Retrospective assessment of rapid outbreak investigation for gastrointestinal diseases using only cases and background exposure data
PHE ePoster Library. Mook P. Apr 9, 2019; 257497
Dr. Piers Mook
Dr. Piers Mook
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Abstract Background:
For outbreaks of gastrointestinal disease, rapid identification of the source or vehicle is crucial to enable public health intervention and prevent further cases. Outbreak investigation comprises collection and analyses of exposure information from cases, microbiological sampling and undertaking analytical epidemiological studies where appropriate. Current analytical investigation practices are resource-intensive and prone to bias mainly due to difficulties in recruiting controls. Moreover, hypothesis generation heavily relies on empirical knowledge of exposures historically associated with a given pathogen and reported frequency of exposures among initial cases. We propose a simple method that compares the information from cases against pre-defined background exposure information.
Data from historical case trawling questionnaires from three outbreaks were used to define background exposures having removed exposures associated with the outbreak pathogens. Case data from three past outbreaks with common (outbreak A: associated with consumption of contaminated herbs) and rare (outbreaks B and C: feeder mice for reptiles) exposures were used, exposures with odds ratios calculated and comparison with original findings made..
The case-background approach showed both sensitivity and specificity, identifying correctly all outbreak-related exposures (outbreak A, consumption of any type of herbs odds ratio (OR): 5.9, 95% CI [1.7-20.0]; B & C, having a reptile as a domestic pet OR: 50.7 [11.0-234.1] & 25.3 [5.7- 111.8], respectively). One additional exposure related to a retailer was also identified. Sensitivity analysis demonstrated that allowing for a proportion of cases incorrectly stating that they had not been exposed (recall bias), the outbreak source would still have been identifiable. ConclusionThe case-background method, a development of the case-case design, is one way in which outbreak investigations can be optimised. The background exposure information ideally needs to be defined based on in-depth interviews of carefully selected controls and updated regularly.
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