Implications of climate change on the distribution of the tick vector Ixodes scapularis and risk for Lyme disease in the Texas-Mexico transboundary region

Abstract

Background: Disease risk maps are important tools that help ascertain the likelihood of exposure to specific infectious agents. Understanding how climate change may affect the suitability of habitats for ticks will improve the accuracy of risk maps of tick-borne pathogen transmission in humans and domestic animal populations. Lyme disease (LD) is the most prevalent arthropod borne disease in the US and Europe. The bacterium Borrelia burgdorferi causes LD and it is transmitted to humans and other mammalian hosts through the bite of infected Ixodes ticks. LD risk maps in the transboundary region between the U.S. and Mexico are lacking. Moreover, none of the published studies that evaluated the effect of climate change in the spatial and temporal distribution of I. scapularis have focused on this region.

Methods: The area of study included Texas and a portion of northeast Mexico. This area is referred herein as the Texas-Mexico transboundary region. Tick samples were obtained from various vertebrate hosts in the region under study. Ticks identified as I. scapularis were processed to obtain DNA and to determine if they were infected with B. burgdorferi using PCR. A maximum entropy approach (MAXENT) was used to forecast the present and future (2050) distribution of B. burgdorferi-infected I. scapularis in the Texas-Mexico transboundary region by correlating geographic data with climatic variables.

Results: Of the 1235 tick samples collected, 109 were identified as I. scapularis. Infection with B. burgdorferi was detected in 45% of the I. scapularis ticks collected. The model presented here indicates a wide distribution for I. scapularis, with higher probability of occurrence along the Gulf of Mexico coast. Results of the modeling approach applied predict that habitat suitable for the distribution of I. scapularis in the Texas-Mexico transboundary region will remain relatively stable until 2050.

Conclusions: The Texas-Mexico transboundary region appears to be part of a continuum in the pathogenic landscape of LD. Forecasting based on climate trends provides a tool to adapt strategies in the near future to mitigate the impact of LD related to its distribution and risk for transmission to human populations in the Mexico-US transboundary region.

Figure 1

Geographic area of study. Each county within Texas, Tamaulipas, Nuevo León and Coahuila [72] from which we obtained Ixodes tick samples has been highlighted in gray. The locations of the positive samples for B. burgdorferi are marked with a target sign.

Figure 2

Geographic distribution of I. scapularis by biogeographic regions (yellow shadow) and states in the US and Mexico. Each locality represent a location in which I. scapularis was detected in this study as well as those previously published [56]. Localities are represented as black dots.

Figure 3

Borrelia burgdorferi strains detected in Texas. For constructing the dendrogram the IGR sequences were analyzed using MEGA 5.2 (Molecular Evolutionary Genetics Analysis, http://www.megasoftware.net/). A phylogenetic reconstruction analysis was obtained through maximum likelihood using the Tamura–Nei nucleotide substitution model.

Figure 4

Present suitable habitat for Ixodes scapularis obtained with a maximum-entropy approach using the localities recorded of infected and non-infected I. scapulairs ticks (black dots) considering 17 climatic variables (temperature-precipitation). Red = high suitable habitat vs. blue = no suitable habitat for I. scapularis.

Figure 5

Future (year 2050) suitable habitat for Ixodes scapularis obtained with a maximum-entropy approach. (A-F) potential distributions for I. scapularis. Black dots = geographic locations of infected and non-infected ticks. Red = high suitable habitat vs. blue = no suitable habitat for I. scapularis. General circulatory models and climatic scenarios: (A) CCCMA-A2A; (B) CCCMA-BA2; (C) CSIRO-A2A; (D) CSIRO-BA2; (E) HADCM3-A2A; (F) HADCM3-B2A. Applied to general circulatory models and climatic scenarios.

Figure 6

Stable area for the distribution of I. scapularis in the geographic area of study. Highlighted is the area in the Texas-Mexico region that will remain suitable for the maintenance of Ixodes scapularis populations present and future (2050) regardless of the prediction made (IPCC scenario and GCM). The total area is estimated in 569,910 Km². Black dots denote the localities we recorded for infected and non-infected I. scapularis ticks.