Environmental surveillance of zoonotic protozoa and fecal indicators in surface waters of a subtropical transition zone in Southern Brazil

Waterborne protozoa pose a significant challenge to public health, particularly in regions with deficient sanitation. This study aimed to evaluate the occurrence of Giardia lamblia , Cryptosporidium spp., and Cystoisospora spp. in surface waters of the Pelotas microregion, Southern Brazil, and their correlation with fecal indicator bacteria ( Escherichia coli ) and rainfall events. From March to August 2025, eight sampling points (streams, urban canals, and recreational beaches) were monitored. Water samples were analyzed for physico-chemical parameters, E. coli quantification, and molecular detection (PCR) of protozoa. Giardia lamblia and Cystoisospora spp. were frequently detected in urban drainage canals and streams, showing a persistence that contrasted with the variable counts of E. coli . Notably, extreme rainfall events (> 180mm accumulated) and low water temperatures (< 14°C) in the final sampling campaign appeared to influence the detection rates, suggesting a dilution effect or reduced bacterial viability, while protozoan DNA persisted. Cryptosporidium spp. was not detected. These findings highlight the environmental resilience of protozoan cysts and the limitations of relying solely on bacterial indicators for water quality assessment in subtropical transition zones.