The number of reported amebiasis cases in Japan decreased significantly in 2018 and 2020 (Figs. 1 and 2, Tables 1 and 2), and these significant decreases were seen regardless of place of infection (domestic or imported) and disease type (intestinal or extraintestinal amebiasis). Hadano et al. showed significant decreases in the numbers of total, intestinal, and extraintestinal weekly reported cases of amebiasis in 2018 and 2019 compared with the preceding years (2014–2017) [4]. Of note, the present study showed that the same period saw significant reductions of the numbers of both domestic and imported cases. In this period, the antibody test reagent stopped being distributed commercially [4, 10], which might have reduced the number of reported amebiasis cases, regardless of the place of infection. Hirae et al. demonstrated a significant decrease in the total number of amebiasis cases in Japan in the intra-pandemic period (2020–2021) compared with the pre-pandemic period (2015–2019) [5]. The present results show that the COVID-19 pandemic might have reduced the number of amebiasis cases regardless of place of infection (domestic or imported) and disease type (intestinal or extraintestinal amebiasis). One can infer that the reduction of amebiasis cases in the intra-pandemic period was brought about not only by the travel ban to and/or from other countries [5], but also by the reduction of socioeconomic activity within the country during that period.
Recent epidemiological trends in amebiasis cases in Japan during and after the COVID-19 pandemic period might have behaved differently from other STDs such as syphilis and HIV. Reported cases of amebiasis in Japan decreased after 2018 and 2020, and the number has not rebounded up to 2022 (Fig. 1a, Table 1). The annual report has not been published yet as of July 16, 2024, but according to the year-end issue of the weekly report [7] of 2023, the tentative accumulated number of reported amebiasis cases in that year was 485 (the number for 2022 was 533, according to the annual report). Reduced numbers of cases in 2020 were also reported for HIV [11] and syphilis [12]. Komori et al. [12] showed a rapid resurgence of the number of syphilis cases reported in Japan in 2021. The rebound trend has also been reported for HIV [13] in 2023. The reason why epidemiological trends differ between amebiasis and these diseases is unclear, but amebiasis as a fecal-oral transmitted disease might have been the key.
Japan is among a few countries where nationwide epidemiological data for amebiasis are available, as well as Taiwan, and trends are similar between the two nations. In Japan, the mode age class of male amebiasis cases was in the 40s and 50s (Fig. 4), which is older than that of female cases, with most cases reported in metropolitan areas (Fig. 5). In Taiwan, the age class mode of male domestic cases was in the 30s, older than that of female cases [14]. In addition, about 40% of male domestic cases were reported in the capital city, Taipei. For comparison, in Western Sydney, Australia, 88% of confirmed cases of amebiasis were male cases, with a median age of 49 years [15]. Although nationwide epidemiological information is scarce for developed countries other than Japan and Taiwan, we can infer that the same trends in amebiasis cases (male-to-female difference and urban occurrence) might apply to other developed nations.
Inside Japan, there exists epidemiological similarity between amebiasis and HIV infection. Ruzicka et al. analyzed a hospital claims database in Japan, showing that 90% of people living with HIV on antiretrovirals are male, and the most frequent age class is in the 40s [16]. This feature coincides with the epidemiology of amebiasis shown in the present study (Figs. 3 and 4), which has been partially mentioned in the previous literature [3, 17]. To the best of our knowledge, no reports have shown that amebiasis is spread among MSM using nationwide epidemiological data, but, given the similarity with HIV epidemiology, one can infer that one aspect of the spread of amebiasis is as an STD among MSM.
There are several limitations in this study. First, even though the government has not taken any measures to prevent amebiasis from spreading in recent years, significant reductions of case numbers in 2018 and 2020 might have been due to a natural decrease caused by improvement of general hygiene. Second, the NESID reporting system is well established and considered to be reliable, but there may be a discrepancy between the reported numbers and the actual numbers of cases. Of concern, amebiasis cases without subjective symptoms are not diagnosed and not reported. Furthermore, though amebiasis as an STD among MSM was discussed, based on the similarity of its epidemiology with that of HIV, the most frequent reported route of infection [3] is “unknown”; thus, the actual status of amebiasis as an STD remains unclear. Future studies are needed to clarify actual routes of infection of amebiasis, and if possible, describe routes of infection by sex and by age class.