Epidemiology, microbiology, and risk assessment of waterborne pathogens including Cryptosporidium

Waterborne microbial risk assessment: a population-based dose-response function for Giardia spp. (E.MI.R.A study)

BACKGROUND

Dose-response parameters based on clinical challenges are frequently used to assess the health impact of protozoa in drinking water. We compare the risk estimates associated with Giardia in drinking water derived from the dose-response parameter published in the literature and the incidence of acute digestive conditions (ADC) measured in the framework of an epidemiological study in a general population.

METHODS

The study combined a daily follow-up of digestive morbidity among a panel of 544 volunteers and a microbiological surveillance of tap water. The relationship between incidence of ADC and concentrations of Giardia cysts was modeled with Generalized Estimating Equations, adjusting on community, age, tap water intake, presence of bacterial indicators, and genetic markers of viruses. The quantitative estimate of Giardia dose was the product of the declared amount of drinking water intake (in L) by the logarithm of cysts concentrations.

RESULTS

The Odds Ratio for one unit of dose [OR = 1.76 (95% CI: 1.21, 2.55)] showed a very good consistency with the risk assessment estimate computed after the literature dose-response, provided application of a 20 % abatement factor to the cysts counts that were measured in the epidemiological study. Doing so, a daily water intake of 2 L and a Giardia concentration of 10 cysts/100 L, would yield an estimated relative excess risk of 12 % according to the Rendtorff model, against 11 % when multiplying the baseline rate of ADC by the corresponding OR. This abatement parameter encompasses uncertainties associated with germ viability, infectivity and virulence in natural settings.

CONCLUSION

The dose-response function for waterborne Giardia risk derived from clinical experiments is consistent with epidemiological data. However, much remains to be learned about key characteristics that may heavily influence quantitative risk assessment results.

The potential for acquiring cryptosporidiosis or giardiosis from consumption of mung bean sprouts in Norway: a preliminary step-wise risk assessment

The current work evolved from a microbial survey of fruits and vegetables conducted in Norway between 1999 and 2001. This survey found that mung bean sprouts were more likely to be contaminated with Cryptosporidium and Giardia than the other produce included in the survey. To support this observation and to demonstrate to public health officials that this might be a risk warranting further attention, a simple risk assessment was initiated. Assuming that 60,000 people in Norway consume a single serving of bean sprouts per week, and contamination levels are similar to those found in the survey, it was calculated that there could be in the order of 20 cases of Giardia or Cryptosporidium infection per 100,000 population attributable to consumption of mung bean sprouts. A number of assumptions were made for the calculations, including parasite factors (e.g. viability, genotype), product factors (e.g. extent of product contamination) and host factors (e.g. composition and extent of consumer group). These assumptions and areas of uncertainty, where further data would improve the risk assessment, are highlighted throughout. Not only does the risk assessment identify new areas of research, but it also demonstrates how risk assessment can be used as a tool to try to influence public health surveillance.

A quantitative risk assessment of waterborne cryptosporidiosis in France using second-order Monte Carlo simulation

A pragmatic quantitative risk assessment (QRA) of the risks of waterborne Cryptosporidium parvum infection and cryptosporidiosis in immunocompetent and immunodeficient French populations is proposed. The model takes into account French specificities such as the French technique for oocyst enumeration performance and tap water consumption. The proportion of infective oocysts is based on literature review and expert knowledge. The probability of infection for a given number of ingested viable oocysts is modeled using the exponential dose-response model applied on published data from experimental infections in immunocompetent human volunteers challenged with the IOWA strain. Second-order Monte Carlo simulations are used to characterize the uncertainty and variability of the risk estimates. Daily risk of infection and illness for the immunocompetent and the immunodeficient populations are estimated according to the number of oocysts observed in a single storage reservoir water sample. As an example, the mean daily risk of infection in the immunocompetent population is estimated to be 1.08 x 10(-4) (95% confidence interval: [0.20 x 10(-4); 6.83 x 10(-4)]) when five oocysts are observed in a 100 L storage reservoir water sample. Annual risks of infection and disease are estimated from a set of oocyst enumeration results from distributed water samples, assuming a negative binomial distribution of day-to-day contamination variation. The model and various assumptions used in the model are fully explained and discussed. While caveats of this model are well recognized, this pragmatic QRA could represent a useful tool for the French Food Safety Agency (AFSSA) to define recommendations in case of water resource contamination by C. parvum whose infectivity is comparable to the IOWA strain.

Position of the American Dietetic Association: food and water safety

It is the position of the American Dietetic Association that the public has the right to a safe food and water supply. The Association supports collaboration among dietetics professionals, academics, representatives of the agriculture and food industries, and appropriate government agencies to ensure the safety of the food and water supply by providing education to the public and industry, promoting technologic innovation and applications, and supporting further research. Numerous bacterial, viral, and chemical food and water threats exist with certain populations such as the elderly, children, pregnant women, those in institutionalized settings, and the immune compromised being at high risk. Recent outbreaks of food and waterborne disease and threats of bioterrorism have focused attention on the safety of US food and water systems. The US government and other entities have developed programs to address challenges associated with maintaining food and water safety. Safety initiatives such as the Pathogen Reduction/Hazard Analysis Critical Point (HACCP), revisions to the Food Code, and the National Primary Drinking Water Regulations provide a framework to evaluate current and future challenges to the safety of food and water systems. Dietetics professionals should take a proactive role in ensuring that appropriate food and water safety practices are followed and can also assume major roles in food and water safety education and research.

The future impact of societal and cultural factors on parasitic disease -- some emerging issues

A variety of societal and cultural factors will increase host exposure or susceptibility to infectious agents, particularly parasites. Such factors have already had a major impact on the emergence of infectious diseases and the situation is likely to worsen further as we enter the new millennium. The changes that are enhancing the spread and transmission of parasitic diseases, as well as those which are adversely affecting host responsiveness, are examined with reference to specific parasites.