PhD thesis - University of Hertfordshire Research Archive

fundamentally flawed. Since the application of case-case methodology to
infectious disease epidemiology is in its infancy, no consensus on a gold-
standard method for analysis exists currently. It is hoped that the
methodological advances detailed in these studies will inform future case-
case studies.
Case-crossover studies are an alternative approach which could have been
applied at the outset of this study (McCarthy & Giesecke, 1999). These
involve capturing information on patients exposures in the incubation period
for a particular disease and comparing these to exposures from a time
outside the incubation period. They therefore employ „control times‟ rather
than „control persons‟ with the patient perfectly matched to themselves. They
obviate the need for ethical approval if conducted within a primary
surveillance framework and are relatively simple to conduct. Ideal in theory.
They assume, however, that the individual has done something „out of the
ordinary‟ which has resulted in their disease episode and this assumption is
debatable for Campylobacter infection. Whilst there are documented
instances where a „change from the norm‟ has resulted in campylobacteriosis
(e.g. household illness following the introduction of a puppy) it is equally
possible that infection occurs as a result of indirect actions in everyday life
(e.g. buying a contaminated sandwich from a sandwich shop where one
routinely buys the same sandwich which has not previously been
contaminated). Furthermore, the incubation period for Campylobacter
infection would impact on the suitability of this study design. Firstly, it is long
and therefore the control period would have to be some time prior to onset,
increasing the likelihood of recall bias for the „control‟ over the „case‟ period.
Secondly, it is variable, and therefore if the chosen control period is too
recent then it is possible that the illness-causing exposure could be included
in the control period and hence the case would be misclassified as a control.
In the absence of a control group, descriptive studies can only tell us so
much about the role of particular exposures in disease, and policy makers
and public health practitioners generally require a higher level of evidence for
60