Social marginalisation, environmental degradation and Toxoplasma gondii exposure in urban informal settlements in Brazil.

Toxoplasma gondii infection poses a substantial global health burden, yet transmission pathways and population susceptibility in urban informal settlements remain poorly characterised, particularly for women of childbearing age. We analysed archived samples from a cross-sectional serosurvey of 728 children and adolescents aged 4-18 years living in a marginalised urban community in Salvador, Brazil, to characterise exposure patterns and identify demographic, socioeconomic, behavioural, household, and environmental factors associated with seropositivity and to assess spatial heterogeneity in exposure risk. Overall seroprevalence was 49%, increasing with age and higher in males than females; Bayesian serocatalytic models estimated sex-specific forces of infection of 0.078 for males and 0.050 for females, with approximately half of female participants still susceptible upon reaching childbearing age, highlighting the risk of congenital toxoplasmosis. In regression analyses, seropositivity was associated with male sex, lower household income, cat ownership, and residence at lower elevation, greater distance from the main road, and reported contact with sewer water. Notably, most seropositive participants (77.3%) did not live in households with cats. Geostatistical modelling demonstrated fine-scale spatial heterogeneity, with clustered hotspots exceeding 50-60% predicted prevalence. Adjustment for measured covariates attenuated but did not eliminate spatial clustering, indicating residual fine-scale spatial structure consistent with unmeasured environmental processes operating beyond individual households, alongside additional unstructured variation that may reflect household-level or peridomestic differences not captured by the measured covariates. Together, these findings provide evidence consistent with an important role for household and peridomestic environmental exposure pathways in T. gondii transmission in informal settlements, extending beyond households with domestic cats and shaped by social marginalisation and environmental vulnerability.