Comprehensive public health surveillance identifies novel human cryptosporidiosis subtypes and genetic diversity associated with swimming pool outbreaks in Australia, Victoria (2018-2024)

Abstract

Cryptosporidium and Giardia are major causes of gastrointestinal illness globally. In Australia, cryptosporidiosis is a nationally notifiable disease, yet molecular characterisation of clinical cases is rarely performed, limiting the capacity to identify outbreaks, trace sources and assess zoonotic risk. During 2024 there was a 273% cases increase in Australia, the third country with the highest increase. We present the first comprehensive molecular investigation of human Cryptosporidium and Giardia infections in the state of Victoria, Australia. We analysed faecal samples collected between 2018 and 2024. Positive samples were subtyped and parasite load was estimated. Of the 2,330 samples tested, 225 were positive for Cryptosporidium and nine for Giardia. Seven Cryptosporidium species and 24 subtypes were identified, including multiple novel or regionally unique subtypes. C. hominis was the predominant species (85%), and three subtypes associated with 11 recreational water outbreaks in 2024. Based on spatiotemporal overlap and subtypes, 52 cases were inferred to represent undetected outbreak-associated infections. Several C. parvum subtypes reflected probable zoonotic transmission, two subtypes were associated with a childcare and camp outbreak. Six C. hominis subtypes and eight subtypes overall were reported for the first time in Australia. Globally novel subtypes of C. occultus, C. fayeri and C. meleagridis in human hosts is reported. The study reveals high diversity of Cryptosporidium subtypes in human infections in Victoria. This study demonstrates the potential for molecular surveillance to inform public health interventions when integrated with epidemiological data. These findings support One Health approaches to outbreak detection and source attribution. IMPORTANCECryptosporidium is a nationally notifiable pathogen in Australia, yet routine genotyping is not performed, limiting outbreak detection and source attribution. This study represents the first large-scale molecular surveillance of human Cryptosporidium and Giardia infections in southern Australia. By integrating species- and subtype-level typing with epidemiological metadata, we demonstrate the substantial diversity of infective lineages, including several novel or zoonotic genotypes. Our findings highlight the critical role of molecular tools in tracking transmission pathways, supporting outbreak investigation and informing public health responses. These data provide a foundation for incorporating routine genotyping into national surveillance strategies for parasitic enteropathogens. This study presents the first epidemiological study of the 2024 outbreaks in Australia. The global comparison of cases during this period also highlights potential large-scale disease dynamic of public health importance.