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

BackgroundLaboratory surveillance detected an unprecedented increase in <i>Cryptosporidium</i> spp. (predominantly <i>Cryptosporidium hominis</i>) 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.AimTo perform a retrospective case-case study investigation of the increase in <i>Cryptosporidium</i> spp. in England and Wales.MethodsWe conducted an unmatched case-case study with 203 cases of laboratory-confirmed <i>C. hominis</i> and 614 comparator cases of laboratory-confirmed <i>Campylobacter</i> 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).ResultsMultivariable 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.ConclusionsFollowing the largest recorded increase of <i>Cryptosporidium</i> spp. and in particular <i>C. hominis</i> 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.