Geographic distributions of medically important ticks and tick-borne diseases in North America in changing climates

Recently, the geographic distributions of ticks and tick-borne diseases have come under public health concern in light of broad geographic expansions of species’ ranges and dramatic increases in human cases of tick-borne diseases. Here, I took advantage of the data available from online biodiversity data portals, published papers, and human case records from the CDC; using ecological niche modeling approaches, I was able to assess and estimate potential geographic distributions of species and highlight future risk areas related to several medically important tick species in face of climate change. I also compared niches of tick vectors and associated pathogens using new statistical method protocols. I obtained environmental variables from various open-sources data archives such as WorldClim for current climates, numerous general circulation models (GCMs) under two representative concentration pathway scenarios from Climate Change Agriculture and Food Security (CCAFS) for future climates, and PRISM for comparisons within the United States. I used ecological niche modeling routines implemented in Maxent, including model calibration, evaluation, and selection based on statistical comparisons against hypotheses of null predictive ability, predictive ability in terms of omission rates, and minimal model complexity via the kuenm R package. For comparisons of ecological niches, I used two statistical analyses, univariate non parametric analysis, and permutational multivariate analysis (PERMANOVA) of variance to delineate niches of two tick vectors and their associated pathogens. Chapter 1. This study investigates the most widespread tick species Rhipicephalus sanguineus sensu lato in the world. I used environmental variables under present and future conditions using 11 different GCMs under two RCPs. I calibrated models for five different regions (North America, Brazil, Europe and North Africa, Pakistan, and Indonesia and Australia. I used similarity analysis to detect similar environmental conditions among those different regions. I also used ecological niche modeling to characterize environmental requirements and potential geographic distribution of R. sanguineus sensu lato under present and future conditions in different regions. Perhaps most importantly, I also assessed uncertainty associated with different model projections in detail. I presented this analysis as both a first global assessment of the species’ distributional potential, and an illustration of how best to assess and understand uncertainty in highly dimensional model outputs. The model results showed high agreement of suitable areas among model predictions from the eastern United States, southern Mexico, South America, Europe, North Africa, sub-Sharan countries, Asia, and Australia. Note: this chapter has been published in Perspective in Ecology and Conservation, in 2020. Chapter 2. This study investigates the impact of climate change on the geographic potential distribution of Powassan virus tick vector Ixodes cookei. This tick species is known to be found in eastern United States, but recently has been found in western Canada (British Colombia). I used ecological niche modeling to link occurrence points to climate variables to assess the potential geographic distribution under current and future climate change using two strategies: First, without including occurrence points from western Canada. Second, I included those occurrence points from western Canada. My models identified western Canada (British Columbia) as suitable for I. cookei even when we did not include those occurrence points; suitable areas were identified in the eastern United States from Tennessee and North Carolina north to southern Canada. We also compared POWV case data that we obtained from CDC (with special request) on our model outputs, and found out that some of those cases overlapped with suitability areas in eastern United States (New England states). However, some of those cases fell outside the predicted suitable areas in Michigan and Wisconsin, which opens questions about the vector species and type of strain that could be different from the one that is transmitted by I. cookei. Note: this chapter has been published in Journal of Vector Ecology, in 2021. Chapter 3. I used a comprehensive occurrence point data to estimate the potential geographic distribution of the two important tick vectors Amblyomma maculatum and Dermancentor andersoni that are responsible for transmitting bacteria that causes spotted fever. I used ecological niche modeling to assess the geographic potential of these two medically important vectors in North America under present conditions and then transfer models to the future under different scenarios from five different GCMs, and two RCPs. Areas across the southern and midwestern United States and eastern, western, and southern Mexico were suitable for A. maculatum. For D. andersoni, our models showed broad suitable areas across northwestern United States. Range expansions was anticipated for both tick species northward in response to climate change, extending across the Midwest and New England for A. maculatum, and still farther north into Canada for D. andersoni. Interestingly, our model for A. maculatum predicted the existence of suitable areas that were recently revealed to hold new populations in Illinois, and Connecticut. Note: this chapter has been published in PeerJ, in 2022. Chapter 4. Here, I developed a review paper providing an overview of climate change influences on eight medically important ticks (Ixodes scapularis, I. pacificus, I. cookei, Dermacentor variabilis, D. andersoni, Amblyomma americanum, A. maculatum, and Rhipicephalus sanguineus) in North America, and included tick-borne disease (TBDs) cases from United States, to make this body of new information available to the public health community. The Centers of Disease Control and Prevention (CDC) provided detailed human case data for several important TBDs (Lyme disease, spotted fever rickettsiosis, babesiosis, anaplasmosis, tularemia, and Powassan virus). I explored how those tick species will likely shift their geographic distributions in coming decades in response to climate warming, and we interpret TBDs patterns in terms of likely implications for tick-associated diseases in North America. Note: this chapter has been published in Insects, in 2021. Chapter 5. Finally, in this chapter, I took advantage of a novel data set on two tick species (Amblyomma americanum, Ixodes scapularis) and their associated pathogens from the National Ecological Observatory Network, using univariate nonparametric tests and multivariate permutation analyses to compare niches of pathogens with those of the vectors that carry them. I used different sets of climate data from the PRISM climate archive (maximum and minimum temperature, mean temperature, and maximum and minimum vapor pressure deficit). After careful quality control of the data, we had 71,113 and 16,800 individual counts of A. americanum, and I. scapularis, which were tested for 12 and 13 pathogens, respectively; sample sizes were sufficient for 4 and 6 pathogens, respectively, for the tests that I developed. My results showed a diversity of outcomes: for instance, the niches of the tick A. americanum and the pathogen Borrelia lonestari could not be distinguished environmentally. In contrast, the niches of the pathogens Babesia microti and Borrelia burgdorferi sensu lato contrasted with that of the Ixodes scapularis vector tick.

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