Quantitative Microbial Risk Assessment of Cryptosporidiosis and Giardiasis from Very Small Private Water Supplies

Abstract

This article reports a quantitative microbial risk assessment of the risk of Giardia and Cryptosporidium in very small private water supplies. Both pathogens have been implicated in causing outbreaks of waterborne disease associated with such supplies, though the risk of endemic disease is not known. For exposure assessments, we used existing data to derive regression equations describing the relationships between the concentration of these pathogens and Escherichia coli in private water supplies. Pathogen concentrations were then estimated using national surveillance data of E. coli in private water supplies in England and France. The estimated risk of infection was very high with the median annual risk being of the order of 25-28% for Cryptosporidium and 0.4% to 0.7% for Giardia, though, in the poorer quality supplies the risk could be much higher. These risks are substantially greater than for public water supplies and well above the risk considered tolerable. The observation that observed infection rates are generally much lower may indicate increased immunity in people regularly consuming water from private supplies. However, this increased immunity is presumed to derive from increased disease risk in young children, the group most at risk from severe disease.

Cryptosporidium Pathogenicity and Virulence

“…Using mathematical modeling, Swift and Hunter showed how an increased probability of exposure could lead to earlier infection but reduced risk later in life (263), a hypothesis supported by data from seroepidemiology studies (90). Indeed, if, as recently hypothesized, repeated exposure serves to maintain an otherwise short-lived immunity, there would be an optimal daily probability of exposure that would minimize illness rates; however, any increased exposure would increase the risk to the youngest individuals, who are likely to be the most vulnerable (264). In a study modeling the risk of cryptosporidiosis associated with unreliable drinking water supplies in Africa, Hunter and colleagues argued that the main impact of increased exposure would be an earlier age of first infection (265).…”

Development of a microbial dose response visualization and modelling application for QMRA modelers and educators

“…Stressors such as global climate change, global population growth, water stress and food scarcity have increased QMRA's importance for mitigating risks from emerging or reemerging pathogens (Hunter et al, 2011; Schijven et al, 2011). Given the need for credible QMRAs across multiple domains, it is the vision of the authors to make QMRA modelling freely accessible to all interested parties.…”

A spatial analysis of private well water Escherichia coli contamination in southern Ontario

“…The maximum sample size of previous bacteriological well quality studies was limited to 4,000 wells from the state of Wisconsin (Knobeloch et al, 2013). Hunter et al (2011) studied 14,000 private water supplies (servicing less than 50 people) in England and France (2011). Additionally, a study by Perkins et al (2009) investigated the quality of rural tap water of 5,000 dairy farms in all of southern Ontario using spatial analysis techniques.…”