Giardiasis in Japan, the United States of America, and Europe from 2007 to 2023, highlighting a decline during the COVID-19 pandemic

Background Giardiasis is one of the most common enteric protozoal infections worldwide. It is frequently reported as an enteric parasitic infection in the United States of America (USA) and Europe, whereas reported incidence in Japan is substantially lower. This study characterized age-specific, geographic, and temporal patterns of reported giardiasis incidence in Japan, the USA, and Europe using publicly available surveillance data. Methodology/principal findings This retrospective study analyzed surveillance data from the National Institute of Infectious Diseases (NIID) in Japan, the Centers for Disease Control and Prevention (CDC), and the European Centre for Disease Prevention and Control (ECDC) to describe epidemiological patterns, possible explanatory factors, and temporal trends before and during the COVID-19 pandemic. Annual trends were examined using all available surveillance years: 2007–2023 for Japan and Europe and 2007–2022 for the USA. Age-specific patterns were compared between the pre-pandemic (2017–2019) and pandemic (2020–2022) periods, and geographic patterns were examined using available regional data. Incidence rates in the USA and Europe were approximately 5 per 100,000 population, whereas reported incidence in Japan remained below 0.1 per 100,000. Age-specific patterns differed: incidence peaked in children aged 0–14 years in Europe, and in children aged 0–4 years and adults aged 25–39 years in the USA, whereas in Japan, incidence was highest among adults aged 25–44 years and relatively high among those aged ≥65 years. Geographic patterns also differed, with higher incidence in Tokyo, Nordic and Baltic countries and selected European countries, and New England and Midwestern states. Annual incidence was significantly lower in the COVID-19-onset period than in the pre-COVID-19 period across all regions.