The Survey of Infection Status of Clonorchis sinensis Metacercariae of Freshwater Fish in 2022

Abstract

The Division of Vectors and Parasitic Diseases, Korea Disease Control and Prevention Agency have been investigated the infection status of Clonorchis sinensis metacercariae (CsMc) in freshwater fish as part of an effort for reducing the parasite infection rate in the Republic of Korea. In this survey, all fishes were collected in 10 regions and examined infection rate using the artificial digestion method. The infection rate of CsMc was highest in Youngjeon Stream (50%) located in Chungsong-gun, Gyeongsangbuk-do. The infection rate of CsMc by fish species was highest in Pungtungia herzi with 1,258 CsMc followed by Squalidus japonicus Koreanus, Microphysogobio yaluensis, Pseudorasbora parva, Acheilognathus lanceolatus. Infection risk that infection intensity per fish was highest in Gilan Stream with 62.3. To reduce infection rate of Clonorchis sinensis (C. sinensis, Cs), it is necessary to conduct survey on infection risk in C. sinensis prevalent areas and provide information to local residents people about at the level of infection risk.

Key messages

① What is known previously?

In some areas, it has been confirmed that the infection intensity of C. sinensis metacercariae still very high and this result showed that C. sinensis life cycle was continuing.

② What new information is presented?

The infection risk that infection intensity per fish was the highest in Gilan Stream located in Andong-si and Youngjeon Stream in Chungsong-gun was increased 3 times than last year.

③ What are implications?

To reduce the infection rate among residents, it is necessary to investigate the infection risk of C. sinensis metacercariae in freshwater fish and inform the local residents about the risk of infection, so it need to change their dietary habit of eating raw fish.

Introduction

Soil-transmitted parasites accounted for most of the parasitic infections in the Republic of Korea (ROK) in the past, but currently, they are maintained at the eradication level [1]. However, according to the Surveillance project on intestinal parasitic infections in high endemic area conducted by the Division of Vectors and Parasitic Diseases of the Korea Disease Control and Prevention Agency (KDCA), the infection rate is approximately 3%, showing that it is still prevalent in some regions [2]. In particular, Clonorchis sinensis (C. sinensis, Cs), which has the highest infection rate among food-borne parasites, can cause pathological changes such as dilatation of the hepatobiliary tract, thickening and inflammation of the bile duct wall, proliferation of biliary epithelial cells, and biliary fibrosis; furthermore, this infection might lead to the development of bile duct cancer [3]. Infection by Cs is owing to the consumption of freshwater fish, an intermediate host. Hence, in addition to testing and medication management, it is necessary to investigate the level of infection in freshwater fish, which is the source of infection, and inform residents about the level of infection risk to reduce the chance of infection.

Methods

The Cs intermediate host investigation region was selected in 10 regions of Gangwon-do (Seom River, Wonju-si and Togyo Reservoir, Cheorwon-gun), Gyeongsangnam-do (Deokcheon River, Sancheong-gun and Hoengcheon River, Hadong-gun), Gyeongsangbuk-do (Gilan Stream, Andong-si and Yongjeon Stream, Cheongsong-gun), Jeollanam-do (Seomjin River, Gurye-gun and Seomjin River, Gokseong-gun), Jeollabuk-do (Seomjin River, Sunchang-gun), and Chungcheongbuk-do (Yeongdong Stream, Yeongdong-gun), Which is river or stream within the Cs prevalent area among the intestinal parasitic disease investigation project regions of the KDCA. The investigation was conducted mainly from May to October, when it was able to catch fish a lot. The subjects of the investigation were freshwater fish such as Pungtungia herzi (P. herzi) and Pseudorasbora parva (P. parva), which are known to be major hosts of Cs, and approximately 100 individuals were collected in each region. After morphological identification of the collected freshwater fish, the length and weight of each individual were measured. Each individual was placed in artificial digestive juice and digested for two hours at 36°C to detect and identify metacercariae (Mc) for each individual. The entire digests were filtered with a 1 mm mesh sieve, allowing Mc to precipitate, and then washed with 0.85% saline solution. After the supernatant was sufficiently clear, the supernatant was removed. Subsequently, the precipitate that sank to the bottom was observed under a stereomicroscope for identification based on the morphological characteristics of Clonorchis sinensis metacercariae (CsMc). After identification, Mc were measured to analyze by region (by water system), type, infection rate, and infection density. The infection risk is expressed as (number infected with Mc/number of freshwater fish tested)×mean number of Mc per infected fish.

Results

We collected a total of 1,069 freshwater fishes in 36 species including P. herzi from 10 regions and determined Cs infection. The infection rate of CsMc in the collected freshwater fish by region was the highest in the Yongjeon Stream in Cheongsong-gun, Gyeongsangbuk-do at 50% (51/103 individuals), followed by Gilan Stream in Andong-si, Gyeongsangbuk-do at 49% (49/101), Togyo Reservoir in Cheorwon-gun, Gangwon-do at 18% (38/211), Seom River in Wonju-si, Gangwon-do at 11% (11/100), Deokcheon River in Sancheong-gun, Gyeongsangnam-do at 7% (7/103), Seomjin River in Gokseong-gun, Jeollanam-do at 6% (5/88), Yeongdong Stream in Yeongdong-gun, Chungcheongbuk-do 4% (4/105), and Seomjin River in Gurye-gun Jeollanam-do at 2% (2/98). The CsMc were not detected in the Hoengcheon River in Hadong-gun, Gyeongsangnam-do, and the Seomjin River in Sunchang-gun, Jeollabuk-do. Analysis of the infection risk by region (average level of Cs infection per fish) showed that the Gilan Stream had the highest risk at 62.3, followed by the Yongjeon Stream (40.3), Togyo Reservoir (21.6), and Deokcheon River (1.9). The infection risk was less than one in the other regions (Table 1). The level of CsMc infection in each freshwater fish ranged from a minimum of 1 to a maximum of 1,258. The fish most infected with CsMc was P. herzi, with 1,258 Mc from one fish, followed by 962 in Squalidus japonicus Koreanus, 891 in Microphysogobio yaluensis, 592 in P. parva, and 267 in Acheilognathus lanceolatus. The remaining 11 species of infected fish had relatively low infection rates, with the number of Mc per fish being less than 30 (Figure 1).

Table 1. The infection rate of Clonorchis sinensis metarcercariae in each river.

Area	No. of fish examined	No. of fish infected (%)	No. of metarcercariae detected			Infection risk
			Total	Range	Average
Seom River (Wonju-si, Kwangwon-do)	100	11 (11)	40	1–12	4	1.0
Togyo Reservoir (Chulwon-gun, Kwangwon-do)	211	38 (18)	1,659	1–592	44	21.6
Deokcheon River (Sancheong-gun, Gyeongsangnam-do)	103	7 (7)	116	1–110	17	1.9
Hoengcheon River (Hadong-gun, Gyeongsangnam-do)	98	0 (0)	0	0	0	0
Gilan Stream (Andong-si, Gyeongsangbuk-do)	101	49 (49)	6,105	1–1,258	125	62.3
Yongjeon Stream (Chungsong-gun, Gyeongsangbuk-do)	103	51 (50)	4,030	1–1,177	79	40.3
Seomjin River (Gurye-gun, Jeollanam-do)	98	2 (2)	4	1–3	2	0.2
Seomjin River (Gokseong-gun, Jeollanam-do)	88	5 (6)	7	1–3	1	0.1
Seomjin River (Sunchang-gun, Jeollabuk-do)	62	0 (0)	0	0	0	0
Yeongdong Stream (Yeongdong-gun, Chungcheongbuk-do)	105	4 (4)	5	1–2	1	0.3

Figure 1. Infection intensity of Clonorchis sinensis (C. sinensis) per fish.

Comparison of the infection risk in 2022 with that in the previous year indicated the region with the most significant increase was the Yongjeon Stream in Cheongsong-gun, where the infection risk was 13.57 in 2021 but increased by approximately three times to 40.3 in 2022. In addition, in the Togyo Reservoir in Cheorwon-gun, it was 21.55, an increase of 6.47 compared to that in the previous year. However, infection risk the Deokcheon River in Sancheong-gun decreased from 7.42 in 2021 to 1.93 in 2022. The extent of change was not significant in other regions. The Gilan Stream in Andong-si, Gyeongsangbuk-do, was investigated in 2022 for the first time and was the region with the highest infection risk among regions (Figure 2).

Figure 2. Infection risk of investigated area in 2021 and 2022 year.

Discussion

To reduce fish-borne parasitic infections such as Cs in the ROK, an approach different from the method of preventing and managing soil-transmitted parasites that were prevalent in the past is required. For soil-transmitted parasites, the focus was on improving hygiene and environmental issues. However, as fish-borne parasites are transmitted to humans by consuming raw or undercooked freshwater fish, an intermediate host; the source of infection should be thoroughly investigated and residents in regions at risk of infection should be simultaneously informed about the risks of infection such that they can avoid freshwater fish consumption. This investigation identified the level of CsMc infection in freshwater fish, focusing on regions where Cs is prevalent. P. herzi, which is the most infected with CsMc, was collected from all rivers except the Togyo Reservoir to determine the level of infection. In particular, although P. herzi was collected at a similar level in the Gilan Stream and Seomjin River, the number of CsMc infections was found to be markedly different regionally, with an average of 125 in the Gilan Stream compared to 1 in the Seomjin River. The infection rate of Cs among residents living near the Yongjeon Stream in Gyeongsangbuk-do, where the infection rate of Mc in freshwater fish was high, was significantly higher than the average infection rate in other regions, showing a close correlation between the infection risk index of intermediate hosts and the infection rate of humans, the definitive host. In addition, the Cs infection rate of freshwater fish in the Gilan Stream in Andong-si was 49%, and the average number of infected Mc was 125, indicating that Cs infection was the highest among the 10 regions investigated (Table 1). Gilan-myeon, where the Glian Stream in Andong-si passes, was found to be a highly infected region with a Cs infection rate of 27.9% in 2021 and 14.9% in 2022 [4]. Despite the infection risk in the Togyo Reservoir in Cheorwon-gun, Gangwon-do, continuing to rise compared to that in the past, the human infection rate is very low, which is thought to be because residents rarely eat raw freshwater fish. Therefore, as part of efforts to lower the infection rate, local governments should analyze the infection risk of CsMc in freshwater fish and inform residents about the risks of eating raw freshwater fish. Furthermore, continued efforts should be made to reduce the Cs infection rate implementing education for residents.