Across the UK, routine surveillance of cryptosporidiosis relies on mandatory laboratory notification. Since the International Organization for Standardization (ISO) week 33 2023 (starting on 14 August), and as at week 39 2023 (ending on 1 October), the combined weekly number of laboratory notifications of Cryptosporidium spp. detections in England, Wales and Northern Ireland has exceeded the expected upper threshold (Figure); an exceedance in Scotland has only been present since ISO week 39.
Observed case numbers are compared to an exceedance threshold as derived from the Farrington Flexible algorithm, using published methodology [10]; the model can account for seasonal effects and changing trends. The upper threshold limit reflects the 99.5% percentile of a negative binomial distribution for the expected case frequency.
Given the scale and geographical spread of the exceedance across regions and nations of the UK, a single local exposure is an unlikely cause.
In England, local practice varies in the routine collection of exposure information in cryptosporidiosis cases. To allow for consistent capture of information as part of this investigation, a standardised electronic questionnaire was deployed. This includes questions on foreign travel, food and water exposures and interaction with animals. Data collection through this method has been ongoing since ISO week 38 (and in retrospect), for cases meeting the following definition: any person resident in England, with a clinical specimen dated 14 August 2023 or later, positive for Cryptosporidium (any species). In Wales, a standard questionnaire for gastrointestinal disease has been in place since before the exceedance.
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