Clonorchiasis mainly occurs in East Asia and is associated with eating raw freshwater fish that carry the parasite [22]. Guangxi Province, where our patients resided, is part of southern China, and residents enjoy raw freshwater fish. In this study, clonorchiasis was diagnosed in 19.2% of the 1119 patients as a result of detected C. sinensis eggs in the fecal and/or bile samples. In addition, clonorchiasis mainly occurred in patients younger than 60 years old (155/215, 72.1%), with a 2.875-fold higher incidence in male than female patients. Clonorchiasis incidence in this cohort was significantly higher than that reported in the general Chinese population. Fang et al. [26] reported that clonorchiasis prevalence was 2.94% and 1.84% in males and females, respectively, with the highest prevalence found in the 50- to 59-year-old age group. Hoang et al. [27] reported that the prevalence in the male was 2.33 times higher than that in females in Vietnam, which supports our finding that more male clonorchiasis patients identified in southern China were susceptible to biliary or pancreatic disorders, including jaundice and elevated hepatic biochemical markers of TBil, DBil, ALT, AST, ALP, and GGT.

Detected C. sinensis eggs is direct evidence of clonorchiasis. The egg detection methods include Kato-Katz method (KK) and direct smear microscopy (DM) [18, 28]. The sensitivities of KK, FEC, and DM reported by Manuel et al. [29] were 71%, 50%, and 3%, respectively. The so-called “gold” standard combines the following four methods: KK, spontaneous sedimentation, FEC, and DM. However, Men et al. [19] referred to a combination of six KK plus two FECT methods as the “gold” standard. However, those standards involve multiple detection methods and a collection of consecutive fecal sample detection methods, which make them difficult to apply. Furthermore, when the infection activity is low or biliary obstruction is present, the probability of detecting eggs in fecal samples is extremely low. In our study, FEC was used for the detection of C. sinensis eggs in fecal and bile samples. We found that C. sinensis eggs were significantly more frequently detected in bile (79/92, 85.9%) than in fecal (54/92, 58.7%) samples, indicating that bile detection of C. sinensis eggs is preferable to fecal detection for the diagnosis of clonorchiasis in patients with biliary obstruction. Thus, bile detection should be incorporated into the established gold standards for diagnosing clonorchiasis.

Previous studies have identified male, eating raw fish, lower educational levels, and location of the villages as demographic or epidemiologic risk factors for clonorchiasis [22, 23, 30, 31]. Multivariate analysis showed that endoscopic diagnosis of clonorchiasis was significantly different between two groups, but it was not an independent risk factor for clonorchiasis. In addition, it could be influenced by subjective judgment (such as flocs and other parasites), resulting in a low sensitivity for direct diagnosis (15/41, 36.6%). Our research suggested both demographic factors of male and age ≤ 60 years old and CBD diameter < 12 mm as independent risk factors for clonorchiasis, while papilla fistula emerged as protective factor in the analysis. These findings are expected to identify a group of patients at high risk for clonorchiasis if they develop biliary or pancreatic disorders, especially in endemic regions.

When infected raw or undercooked fish is ingested by humans, the metacercariae excyst in the duodenum migrates into the intrahepatic bile ducts where eggs are laid. We wondered whether the excyst migration efficiency is related to papilla type. Our results showed that patients with normal papilla more frequently had clonorchiasis than patients with papillary fistula. As recently reported, positron emission tomography-computed tomography [32] can view the migration route within the host. This device may help investigate this issue among subjects with or without normal papilla.

We also compared the endoscopic procedures and outcomes between patients with and without clonorchiasis and found significant differences in conducting procedures including the cannulation method, EST, ENBD, stent implantation, balloon, and bile culture between the two groups. Patients with clonorchiasis were inclined to require guidewire cannulation, EST, ENBD, a single plastic stent, balloon, and bile culture. It is established that infection with C. sinensis is one of the most important factors for cholangiocarcinoma [33, 34], but our study found no significant differences in the brush results between clonorchiasis and nonclonorchiasis. The reason could be that patients with biliary or pancreatic disorders may not be representative; thus, future studies are needed to investigate the relationship between clonorchiasis and cholangiocarcinoma in a population without biliary or pancreatic disorders. There was no correlation between clonorchiasis and bile duct stones. As noted in this study, patients with clonorchiasis did not show special endoscopic features, so endoscopists must choose procedures after fully evaluating patients.

A few limitations of this study are worth mentioning. First, this was a retrospective study; thus, it may inevitably produce bias in data collection and selection. Prospective, large-cohort, multicenter studies are needed to confirm our findings. Second, only one assay (i.e., FEC) was used for the detection of C. sinensis in our study, and additional methods like KK were not used. Third, as bile samples were obtained during the ERCP procedure, the presence of C. sinensis eggs near the duodenal papilla may cause a positive bile finding, and ERCP procedures may have also increased the positive fecal finding. Fourth, the cohort we studied represented patients with biliary or pancreatic disorders, so these findings cannot be directly inferred to the general population.