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Zoonoses Public Health. 2018 May;65(3):291-303. doi: 10.1111/zph.12388. Epub 2017 Oct 5.

Temporal trends in Giardia occurrence in the Grand River and surrounding tributaries, Waterloo, Ontario (2005-2013), a retrospective analysis of surveillance data.

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Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada.
Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada.
Environmental Monitoring and Reporting Branch, Ontario Ministry of the Environment and Climate Change, Toronto, ON, Canada.
Centre for Food-Borne, Environmental and Zoonotic Infectious Diseases, Public Health Agency of Canada, Ottawa, ON, Canada.


Giardia contamination in the Grand River Watershed (south-western Ontario, Canada) was monitored from 2005 to 2013 as part of FoodNet Canada. Our study objectives were to describe the temporal pattern of Giardia occurrence and determine whether water quality parameters and bacterial indicators could act as effective markers for Giardia occurrence. Water samples were collected monthly from the Grand River near a drinking water intake point (2005-2013) and also collected intermittently from other areas in the watershed during the study period. Samples were tested for Giardia cysts using the US EPA method 1623. Samples were also tested for chemical and microbial water quality indicators. Univariable and multivariable linear regression models were built to examine whether temporal, water quality and bacterial indicators were associated with Giardia cyst concentration. Giardia cysts were identified in 89% of samples (n = 228), with highest measured concentrations downstream of a waste water treatment plant outfall. Year and season were found to be predictors for Giardia occurrence. Concentrations were significantly higher in the winter and fall compared to the summer, and significantly higher in 2007 compared to other study years. After controlling for season, year and sampling location, dissolved oxygen was the only variable significantly associated with Giardia cyst concentration. Seasonal peaks in Giardia cyst concentrations in samples collected near the intake for the drinking water plant did not align with the seasonal peak in human Giardiasis cases in this region that are reported annually by public health authorities. This suggests that the risk of contracting Giardiasis from treated drinking water in this community is possibly low when the treatment plant is functioning adequately. Instead, waterborne exposure is likely the result of seasonal behaviours surrounding recreational water use. Therefore, the collective findings of our study are important to help inform future risk management studies and guide public health protection policies.


E. coli ; Giardia ; Enterococci; dissolved oxygen; surface water; surveillance

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