In this study, by analysing cross-sectional data from an endemic area in China, we found that the infection proportion of clonorchiasis was high and that a large proportion of infected persons had moderate and heavy intensity infections. People aged 30–59 years had a higher infection proportion and a higher percentage of moderate and heavy infections than those in the other age groups. Males experienced a higher percentage of moderate and heavy intensity infections than females, which might be attributed to the longer duration, higher frequency and greater quantity of their raw fish consumptions. Additionally, people who had been previously diagnosed with clonorchiasis or had previously taken anti-parasitics had longer durations and higher frequencies of raw fish consumptions, while no differences were identified among people with different knowledge of clonorchiasis. Furthermore, a high frequency of raw fish consumption was a risk factor for a high intensity of infection.
The distribution characteristics of infection intensity and infection proportion among the sexes and age groups that were identified in this study were mainly related to living customs, i.e., raw fish consumption. Raw fish are often consumed in social gathering or at restaurants, and offering raw fish to guests is deemed a hospitable gesture; males have more opportunities to participate in these practices [8, 26, 27]. Moreover, raw fish are often enjoyed with alcoholic beverages, which is more common among males than females, except for among children [5, 28]. Furthermore, adult worms can survive in the body for decades. Consequently, adult males’ exposure and, consequently, their worm load are higher, which leads to higher infection intensity and higher prevalence. The decline in the overall prevalence of infection and the three types of infection intensity in those older than 60 is probably due to early death caused by clonorchiasis-related complications [5, 15]. Additionally, elderly individuals seek medical services more frequently due to clonorchiasis-related complications or unrelated diseases and then accept diagnosis and treatment [5].
At present, the management of clonorchiasis is focused on morbidity control with praziquantel [29]. In moderately endemic areas (prevalence rate: 20%-40%) [30], such as the area where our study site is located, yearly administration of praziquantel under selective chemotherapy resulted in a substantial decrease in the prevalence and intensity of infection within 3 years [30]. However, the sustainability of long-term achievements is challenging, as re-infection cannot be avoided in chemotherapy, especially in older age groups [24, 28, 31]. Information, education and communication (IEC) is usually combined with chemotherapy to enhance sustainability [6, 32]. However, according to our study, no significant difference in raw fish consumption behaviour was identified among people with different knowledge of clonorchiasis. Hence, other control measures should be implemented in endemic areas. Currently, in local endemic areas of China, particularly in village regions, toilets are built directly above or beside fish ponds, and unprocessed faeces can contaminate the water, which increases Clonorchis sinensis infection in snails and freshwater fish [33]. Sanitary toilets with a harmless processing design, which have been previously implemented in one epidemic area in China with promising effects [33], should be tried in other endemic areas. Moreover, residents in epidemic areas find it difficult to change their raw fish consumption habits. Therefore, more attention should be paid to the safety of freshwater fish. The infection rates and distribution of freshwater fish should be investigated in endemic areas, and infected ponds should be placed under surveillance. Additionally, metacercaria-tainted fish should be barred from markets [34]. Currently, the direct compression or artificial digestion of fish followed by detection under a microscope is used to examine Clonorchis sinensis metacercaria in freshwater fish, which is time consuming and labour-intensive and can allow Clonorchis sinensis to be easily confused with other parasites [34–36]. Meanwhile, PCR-based molecular biology techniques are expensive [34]; thus, rapid, convenient, inexpensive and accurate detection methods are urgently needed. Moreover, a notable avenue of vaccine research is the vaccination of the second intermediate host, i.e., freshwater fish, with feed probiotics. Indeed, an oral vaccine based on Bacillus subtilis expressing enolase is being tested in freshwater fish [6, 37].
One limitation exists in this study. Among the 728 persons who were infected with Clonorchis sinensis, only 538 (73.9%) participated in our questionnaire investigation. Therefore, the results might have been influenced. However, we compared the basic demographic information (sex and age) between these 538 subjects and the 190 non-participating persons, and no significant difference was identified. Thus, we believe this had a limited effect on the results.
Based on our results, the overall infection proportion and intensity of clonorchiasis were high at the study site, with males experiencing higher intensity of infection than females. People 30–59 years old had a higher burden of clonorchiasis than those in the other age groups. In addition, males showed a higher risk of raw fish consumption than females, and no difference in raw fish consumption behaviour was noted among people with different knowledge of clonorchiasis. A high frequency of raw fish consumption was associated with a high intensity of infection. More measures focused on contaminated faeces and intermediates, such as the reconstruction of toilets and the examination of freshwater fish, should be implemented along with chemotherapy and IEC in local endemic areas.