Cryptosporidium species and subtypes in Norway: predominance of C. parvum and emergence of C. mortiferum

PCR-based diagnostics has revealed the previously largely unknown Cryptosporidium transmission and infections in high-income countries. This study aimed to determine domestic and imported subtypes of Cryptosporidium species in Norway, evaluate their demographic distribution, and identify potential small outbreaks. Cryptosporidium-positive human faecal samples were obtained from six medical microbiology laboratories between February 2022 and January 2024, together with 22 Cryptosporidium-positive animal samples. Species and subtypes were identified by sequencing PCR products from gp60 and SSU rRNA genes. Most cryptosporidiosis cases occurred during late summer/early autumn, primarily in children and young adults. Of 550 human samples, 359 were successfully characterized molecularly (65%), revealing infection with 10 different Cryptosporidium species. C. parvum occurred in 245 (68%) human isolates with IIa and IId being major allele families, with distinct regional distribution patterns of common subtypes. A kindergarten outbreak with 5 cases was due to C. parvum IIaA14G1R1. C. mortiferum was identified in 33 (9.2%) human cases of which 24 were known to be of domestic origin, making it the second most common species in human autochthonous cases in Norway. All C. mortiferum isolates were of the same genotype; XIVaA20G2T1, including 13 cases from a suspected small outbreak in Trøndelag. C. hominis occurred in 68 typed cases (19%), but mostly in infections acquired abroad, with allele families Ib and If occurring most often. In conclusion, this study of recent Cryptosporidium spp. and subtypes in Norway, highlights the predominance of C. parvum and the emergence of C. mortiferum among autochthonous cases.