From rare to recognized: enhanced detection uncovers Cryptosporidium endemicity and species diversity in Denmark

Human cryptosporidiosis, a diarrheal disease caused by Cryptosporidium species, has previously been considered rare in Denmark and primarily associated with travel abroad. Cryptosporidium in humans came under national surveillance in 2023. Here, we assess all cases identified in Denmark from 2010 to 2024, presenting the identified species, the trends in time and place, and relate the findings to recent changes in diagnostic methods. After 2021, the number of new cases increased substantially, coinciding with the adoption of gastrointestinal syndromic testing in several local hospitals. During seasonal peaks (August-October), Cryptosporidium was detected in the stool of >2% of patients tested. Infections predominantly occurred in individuals without known comorbidities, and hospitalization rates exceeded 10% in recent years. Co-infections with enteropathogenic bacteria were rare (6%), suggesting that Cryptosporidium alone was the causative agent in the patients. Most cases had no history of travel outside Denmark. Beyond C. parvum (56.9%), and C. hominis (11.3%), species of zoonotic relevance were implicated, e.g., C. mortiferum (2.5%), C. meleagridis (1.7%), C. felis (1.2%) and C. erinacei (0.8%). The transition to high-throughput molecular diagnostic methods and the testing of more patients, including those without recent travel history, has dramatically improved the detection of Cryptosporidium in stool samples in Denmark. Cryptosporidiosis appears to be a common and endemic disease in Denmark. The wide heterogeneity of infecting species suggests a number of transmission routes; these are yet to be uncovered. Cryptosporidiosis should be considered a common gastrointestinal infection by clinicians in Denmark and preventive measures should be prioritized.