Quantitative microbial risk assessment: an integrated probabilistic modeling of human exposure to norovirusin wastewater
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Date
2015-11-04
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Abstract
The call for models applying quantitative data of pathogens that are of interest to
replace the otherwise commonly applied models usingfecal indicator conversion
ratio has gained prominence, challenges of analytical studies on virus enumeration
(genome copies or particles) have contributed further to the low availability of data
in Quantitative Microbial Risk Assessment (QMRA) modelling. In this thesis, a
probabilistic stochastic model was developed to respond to the call for virus of
interest based models. Quantitative data on genome copies of Norovirusand
oocyste of Cryptosporidium spp. were applied in a QMRA model. The model was
extended to include an induced immunity for Dose Response Incidence (DRI) of
illness reduction in individual and population exposures, five different scenarios were
modelled for Norovirusbased on the epidemiological understanding of immunity
within an individual and Norovirustransmission dynamics. A third model was
developed to measure the uncertainty of compliance and reliability of wastewater
effluent with integrated policy standards. The probabilistic QMRA model revealed
fecal indicator ratio conversion method underestimated the Disability Adjusted Life
Years (DALYs) with more than two (2) orders of magnitude and were confirmed using
theCryptosporidium spp. data. For immunity extended DRI models, results shows,
illness incidence is much reduced when both dose-dependent and immunity are
integrated into risk assessment models. Integrationof immunity only into DRI model
also performed better than dose-DRI model only. It was also revealed that,
irrespective of the epidemiology transmission dynamics within the population, DRI
models predictions were similar and dose-immunity DRI model was better predictor.
Finally, the analyses of compliance and reliabilityof wastewater effluent
measurements revealed that results from wastewater effluents which met the policy
standard values, in some cases could not meet the compliance level needed for
effluent discharge. A chart was developed for the various wastewater treatment
effluent discharge parameters for easy comparison with effluent discharges
Description
A thesis submitted to the Department of Mathematics in partial fulfillment of the requirement for the award of the degree of Doctor of Philosophy in Mathematical Statistics,