Clinical and epidemiological characteristics of hospitalized patients with Diphyllobothriasis in Japan: A nationwide administrative database study.

Diphyllobothriasis, a fish-borne infection caused by broad tapeworms and transmitted through the consumption of raw or undercooked fish, continues to pose a public health concern in Japan. However, nationwide epidemiological data on hospitalized patients are limited. We conducted a retrospective analysis using the Japanese Diagnosis Procedure Combination (DPC) inpatient administrative database from April 2014 to March 2021. Hospitalized patients with diphyllobothriasis were identified using the ICD-10 code B700. Demographic characteristics, clinical presentations, treatment patterns, temporal trends, and geographic distribution were evaluated. Population-standardized incidence rates were calculated per 100,000 population by prefecture. A total of 340 unique hospitalized patients were included. The mean age was 36.6 years, and 65.0% were male. The highest incidence rates were observed in Toyama, Kyoto, Kagawa, Ishikawa, and Nara prefectures. Seasonality was evident, with peaks in late spring and summer. Overt anemia and severe complications were uncommon. Praziquantel was administered in 73.2% of cases, and endoscopic procedures for worm removal were rare. This nationwide DPC-based study demonstrates marked geographic heterogeneity and clear seasonal patterns in hospitalized diphyllobothriasis in Japan, with a predominance among working-age adults. Population-standardized incidence rates provide an alternative perspective on regional patterns among hospitalized cases than absolute case counts. These findings enhance understanding of the inpatient burden and real-world management of diphyllobothriasis in contemporary Japan. (212 words).