<i>Cryptosporidium</i> species and subtypes in Norway: predominance of <i>C. parvum</i> and emergence of <i>C. mortiferum</i>

PCR-based diagnostics has revealed the previously largely unknown <i>Cryptosporidium</i> transmission and infections in high-income countries. This study aimed to determine domestic and imported subtypes of <i>Cryptosporidium</i> species in Norway, evaluate their demographic distribution, and identify potential small outbreaks. <i>Cryptosporidium</i>-positive human faecal samples were obtained from six medical microbiology laboratories between February 2022 and January 2024, together with 22 <i>Cryptosporidium</i>-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 <i>Cryptosporidium</i> species. <i>C. parvum</i> 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 <i>C. parvum</i> IIaA14G1R1. <i>C. mortiferum</i> 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 <i>C. mortiferum</i> isolates were of the same genotype; XIVaA20G2T1, including 13 cases from a suspected small outbreak in Trøndelag. <i>C. hominis</i> 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 <i>Cryptosporidium</i> spp. and subtypes in Norway, highlights the predominance of <i>C. parvum</i> and the emergence of <i>C. mortiferum</i> among autochthonous cases.