A retrospective study to understand the impact of flooding on natural selection and physical growth dynamics

Flooding ranks among the most devastating natural disasters, causing substantial economic disruption and significant loss of life. Its impact on human populations has both immediate and long-term consequences that can extend from birth through adulthood. Studies on the effects of flooding have primarily focused on preschool children, often within diverse populations characterized by heterogeneous genetic ancestry. Ultimately it has left a gap in understanding its long-term developmental effects, particularly during adolescence and within genetically cohesive populations. Notably, no prior study has examined its potential evolutionary significance through natural selection, and finally its impact on adolescent physical growth dynamics as well as the prevalence of stunting within endogamous population groups. To address this gap, the Mishing population from Assam has been selected, which exhibits a degree of genetic homogeneity due to their shared ancestry and endogamous practices. They are characterized by differential habitation patterns in relation to perennial flooding. This variation in exposure to flood-affected and non-affected areas within the same ethnic group provides a unique opportunity to conduct a comparative analysis, minimizing genetic variability while assessing the environmental impact on health and related parameters. The present study thus investigates the effects of flooding across the life course among the Mishing, with specific focus on the opportunities for natural selection and the long-term impacts on stunting and physical growth parameters viz final height, peak height velocity (PHV), and age at peak height velocity (APHV). Using stratified random sampling, two flood-affected and two non-affected Mishing villages were selected from four districts of Assam, covering 1687 households. All 309 post-menopausal women (186 from flood-affected and 123 from non-affected villages) were interviewed for their fertility records, achieving a 100% response rate. A modified index for natural selection (Sikdar's index) was used to estimate selection pressure across life stages. Out of 3761 children and adolescents (aged 6-20 years) identified in these households, a cross-sectional study was conducted on 2970 individuals (1464 boys and 1506 girls). With a 78.9% response rate, the study assessed height growth variation using the Preece and Baines Growth Curve Model 1. Additionally, the prevalence of stunting was assessed among 2752 individuals (1353 boys and 1399 girls aged 6-19) using the WHO 2007 LMS approach. Results show a substantial selection pressure on the infants in flood-affected villages which accentuates the urgent need for targeted public health interventions and climate-resilient infrastructure. However, among surviving children and adolescents, flood exposure had a negligible impact on postnatal growth parameters like final height, PHV, APHV and stunting, possibly reflecting cultural adaptation, community resilience, and the hygiene hypothesis. Additionally, the findings underline the protective role of higher household income and better maternal education, both of which were significantly associated with reduced odds of stunting which suggest that in flood-prone regions interventions must go beyond emergency response and include enduring as well as development focused policies.