Retrospective case-case study investigation of a significant increase in Cryptosporidium spp. in England and Wales, August to September 2023

Background: Laboratory surveillance detected an unprecedented increase in Cryptosporidium spp. (predominantly Cryptosporidium hominis) in England and Wales in August 2023. Cases are not routinely followed up in all of England and Wales, and initial investigations identified no common exposures. Aim: To perform a retrospective case-case study investigation of the increase in Cryptosporidium spp. in England and Wales. Methods: We conducted an unmatched case- case study with 203 cases of laboratory-confirmed C. hominis and 614 comparator cases of laboratoryconfirmed Campylobacter spp. reported between 14 August and 30 September 2023. We fitted a multilevel logistic regression model, with random intercepts for geographical region, to estimate adjusted odds ratios (aOR) for exposures. We present the final model as aOR and 95% confidence intervals (CI). Results: Multivariable analysis identified associations with swimming pool use (aOR: 5.3, 95% CI: 2.3-9.3), travel to Spain (aOR: 6.5, 95% CI: 3.5-12.3) and young age, with children 0-4 years having the strongest association of being a case (aOR: 3.6, 95% CI: 1.5-8.6). We also identified associations with swimming in a river, and travel to France or Türkiye, but there was low frequency of exposure among cases and comparator cases. Conclusions: Following the largest recorded increase of Cryptosporidium spp. and in particular C. hominis cases in England and Wales, we identified several exposures, suggesting that causation was likely to be multifactorial. We recommend development of a standardised questionnaire to enable rapid investigation of future case increases, which will improve existing surveillance and inform public health actions.