The feasibility of testing whether Fasciola hepatica is associated with increased risk of verocytotoxin producing Escherichia coli O157 from an existing study protocol

The parasite Fasciola hepatica is a major cause of economic loss to the agricultural commu-nity worldwide as a result of morbidity and mortality in livestock, including cattle. Cattle arethe principle reservoir of verocytotoxigenic Escherichia coli O157 (VTEC O157), an impor-tant cause of disease in humans. To date there has been little empirical research on theinteraction between F. hepatica and VTEC O157. It is hypothesised that F. hepatica, whichis known to suppress type 1 immune responses and induce an anti-inflammatory or regu-latory immune environment in the host, may promote colonisation of the bovine intestinewith VTEC O157. Here we assess whether it is statistically feasible to augment a prospectivestudy to quantify the prevalence of VTEC O157 in cattle in Great Britain with a pilot studyto test this hypothesis. We simulate data under the framework of a mixed-effects logisticregression model in order to calculate the power to detect an association effect size (oddsratio) of 2. In order to reduce the resources required for such a study, we exploit the factthat the test results for VTEC O157 will be known in advance of testing for F. hepatica byrestricting analysis to farms with a VTEC O157 sample prevalence of >0% and <100%.From a total of 270 farms (mean 27 cows per farm) that will be tested for VTEC O157,power of 87% can be achieved, whereby testing of F. hepatica would only be necessary foran expected 50 farms, thus considerably reducing costs. Pre-study sample size calculationsare an important part of any study design. The framework developed here is applicable tothe study of other co-infections.© 2015 The Authors. Published by Elsevier B.V. This is an open access article under theCC BY license (http://creativecommons.org/licenses/by/4.0/).