Fish-borne trematode metacercarial infections as bioindicators for heavy-metal contamination in cyprinoid fish from Northeastern Thailand

In Southeast Asia, consumption of raw or undercooked cyprinoid fish poses dual health concerns: transmission of zoonotic fish-borne trematodes (FBTs) and exposure to heavy metals that have bioaccumulated in fish. We investigated the association between heavy-metal contamination and FBT metacercarial infections in cyprinoid fish from canals connected to the Huay Luang River, Udon Thani Province, Thailand, during the 2024 rainy season. In total, 1377 fish were examined for metacercariae using the pepsin digestion method. Concentrations of five heavy-metals (Cd, Zn, Cu, Ni, and Pb) were quantified by atomic absorption spectrometry (AAS) in infected fish, uninfected fish, water, sediment, and <i>Bithynia</i> snails. The overall prevalence of infection was 8.5%, with mean intensities of 1.6-3.6 metacercariae/infected fish. Infected fish contained significantly higher Cd (1.63 ppm), Cu (11.06 ppm), and Zn (68.51 ppm) than uninfected fish (<i>P</i> ≤ 0.001). Pearson correlation revealed positive associations between metacercarial prevalence and all metal concentrations, except Pb (<i>P</i> < 0.05). Multivariable regression identified Cd as the strongest predictor of infection intensity, with each 1-ppm increase associated with 5.36 additional metacercariae/infected fish (<i>P</i> < 0.001). Bioaccumulation factors were higher in infected fish for Cd (ratio 1.3), Zn (ratio 1.6), and Cu (ratio 1.4) compare to uninfected fish, while Pb was lower. Spatial analysis identified three sites where elevated Cd overlapped with high parasite intensity. These findings establish FBT metacercariae as site-specific bioindicators of heavy-metal accumulation, highlighting the public-health risk from co-exposure and their potential use in food safety and ecosystem monitoring.