Diversity of domestic and imported assemblages of Giardia in Norway and multi-locus sequence typing of domestic assemblage A isolates

Giardia duodenalis is a gastrointestinal parasite and one of the most frequently reported parasitic infections associated with contaminated water. This study investigated the diversity of domestic and imported Giardia assemblages in Norway, with a focus on the genetic characterization of domestic assemblage A isolates using multi-locus sequence typing (MLST) targeting six genome markers. We analysed 340 human, and 40 animal faecal samples collected between February 2022 and January 2024 from six medical microbiology laboratories and one veterinary diagnostic center across four Norwegian health regions. All the samples were analysed using nested PCR targeting part of the triose-phosphate isomerase (tpi) gene, and MLST was performed on 33 assemblage A isolates, targeting six polymorphic markers. The results revealed that assemblage B was most prevalent in humans (59 %), followed by assemblage A (41 %). Among the sub-assemblages, AII was the most frequently identified (37 %), followed by BIII (32 %), and BIV (27 %). Regarding the origin of infection, 30 % were of domestic origin, while 33 % were reported as imported-primarily originating from Africa (48 %) and South-East Asia (22 %). Sub-assemblage AII (46 %) was more common in domestic cases, whereas BIV (42 %) predominated in imported cases. We found a regional clustering of sub-assemblages, with AII frequently identified in Western Norway, BIII in South-Eastern Norway, and BIV in Mid Norway. The MLST analysis of domestic assemblage A isolates demonstrated high genetic variation, identifying 20 distinct MLST types among 21 isolates, including five novel variants. In animals, assemblage E was most common (62 %). This study provides a comprehensive overview of Giardia assemblages in human infections in Norway, offering insights into their genetic diversity. It also underscores the feasibility of employing MLST as a tool to evaluate potential epidemiological links of Giardia assemblage A isolates.