Fish and shellfish as sentinels of aquatic contamination: Global distribution and One Health implications of zoonotic Cryptosporidium and Giardia

The zoonotic protozoans <i>Cryptosporidium</i> and <i>Giardia</i> are major drivers of the global gastrointestinal disease burden, circulating through complex human-animal-environment interfaces. This review synthesizes global epidemiological data to evaluate the role of fish and shellfish as reservoirs and potential conduits for zoonotic transmission. The pooled prevalence of <i>Cryptosporidium</i> spp. was 8.7% (858/9878) in fish and 16.5% (1459/8820) in shellfish. Likewise, the pooled prevalence of <i>Giardia</i> spp. in fish and shellfish were approximately 8.3% (30/361) and 6.2% (332/5392), respectively. Zoonotic <i>Cryptosporidium</i> species in fish included <i>C. parvum</i>, <i>C. hominis</i>, <i>C. ubiquitum</i>, <i>C. scrofarum</i>, and <i>C. xiaoi</i>. In shellfish, there were <i>C. parvum</i>, <i>C. hominis</i>, and <i>C. meleagridis</i>, with <i>C. parvum</i> dominant in both hosts<i>.</i> Zoonotic <i>G. duodenalis</i> assemblages A and B were detected in both fish and shellfish, with <i>G. duodenalis</i> assemblage A dominant in shellfish. These findings highlight that fish and shellfish, acting as either biological hosts or mechanical concentrators of environmental contamination, facilitate the dissemination of (oo)cysts through the aquatic food web. The presence of human-pathogenic genotypes in edible tissues poses significant food and water safety risks, exacerbated by anthropogenic pressures such as inadequate wastewater management and urbanization. This review emphasizes the necessity of a One Health framework to bridge the gap between environmental monitoring and public health surveillance. We advocate for integrated preventive measures, including enhanced molecular diagnostics and cross-sectoral management of aquatic ecosystems, to mitigate the transmission of foodborne and waterborne cryptosporidiosis and giardiasis.