Estimating the contribution of different sources to the burden of human campylobacteriosis and salmonellosis : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
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Date
2009
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Massey University
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Abstract
This thesis is concerned with the molecular epidemiology of Campylobacter jejuni and
Salmonella in New Zealand and the development of source attribution tools for these
pathogens. Although campylobacteriosis is the leading enteric zoonosis worldwide, the
pathogen's complex epidemiology and di culties with existing typing schemes, have
posed challenges for the control of this disease.
The rst study of this thesis gives an overview of existing approaches to microbial
risk assessment and source attribution, with particular respect to campylobacteriosis,
and describes their advantages and shortcomings. Further, the chapter discusses phenoand
genotyping techniques for Campylobacter spp. and the value of including microbial
typing data in risk assessments. In the second study, data from a sentinel surveillance
site in the Manawatu region was used to investigate the molecular epidemiology of
human campylobacteriosis cases. This analysis revealed the presence of a dominant C.
jejuni clone, namely sequence type (ST) 474, which accounted for 30.7 % of human
cases in the study and identi ed risk factors for infection with ruminant and poultry
associated STs. The third study investigated the link between C. jejuni in human
cases and samples taken from poultry. By applying epidemiological and population
genetic techniques this part of the thesis provided further evidence that poultry is
a major contributor to human infection. In the fourth study an existing Bayesian
source attribution model was modi ed and consecutively applied to New Zealand's
major foodborne zoonoses: campylobacteriosis and salmonellosis. The majority (80
%) of human campylobacteriosis cases attributable to C. jejuni were estimated to
have been acquired from poultry sources, whereas wildlife source were estimated to
contribute only a minor proportion of cases. In the fth study the Salmonella dataset
was descriptively analysed and a large proportion of human cases was found to be
caused by `exotic' Salmonella types. In the nal study of this thesis four di erent
genetic and epidemiological source attribution methodologies were applied to the same
dataset in a comparative modelling framework.
iv
The studies in this thesis show that epidemiological studies combined with molecular
tools and modeling can provide valuable risk-based tools to inform the surveillance
and control of zoonotic pathogens. Methods from these studies may be readily applied
to the control of other (food borne) zoonoses and provide new opportunities for
epidemiological investigations and source attribution modelling of major pathogens.
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Keywords
Campylobacter jejuni, Salmonella, Molecular epidemiology, Human campylobaceriosis, Human salmonellosis, New Zealand