Overview of a Decade-Plus Surveillance Program
Ensuring safe drinking water from natural catchments requires detailed knowledge of protozoan parasites in wildlife. This long-term study tracks Cryptosporidium and Giardia in Melbourne’s protected catchments using cost-effective molecular methods to identify and classify parasites in host wildlife. Spanning 2009–2024 with updates through 2024, the program provides a comprehensive view of parasite diversity, host associations, and seasonal dynamics in a closed catchment system that feeds Melbourne’s water supply.
Key Findings: Prevalence and Diversity
From 8695 faecal DNA samples, researchers detected Cryptosporidium in 3.15% of samples and Giardia in 0.16%. Of the 274 SSU (small subunit) amplicons obtained, 13 Cryptosporidium species and genotypes were identified, with 23 novel SSU sequences (<100% identity to GenBank entries) expanding the known diversity. A final set of 35 sequences received GenBank accession numbers and informed phylogenetic analysis, situating most sequences within three dominant clades: C. ryanae, C. fayeri, and C. macropodum.
Dominant Cryptosporidium Clades and Host Links
– Deer showed infections largely within the C. ryanae clade, including four SSU sequence variants representing C. ryanae and three deer genotypes. The most common deer genotype was Cryptosporidium sp. deer genotype v3. Other detections included C. muris, C. ubiquitum, and C. occultus in certain deer populations.
– Marsupials bore a rich diversity, especially macropods. Within the C. macropodum clade, five variants appeared across kangaroos and wallabies, with most detections in eastern grey kangaroos. C. ubiquitum and a possum genotype were also observed in marsupials, illustrating cross-species dynamics in catchments.
– Canids, lagomorphs, rodents, and birds contributed additional species, including C. canis in a fox and dingo, C. cuniculus in rabbits (with gp60 subtypes), and C. baileyi in wood ducks. A wallaby sample carried a C. parvum-like genotype, underscoring occasional human-related parasite links.
Giardia Findings: Sub-Assemblages and Novel Variants
Giardia was detected in 14 samples. Sub-assemblage AI appeared in eastern grey kangaroos and some deer, with four novel AI variants expanding the known genetic diversity. Sub-assemblage AIII appeared in several deer and one dog. Notably, no Giardia duodenalis was found in rabbits, rodents, or birds in this study.
Seasonal and Temporal Trends
Cryptosporidium prevalence fluctuated over time, ranging from 0.99% in 2016 to 6.15% in 2023, with an overall mean of 3.15%. Across 16 years (2009–2024), the trend shows a significant year-on-year increase (adjusted R2 = 0.294; P = 0.0175). Seasonal patterns revealed higher prevalence in summer for overall Cryptosporidium, while macropods peaked in summer and spring, and deer peaked in autumn and winter. When looking at species-level trends, C. macropodum tended to peak in summer; C. fayeri and C. ryanae showed winter peaks, with deer showing autumn/winter elevations for C. ryanae. These patterns likely reflect host biology, reproduction cycles, and habitat changes in catchments.
Public Health Context and Water Safety
Cryptosporidium remains the primary concern for water safety in Melbourne’s catchments due to its potential for human cryptosporidiosis, which is harder to treat than bacterial pathogens. The study found very low human-associated species like C. hominis and low prevalence of C. parvum, suggesting a relatively low immediate risk of waterborne transmission to humans in Greater Melbourne. However, the presence of zoonotic species and novel variants highlights the need for ongoing genomic surveillance to inform risk management and treatment strategies.
Comparisons and Implications for Catchment Management
Compared with Sydney’s open catchments, Melbourne’s closed system shows a distinct parasite profile dominated by C. macropodum and C. fayeri, with far fewer human-adapted species. This contrast likely reflects different land use, wildlife composition, and catchment management practices. The identification of several novel SSU sequences underscores substantial undiscovered diversity and suggests that long-term monitoring will continue to reveal new variants and potential zoonotic links.
Concluding Thoughts
This longitudinal, molecular-based surveillance provides a critical knowledge base for water utilities and public health officials. By linking parasite genotypes to host species and catchment areas, the study supports proactive risk assessment, targeted management actions, and preparedness for emerging protozoan threats in protected drinking water sources.
