An ecological study of the effect of white-tailed deer on alpha-gal syndrome in United States counties

Background: Alpha-gal syndrome (AGS) is an emerging disease, increasingly recognized as a public health concern in the United States. The primary cause of AGS in the United States is the bite of Amblyomma americanum ticks. White-tailed deer serve both as a preferred food source and as transport for A. americanum. In this work, we aim to quantify the effect of white-tailed deer abundance on number of AGS cases in United States counties. Methods: To mitigate concerns about confounding, we used the front-door formula, leveraging biological knowledge about the causal process. Due to lack of official data, our analysis relied on data made available by citizen science efforts. Results: We found that a higher number of reported white-tailed deer sightings in 2020 was associated with the county-level presence of A. americanum in 2024. In turn, county-level presence of A. americanum was associated with a higher number of self-reported AGS cases. We estimated that if white-tailed deer abundance had increased by 50%, 75%, or 100% in 2020, there would have been 89 (95%CI: 12, 252), 126 (12, 354) or 159 (6, 448) additional AGS self-reported cases in the US in 2025. Conclusions: The estimated associations are compatible with an effect of white-tailed deer abundance on AGS in the country. Due to measurement error, the low granularity of the available data, the ecological nature of the design, and the modelling choices, our effect estimates should be interpreted cautiously. Further studies are needed to quantify the population-level effect of white-tailed deer on AGS.