Since the late 20th century, Korea has shown a remarkable decline in parasite infection rates because of rapid economic growth and the government’s and parasitologists’ antihelminthic campaigns [2,7]. With regard to historical parasite infection before this change, detailed information could be obtained through research on mummy coprolites from the graves at that time [2]. Nevertheless, for the first half of the 20th century, reports on mummies of the time at archaeological sites have been extremely rare. Although toilet ruins are therefore bound to be the only useful specimen for studies on parasite infection of the time, parasitological reports on archaeological samples have so far been scant.

In 2021, an archaeological investigation was conducted at the Sewoon Reorganization Promotion Zone in Ipjeong-dong, Seoul City. During the Joseon Dynasty period, this area was located southeast of Old Seoul City. During the excavation, archaeologists identified various remains including 20 buildings and concrete red-brick structures. The buildings and structures were estimated to be of the late 19th to the early 20th century, from which fragments of Japanese porcelain, glass bottles, and coins were collected. Specifically, a wooden structure that resembles a toilet could be identified in Building No. 7, which is likely an underground structure of the toilet in terms of form. This toilet-like structure was built by stacking 2 wooden barrels, and the inside of it was full of soil sediments (Fig. 1A).

Investigating the barrels has allowed for the date estimation of the toilet. In our opinion, one barrel was first used as a container for early 20th-century Japanese liquor because the name of a liquor manufacturer, “Masamune,” could be identified on the barrel. Next, the name of a vinegar company, “Sasadahonke,” was also found on the barrel, so the vinegar company seemed to have recycled what was originally a liquor container as their vinegar container (Fig. 1B). If this is actually a toilet ruin, it could be the first report on a toilet during the early 20th-century colonial period. However, archaeologists could not confirm this estimation onsite. They commissioned us to examine the samples by parasitological method.

We decided to collect samples from different levels (layers I–VII) of wooden barrels while removing soil from top to bottom. Each soil specimen (1.33–4.64 g) was rehydrated in 0.5% trisodium phosphate solution and then filtered with multiple-layered gauze. The obtained precipitate was dissolved in 0.5% trisodium phosphate solution again (with a final volume of 20 ml). The specimen slides were examined using a light microscope (BH-2; Olympus, Tokyo, Japan) [8–10].

Among the 7 sample layers, we found ancient parasite eggs of Trichuris trichiura, Ascaris lumbricoides (unfertilized), and Taenia spp. in specimens from layers V, VI, and VII (Fig. 2), suggesting that the barrels could have been used as toilets at the time. Particularly, parasite eggs were found only in the lower 3 layers (V–VII), indicating that feces were deposited at the bottom of the barrel when the structure was used as a toilet. The soil on the upper layers (I–IV) might not be deposits of toilet feces but rather the topsoil that has been pushed inward when this barrel toilet was finally abandoned. Table 1 provides a summary of the parasitological examination’s eggs per gram (EPG) and related information. Our parasitological analysis suggests that the liquor-vinegar barrel appears to have been recycled as a toilet. Since the use of wooden barrels for toilets was a common tradition practiced in Edo-period Japan [11], the Japanese people who used this barrel toilet are likely those who lived in the area between 1910 and 1945.

Although the early 20th century might be deemed too recent of an era to be the subject of archaeoparasitological research, it is an important period to understand how parasite infections have historically changed in East Asia, considering that it was just before the late 20th century, when parasite infection rates sharply dropped. We note that a parasitic examination of early 20th-century specimens has also been reported in Taiwan. Yeh et al. [12] published a report on samples from a Japanese police outpost stationed in Taiwan from 1921 to 1944. In these specimens, they confirmed the presence of ancient parasite eggs of Eurytrema sp., T. trichiura, and A. lumbricoides. Taken together, despite years of archaeoparasitological research, some academic queries about parasitism in East Asian history have yet to be fully answered. More research should focus on early 20th-century ruins to understand the achievements of the anthelminthic campaign after the 1960s on the reduction in parasite infection rates in East Asian countries.