Retrospective mercury exposure and current symptoms of nervous system dysfunction in adults of a First Nation community (Canada)

Abstract

Background: The watershed in Asubpeeschoseewagong Netum Anishinabek (Grassy Narrows First Nation) territory has been contaminated by mercury (Hg) since 1962, resulting in very high Hg concentrations in fish, central to the community’s culture, traditions, economy and diet. Biomarkers of Hg (umbilical cord blood and hair/blood samples), monitored between 1970 and 1997, decreased over time. A recent Grassy Narrows Community Health Assessment (GN-CHA) survey included current symptoms of nervous system dysfunction. The present study aimed to cluster self-reported symptoms and examine their relation to past Hg exposure. Methods: The GN-CHA included 391 Registered adult Band members. Symptom clustering used a two-step segmentation approach. Umbilical cord Hg and yearly measurements of hair Hg were available for 242 participants. Longitudinal Mixed Effects Models (LMEM) served to examine past hair Hg with respect to clusters. Structural Equation Models (SEM) displayed direct and indirect pathways between Hg exposure and clusters, with Hg exposure modelled as a latent variable or in separate time periods (prenatal, childhood and having had hair Hg ≥ 5µg/g at least once). Results: A total of 37 symptoms bonded into 6 clusters, representing Extrapyramidal impairment, Sensory impairment, Cranial nerve disturbances, Gross motor impairment, Neuro-cognitive deficits and Affect/Mood disorders. Median Hg concentrations were 5µg/L (1-78.5) and 1.1µg/g (0.2-16) for umbilical cord and childhood hair, respectively. More than one-third (36.6%) had hair Hg ≥ 5 µg/g at least once. LMEM shows higher long-term hair Hg among those with higher scores for most clusters. In SEM, latent Hg is directly and indirectly associated with all clusters. The contribution of prenatal exposure is mediated by childhood exposure, except for Affect/Mood disorders, where the association is direct. Hair Hg ≥ 5 µg/g is directly related to Extrapyramidal and Sensory impairment. Conclusion: Our findings provide evidence that in this First Nation community, past Hg exposure from fish consumption was associated with later-life clusters of coexisting symptoms of nervous system dysfunction. Given the complexity of the interrelations between the various determinants of chronic health problems in this and other First Nation communities, LMEM and SEM methods provide the opportunity to trace direct and/or indirect effect routes of potential “causal impact”.