PHWR Public Health Weekly Report

Parasitic worm transmission can occur through different routes, including transmission through soil, shellfish, arthropods, food, and direct contact. Most intestinal parasitic infections in the Republic of Korea are transmitted through the soil and food-borne routes. The rate of infection with parasites can be determined by conducting a nationwide survey assessing the prevalence of intestinal parasitic infections among the general population. According to the 8th National Survey of Intestinal Parasitic Infections conducted in the Republic of Korea in 2012, the overall rate of infection with intestinal parasites was 2.6%. The rate was decreased significantly compared to the first survey, which was over 80%. However, while the rate of infection with food-borne parasites decreased from 4.6% in the 1st survey (1971) to 2.8% in the 4th survey, it only slightly decreased to 2.16% in the 8th survey (2012), compared with that in the 4th survey. Furthermore, food-borne infections still accounted for 73% of the overall infection rate of 2.6% [1,2]. Unlike soil-borne parasitic infections, which have reached near-eradication levels, food-borne parasitic infections persist due to the widespread consumption of raw freshwater fish. The Division of Vectors and Parasitic Diseases at the Korea Disease Control and Prevention Agency (KDCA) has been conducting a surveillance project on intestinal parasitic infections in high endemic area affected by food-borne trematodiasis. This paper presents an analysis of infection rates for various parasitic strains and regional infection rates based on a surveillance project conducted in 2022.

The surveillance project for intestinal parasitic infections included testing of 200 to 2,000 participants in the following 36 regions: Goesan-gun, Yeongdong-gun, and Okcheon-gun in Chungcheongbuk-do; Gongju-si and Geumsan-gun in Chungcheongnam-do; Namwon-si, Jinan-gun, Sunchang-gun, and Imsil-gun in Jeollabuk-do; Gurye-gun, Suncheon-si, Naju-si, Gwangyang-si, Damyang-gun, Gokseong-gun, Boseong-gun, Hwasun-gun, Gangjin-gun, Haenam-gun, and Hampyeong-gun in Jeollanam-do; Pohang-si, Andong-si, Sangju-si, Cheongsong-gun, Yeongcheon-si, and Yecheon-gun in Gyeongsangbuk-do; Jinju-si, Miryang-si, Uiryeong-gun, Haman-gun, Changnyeong-gun, Hadong-gun, Sanchung-gun, Hamyang-gun, Geochang-gun, and Hapcheon-gun in Gyeongsangnam-do. The KDCA was responsible for planning, managing, and conducting screening tests as part of the surveillance project. Public health centers played a role in collecting samples, providing education on infection prevention, and supporting the treatment of infected individuals. The Korea Association of Health Promotion was responsible for performing diagnostic work specifically related to parasitic infections. A total of 28,918 collected samples were tested for 11 different types of intestinal parasitic infections [3]. Parasitic eggs were isolated using a sedimentation technique with water-ether and subsequently examined. The infection intensity of Clonorchis sinensis eggs was determined by calculating the egg count per 1 g of sample.

According to the results obtained from the surveillance project on intestinal parasitic infections, the infection rate among the 28,918 residents in the target regions increased by 0.1% to 5.3% (n=1,529) compared with that in the previous year. C. sinensis had the highest infection rate (3.3%, n=951), followed by heterophyid flukes (1.7%, n=506), Trichuris trichiura (0.2%, n=44), Gymnophalloides seoi (0.1%, n=26), and Echinostoma hortense (0.003%, n=1). Additionally, one case of Capillaria philippinensis infection was reported (Table 1). Compared with the infection rate in female (3.7%), that in male was twice as high (7.6%). Furthermore, the infection rate was the highest (6.7%) in individuals in their 50s and 60s, followed by individuals in their 70s (4.8%), 80s (3.8%), and 90s and older (3.1%). Those in their 40s or younger had infection rates of ≤3% (Figure 1). Heterophyid flukes infection was identified in a male resident aged ≤20 years in Gurye-gun, a region with a high rate of heterophyid flukes infection (3.0%). It is speculated that the infection may have transmitted through the consumption of raw freshwater fish, such as trout, which act as intermediate hosts for heterophyid flukes Jeollanam-do had the highest intestinal parasitic infection rate (6.5%), with a 1.3% increase from the previous year. This could be attributed to Gwangyang-si having the highest positivity rate (12.9%) among all cities and counties along with significant increases in infection rates in Suncheon-si (9.8%) and Hwasun-si (8.4%), which was more than double than that in the previous year. Gyeongsangbuk-do had the second-highest intestinal parasitic infection rate, 6.2%, which was lower than the rate in the previous year (7.0%) (Figure 2). Among the 36 surveilled regions, 12 had infection rates higher than the average infection rate of 5.3%. The infection rate was the highest, 12.9%, in Gwangyang-si, followed by Andong-si (10.1%), Suncheon-si (9.8%), Hadong-gun (8.7%), and Hwasun-gun (8.4%). Gwangyang-si has shown a consistent increase in the infection rate since 2019. Additionally, in 2021, Suncheon-si had a 2.4% positivity rate, while Hwasun-gun had a positivity rate of 3.1%, both showing a more than two-fold increase in the infection rate over the previous year. With regard to the infection rates by river basins, the infection rate was the highest for the Seomjin River (6.2%), followed by the Nakdong River (5.9%) and the Yeongsan River (3.0%). Although the Geum River had the highest infection rate in 2021, owing to the high infection rate in Yeongdong-gun (10.0% in 2021), the infection rate for the river was the lowest among all river basins in 2022 because the infection rate in Yeongdong-gun decreased to 3.4% (Figure 3). The intensity of C. sinensis infection, measured as eggs per gram (EPG), showed a proportional increase corresponding to the infection rate for each city (Figure 4). Two individuals in Gyeongsangbuk-do tested positive with an EPG value of 5,001 or higher, while 16 individuals had EPG values ranging from 1,001 to 5,000. A total of 28 individuals had an EPG value of 1,001 or higher. Among them, 18 (64.2%) were from Gyeongsangbuk-do, indicating that Gyeongsangbuk-do had a higher infection rate than those for the other regions.

Figure 1. Comparison intestinal parasites infection rates by age and sex

Figure 2. The infection rates by provinces (do)

Figure 3. Comparison of intestinal parasite infection rates by river basin

Figure 4. Comparison Clonorchis sinensis infection intensity (eggs per gram) by administrative district

Table 1 Egg positive rate of intestinal parasites

Investigation	Intestinal infection cases and rates (%)
	Total	Clonorchis sinensis	Heterophyid flukes	Trichuris trichiura	Gymnophalloides seoi	Echinostoma hortense	Capillaria philippinensis
28,918	1,529 (5.3)	951 (3.3)	506 (1.7)	44 (0.2)	26 (0.1)	1 (0.003)	1 (0.003)

The intestinal parasitic infection surveillance project started in eight regions in 2005 and has since expanded to cover 36 regions as of 2022. A total of 86 regions have participated in the project at least once until 2022, with a cumulative total of 537,497 tests performed. Based on the survey results, the intestinal parasitic infection rate was 11.0% in 2005. Over the years, it has decreased by over two-fold to 5.3% in 2022. However, the infection rate has remained relatively stable at 5% since 2015, with no significant decrease observed [3]. C. sinensis, which is transmitted through food, accounts for the majority of infection cases and has shown persistent prevalence in areas around river basins. The survey identified individuals in their 50s and 60s residing in areas around the Seomjin River and Nakdong River basins as high-risk groups, suggesting that people in this age range continue to maintain the habits of eating raw freshwater fish [4]. C. sinensis, having the highest infection rate among intestinal parasitic worms, accounted for 61.9% of all infections, while heterophyid flukes accounted for 33.4% of all infections. This indicates that food-borne parasites contribute to approximately 95% of all intestinal parasitic infections. C. sinensis settles in the bile ducts, causing dilation of the ducts and the progression of chronic inflammation. Over time, this can potentially lead to the development of bile duct cancer [5]. While Gwangyang-si had the highest overall intestinal parasitic infection rate among the 36 cities and counties surveyed, Andong-si had the highest infection rate specifically for C. sinensis (9.7%). Giran-myeon exhibited a consistently high infection rate for the second consecutive year, emphasizing the need for a more focused investigation in Andong-si. Additionally, Imha-myeon, a newly investigated area, showed a high positivity rate (8.7%), highlighting the necessity for investigations in all regions within Andong-si. Based on these results, it is necessary to implement testing and treatment measures specifically targeting C. sinensis to reduce the rates of intestinal parasitic infections in the Republic of Korea. The Division of Vectors and Parasitic Diseases at the KDCA plans to continue conducting more detailed analyses and testing for each town in high endemic area, as well as adding new areas for monitoring. It also plans to introduce an eradication certification system to further the efforts toward reducing the prevalence of intestinal parasitic infections.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: HIL. Data curation: JWJ. Methodology: SOB. Investigation: YJL, Writing–original draft: MRL. Writing–review & editing: HIL.