith, 1998). Generating more accurate projections for these regions would require experiments investigating effects of future temperatures on aspects of tick‐borne disease transmission.…”

“…As portions of the Northeast and by 2090-2100 (Table 4) These results indicate that climate change will likely contribute to increasing Lyme disease incidence in the Northeast, but the specific numerical projections should be interpreted with caution. While significant increases were projected in the Northeast, many other factors contribute to Lyme disease transmission including host movement and community composition, and human avoidance behaviors (Berry et al, 2018;Brinkerhoff et al, 2011;Brownstein, Skelly, et al, 2005;Larsen et al, 2014;Ogden et al, 2008;Ostfeld, 1997 here may reduce tick survival and host-seeking suitability (Berger et al, 2014;MacDonald et al, 2020;Randolph & Storey, 1999;Schulze et al, 2001;Vail & Smith, 1998). Generating more accurate projections for these regions would require experiments investigating effects of future temperatures on aspects of tickborne disease transmission.…”

“…Accordingly, we found that unobserved county-level heterogeneity, which would encompass these factors, was a predominant driver of incidence in each of our regional models. (Berger et al, 2014;MacDonald et al, 2020;Randolph & Storey, 1999;Schulze et al, 2001;Vail & Smith, 1998). Generating more accurate projections for these regions would require experiments investigating effects of future temperatures on aspects of tickborne disease transmission.…”

Toward climate change refugia conservation at an ecoregion scale

“…This work reviews a pathway toward climate change adaptation planning in a region currently facing tangible threats from climate change. Future climate change refugia conservation in the Sierra Nevada might incorporate regional landscape connectivity work (Buttrick et al, 2015; McRae et al, 2016), considerations for increased human‐wildlife conflict and zoonotic disease (e.g., Hammond, Liebman, Payne, Pigage, & Padgett, 2020; MacDonald, McComb, O'Neill, Padgett, & Larsen, 2020), additional existing climate change refugia conservation approaches in the region (Buhler et al, 2019), and much more. Placing priorities and resources into an actionable framework provides ideas for near‐term application, and can stimulate additional collaboration to meet the challenges of climate change adaptation in the Sierra Nevada ecoregion.…”

Impact of prior and projected climate change on US Lyme disease incidence

“…That is, we assume that the relationship between cumulative temperature, for example, and Lyme disease incidence in a given region will remain the same even as cumulative temperatures exceed prior values. This could generate inaccurate projections for regions near current tick upper thermal limits such as the Southeast and Southwest as further warming and drought here may reduce tick survival and host-seeking suitability (Vail and Smith 1998, Randolph and Storey 1999, Schulze et al 2001, Berger et al 2014, MacDonald et al 2020. Generating more accurate projections for these regions would require experiments investigating effects of future temperatures on aspects of tick-borne disease transmission.…”