Discussion

Among paragonimiasis cases reported by 13 medical institutions in Chongqing, males accounted for a significantly higher proportion than females. This finding aligned with findings from passive surveillance in Sichuan Province (2015–2020) and active surveillance in Anhui Province (2013–2016) (Liu et al. Reference Liu, Guo, Wang, Zhang and Zhu2017; Li et al. Reference Li, Chen, Xu, Zhang and Lu2022). The observed disparity was attributed to greater male exposure to endemic water bodies, coupled with higher engagement in catching or handling freshwater crustaceans due to societal behavioural roles (Li et al. Reference Li, Chen, Xu, Zhang and Lu2022). The age distribution of cases was predominantly concentrated in the <10-year and 10–19-year groups, with occupations primarily involving children and students. Educational attainment was mainly categorized as illiteracy (470 preschool children, 79.12%) or at the elementary school level. This trend toward younger age groups was likely related to adolescents’ insufficient awareness of disease risks, weak protective consciousness and prevalent habits of wading to catch crabs and consuming them raw or undercooked. Additionally, underdeveloped immune systems in children and adolescents increase susceptibility to parasitic infections. Limited disease awareness among younger individuals and their guardians may have contributed significantly to transmission dynamics (Zhang et al. Reference Zhang, Wang, Huang, Chen, Huang, Li and Qin2002).

From 2010 to 2024, the number of paragonimiasis cases reported by medical institutions in Chongqing showed an overall downward trend, consistent with the decline in serological positive rates observed in the 2015–2020 paragonimiasis surveillance results for Sichuan Province (Li et al. Reference Li, Chen, Xu, Zhang and Lu2022). This may indicate that the disease’s prevalence intensity had reached a relatively low level. Economic and social development, widespread health education and improved dietary hygiene practices among residents, particularly standardized cooking methods for potential vectors such as Potamoidea and Cambaridae, likely contributed to reduced infection risks (Lu et al. Reference Lu, Shi, Hu, Zhu, He and Zheng2006). Case reports exhibited a pronounced seasonal pattern, with 56.20% of cases concentrated between March and August. This aligned with a prior retrospective analysis of paragonimiasis incidence in Chongqing, which noted that over 50% of patients experienced peak onset during March–July (Peng et al. Reference Peng, Zhang, Zhang, Wang and Zhang2018). This pattern may be attributed to climatic conditions during March–August, directly enhancing the ecological efficiency of the transmission chain. Studies confirm that warm, humid climates favour the growth and reproduction of intermediate hosts such as freshwater snails, Potamoidea and Cambaridae (Zhu and Su, Reference Zhu and Su2013). Other studies indicated that within a certain temperature range, rising temperatures accelerated the reproduction rate of freshwater snails and shortened their generation time. Simultaneously, warmer environments favoured the survival of metacercariae in the external environment, prolonging their infectivity (Guan et al. Reference Guan, Liu, Wu, Lyu, Lu and Song2016). Furthermore, increased outdoor activities during spring and summer, such as farming, picnicking and water recreation, were associated with heightened exposure to metacercaria-contaminated water. Traditional practices of consuming Potamoidea and Cambaridae crustaceans raw or partially cooked were also prevalent during this period, with more frequent harvesting and consumption of these intermediate hosts significantly elevating the risk of oral infection. Although reported cases of paragonimiasis in Chongqing medical institutions showed a declining trend year by year, continuous strengthening of health education and behavioural interventions targeting adolescents was deemed necessary to enhance protective awareness. Health education and prevention awareness were found to directly influence residents’ understanding of paragonimiasis and their self-protection capabilities (Lu et al. Reference Lu, Shi, Hu, Zhu, He and Zheng2006; Lall et al. Reference Lall, Sahni and Rajput2013).

The regional distribution of paragonimiasis cases revealed that 52.3% originated from Chongqing. Spatially, cases were predominantly concentrated in northeastern Chongqing, consistent with previous surveys identifying the Three Gorges Reservoir area (Wushan, Wuxi, Fengjie and Yunyang) as moderately to highly endemic zones (Luo et al. Reference Luo, Yi, Liu and Gu1990). Analysis indicated that northeastern Chongqing constituted a persistent HH cluster, reflecting sustained high transmission intensity and shared high-risk characteristics among adjacent counties. Despite a declining case trend, the epidemic’s spatial scope continued to expand: the HH cluster expanded from 4 counties (Chengkou, Yunyang, Wuxi and Fengjie) during 2010–2014 to 6 counties by 2020–2024. Wanzhou District transitioned from an LH outlier to an HH cluster member, providing empirical evidence for geographic infection risk spread. Research indicated that rising water levels following the Three Gorges Project reservoir formation created additional stream habitats suitable for freshwater crabs, potentially elevating lung fluke transmission risk (Zhang et al. Reference Zhang, Wang, Wang, Chen, He, Niu, Li, Chen and Wang2012). Northeastern Chongqing lies within this ecological influence zone. Furthermore, studies documented uneven water conservation capacity distribution in the reservoir area, characterized by stronger downstream capacity and weaker upstream capacity. Northeastern Chongqing, dominated by forest ecosystems, possessed the strongest water conservation function (Ye et al. Reference Ye, Yang, Zhang, Wang, He and Zheng2022). This function sustained water resource availability for ecosystems and human populations. Abundant water resources and suitable ecological conditions provided favourable breeding habitats for lung fluke intermediate hosts (snails and stream crabs), explaining the high case clustering in this region.

Conversely, western Chongqing remained a low-incidence area. A stable LL cluster persisted throughout the study period, suggesting limited exposure to intermediate hosts in urban environments and distinct regional dietary habits. Local spatial autocorrelation analysis identified sporadic HL outliers, indicating isolated high-risk points: Jiangjin District reported an HL outlier during 2015–2019 (local cases surrounded by low-incidence counties), while Yubei and Jiulongpo Districts exhibited HL outliers during 2020–2024. These HL outliers indicated localized high-risk points within the overall low-incidence region, potentially arising from specific environmental conditions (e.g. suitable streams or crab populations) or behavioural factors (e.g. raw crab consumption). Although Jiangjin’s HL outlier did not persist in 2020–2024, the emergence of new HL outliers in Yubei and Jiulongpo suggested potential new localized transmission risks near urban centres. Targeted investigations were warranted to address these risks.

To assess spatial pattern robustness, a sensitivity analysis was conducted on clinically diagnosed and confirmed cases. Results showed that persistent HH clustering in northeastern Chongqing and stable LL clustering in western Chongqing remained significant after excluding suspected cases, confirming genuine spatial heterogeneity rather than classification bias. During 2010–2014, global clustering was not significant, but local analysis revealed HH clusters limited to counties like Yunyang and LH outliers in areas such as Wanzhou District. This indicates localized hotspots dominated early-stage distribution, with dispersed patterns preventing citywide global clustering. This aligns with early disease transmission dynamics, where focal outbreaks precede regional spread.

In summary, the spatial patterns indicated that northeastern Chongqing, as a persistent HH cluster, required prioritized cross-county collaborative prevention and control measures. Meanwhile, the LL cluster region in western Chongqing, despite a lower overall case burden, necessitated vigilance toward sporadic HL outliers, with in-depth investigations recommended when necessary to prevent the emergence of new transmission hotspots.

The case composition in this analysis highlights diagnostic challenges in clinical practice regarding Paragonimus infection. Only 1.66% of cases were confirmed by pathogen, with the majority classified as suspected or clinically diagnosed. Pathogen detection remains the gold standard for diagnosing paragonimiasis. Consistent with these findings, parasitological examinations during the 2013–2016 surveillance in Anhui Province (Liu et al. Reference Liu, Guo, Wang, Zhang and Zhu2017), the 2015–2020 surveillance in Sichuan Province (Li et al. Reference Li, Chen, Xu, Zhang and Lu2022) and the 2018–2020 infection survey in Hubei Province (Zhang et al. Reference Zhang, Xia, Zhang, Zhu, Wu, Wan and Cao2021) also failed to detect parasites or eggs. Most cases reported by medical institutions originated from Chongqing and Sichuan, regions endemic for P. skrjabini, where egg detection in sputum or stool samples is technically challenging (Zhu et al. Reference Zhu, Wei, Liu and Yang2016). This limitation arises from complex parasitological procedures and their limited availability in primary healthcare settings. Consequently, misdiagnosis and missed diagnosis remain persistent concerns in clinical management. The initial misdiagnosis rate of 30.40% closely aligned with that reported in a clinical analysis of 110 paediatric paragonimiasis cases (30.91%) (Chen et al. Reference Chen, Wang, Ju, Wang and Li2009), but was lower than misdiagnosis rates documented in some prior studies (45.16%–92.3%). Findings indicated that approximately one-third of paragonimiasis cases were not clinically suspected during initial consultations, suggesting misdiagnosis remains prevalent in clinical practice. This quantifies the extent of diagnostic challenges and underscores the need to enhance clinicians’ awareness of paragonimiasis in endemic regions.

Acknowledgement is made that healthcare system factors, including disparities in diagnostic capacity (e.g. ELISA test availability) and variations in testing methodologies, may have influenced observed spatial patterns. Diagnostic practices for paragonimiasis in Chongqing likely evolved throughout the 15-year study period, with increased clinician awareness and broader immunological testing potentially contributing to higher case detection rates in later years. This may partially explain the expansion of HH clusters in northeastern Chongqing after 2015. Conversely, regions with limited diagnostic tool accessibility may have experienced underreporting of cases, potentially contributing to persistent LL clusters in western Chongqing. However, sensitivity analyses incorporating only clinically diagnosed and confirmed cases demonstrated that the primary spatial patterns, particularly the enduring HH clustering in northeastern Chongqing and stable LL clustering in western Chongqing, remained robust even after excluding suspected cases. This consistency suggests that observed clustering is unlikely to be driven solely by healthcare-related biases and may instead reflect genuine geographic heterogeneity in infection risk, though healthcare system factors may contribute to the intensity and precision of these patterns. Several limitations are acknowledged. First, the data were obtained from reports submitted by medical institutions and did not include community-based populations, which may have contributed to an underestimation of the true infection burden. Second, ecological investigations of intermediate hosts and infection sources were not performed concurrently, thereby hindering the establishment of comprehensive evidence for the ‘environment-host-population’ transmission chain.

In summary, the number of paragonimiasis cases reported by 13 medical institutions in Chongqing from 2010 to 2024 showed a declining trend and exhibited distinct epidemiological characteristics, including concentration in younger age groups, marked seasonality and spatial clustering. Despite this decline, multiple lines of evidence indicate that paragonimiasis remains a public health concern, particularly in the Three Gorges Reservoir area of northeastern Chongqing (Zhang et al. Reference Zhang, Wang, Wang, Chen, He, Niu, Li, Chen and Wang2012, Reference Zhang, Peng, Liu, Fan, Zhang, Chen, He, Wu, Liu, Yang, Yu and Yun2014). It was recommended that in-depth epidemiological investigations be conducted in the Three Gorges Reservoir area of northeastern Chongqing, that community-based health education be strengthened, and that efforts be made to promote behavioural changes related to high-risk dietary practices among residents. Such measures would help reduce the health risks posed by Paragonimus infection, especially among children and adolescents. For medical institutions, specialized training for physicians across departments should be intensified to improve recognition of clinical manifestations of paragonimiasis, thereby reducing misdiagnosis and missed diagnosis resulting from limited awareness. Immunological detection methods, such as ELISA, should be actively promoted and implemented as primary tools for clinical supportive diagnosis and epidemiological screening. Furthermore, case detection and reporting systems require optimization, with enhanced information collection and quality control for reported cases.