Mapping the distribution of Amblyomma americanum in Georgia, USA

Abstract

Background: Amblyomma americanum, the lone star tick, is an aggressive questing species that harbors several pathogens dangerous to humans in the United States. The Southeast in particular has large numbers of this tick due to the combined suitable climate and habitats throughout the region. No studies have estimated the underlying distribution of the lone star tick across the state of Georgia, a state where it is the dominant species encountered.

Methods: Ticks were collected by flagging 198 transects of 750 m² at 43 state parks and wildlife management areas across the state from March to July of 2022. A suite of climate, landscape, and wildlife variables were assembled, and a logistic regression model was used to assess the association between these environmental factors and the presence of lone star ticks and to predict the distribution of these ticks across the state.

Results: A total of 59/198 (30%) transects sampled contained adult or nymph A. americanum, with the majority of transects containing these ticks (54/59, 91.5%) in forested habitats. The presence of A. americanum was associated with elevation, normalized difference vegetation index (NDVI) on January 1, isothermality, temperature seasonality, and precipitation in the wettest quarter. Vast regions of central, eastern, and southern coastal Georgia (57% of the state) were categorized as suitable habitat for the lone star tick.

Conclusions: This study describes the distribution of the lone star tick across the state of Georgia at a finer scale than the current county-level information available. It identifies specific variables associated with tick presence and provides a map that can be used to target areas for tick prevention messaging and awareness.

Keywords: Amblyomma americanum; Georgia USA; Predictive vector map; Species distribution; Tick.

Fig. 1

Density of A. americanum at each site per 100 m² sampled. The total number of A. americanum collected at each site was divided by the total transected distance and multiplied by 100 to display the density of adult and nymph A. americanum. Larger white circles represent sites with higher density of lone star ticks

Fig. 2

Bar graph of the percentage of transects where A. americanum was found, split by habitat in each ecoregion (BR Blue Ridge, RV Ridge and Valley, P Piedmont, SP Southeastern Plains, SC Southern Coastal Plain). In total, 0/20 (0%) forested transects contained lone star ticks in Blue Ridge, 31/51 (60.8%) in Piedmont, 1/17 (5.9%) in Ridge and Valley, 12/24 (50%) in the Southern Coastal Plain, and 10/28 (35.7%) in the Southeastern Plains. For grassland/other transects, 0/2 (0%) had lone star ticks in Blue Ridge, 2/14 (14.3%) in Piedmont, 0/8 (0%) in Ridge and Valley, 0/12 (0%) in the Southern Coastal Plain, and 3/22 (13.6%) in the Southeastern Plains

Fig. 3

Probability of A. americanum occurrence across Georgia, estimated from the best-fit logistic regression model of tick presence. Areas that are yellow to red indicate areas with a higher probability of finding questing lone star ticks. White reflects missing data

Fig. 4

Estimated ecological niche of A. americanum in Georgia using the optimal probability cut point maximizing sensitivity and specificity (0.2508). Red areas indicate locations that would have suitable habitat for A. americanum based on the final model