Social Change, Parasite Exposure, and Immune Dysregulation among Shuar Forager-Horticulturalists of Amazonia: A Biocultural Case-Study in Evolutionary Medicine
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The Hygiene Hypothesis and Old Friends Hypothesis focus attention on the coevolutionary relationship between humans and pathogens, positing that reduced pathogen exposure in economically developed nations is responsible for immune dysregulation and associated increases in chronic inflammation, allergy, and autoimmunity. Despite progress in testing these ideas, few studies have examined these relationships among populations undergoing the transition from traditional to more market-based lifestyles. The present study tests relationships between economic development and social change, altered infectious disease exposure, and immune function among the Shuar forager-horticulturalists of Amazonian Ecuador, a population undergoing rapid economic change associated with increased market participation. Using stool samples to assess soil-transmitted helminth (STHs; parasitic intestinal worms) burden, dried blood spot measurement of the inflammatory marker C-reactive protein (CRP), and interviews to evaluate level of market integration (MI; the suite of social and cultural changes associated with rapid economic development) and disgust sensitivity, this dissertation tests the Hygiene and Old Friends Hypotheses. The first study tests relationships between STH exposure and MI, using geographic location in relation to the regional market center as a proxy for MI. This study documents lower rates of STHs in people living in more market integrated regions. The second study tests the coevolutionary role that STHs and other pathogens have played in shaping human psychology and behavior. Findings suggest that pathogen exposure has acted as a selective pressure, resulting in evolved disgust sensitivity toward pathogen related stimuli. This study provides evidence that disgust sensitivity is calibrated to local environments, acting to decrease STH exposure. The third study tests the role of STHs in immune function. CRP was positively related to age in uninfected individuals. No relationships existed for more traditionally living or infected individuals. These findings suggest that STH exposure may decrease the risk of developing chronic inflammation and associated diseases with advancing age. These studies provide support for the idea that STHs provide stimuli that decrease chronic inflammation, suggesting that altered intestinal microflora in developed nations may be partially responsible for the development of chronic inflammatory disorders like allergy and autoimmunity. This dissertation includes previously published and unpublished coauthored material.