Echinococcosis is a worldwide zoonotic parasitic disease that seriously endangers the health and life safety of people. It also affects social and economic development, brings heavy economic burdens to the families of patients, and is one of the major reasons that lead people in endemic areas of China to become poor and live in poverty due to illness [21]. To deeply understand the prevalence of human echinococcosis in Sichuan province, we herein performed a survey in 35 epidemic counties previously identified in 2012 from 2016 to 2019. We specifically investigated the epidemic areas and human echinococcosis at the township level in Sichuan province. Our results revealed that the echinococcosis patients were distributed in 426 townships, which were primarily located in the northwestern part of Sichuan province and concentrated in high mountain meadows, pastoral areas with a cold, arid climate and little rainfall, and semi-agricultural and semi-pastoral areas. A major reason for this is that Echinococcus eggs are well adapted for cold, dry, and rainless natural environments. In addition, there were abundant animal resources in these environments, which form a relatively suitable food chain for predation and prey, thus enabling Echinococcus spp. to form a complete life history, resulting in the prevalence of echinococcosis [22, 23]. We also found that CE and AE were prevalent in 405 and 167 townships, CE epidemic areas were more widespread compared with AE. This observation may be attributed to the fact that CE is mainly spread in the biological circulation chain composed of domestic dogs as the main definitive host and domestic animals as the main intermediate hosts, while AE is primarily spread in the biological circulation chain composed of foxes and dogs as the main definitive host and small mammals as the main intermediate hosts. In comparison, the distribution range of small mammals is smaller than that of domestic animals [24–27].
Among the 426 townships where patients were distributed, we noted that there were 64 townships with a human population prevalence of ≥ 1.00%, which were predominantly distributed in some townships of Shiqu, Seda, Ganzi, Dege, and Baiyu counties in the northwest part of Sichuan province. Similarly, the spatial distribution of human echinococcosis prevalence map elucidated that echinococcosis was largely spread in Sichuan province, particularly with a high prevalence in the northwest.
Trend surface analysis is a commonly used spatial statistical analysis method for evaluating the trend and gradual change of spatial geographical position on a regional scale. It often establishes a polynomial regression mathematical model and carries out interpolation analysis according to the observed values of observed variables and the polynomial regression results of their sampling locations, so as to obtain one-dimensional, two-dimensional, or three-dimensional continuous line segments, planes, or solid surfaces [28]. Based on the results of the trend analysis, we demonstrated that the predominance of human echinococcosis in Sichuan province followed a trend in both the east-west and north-south directions for most of the years, gradually decreasing from west to east and from north to south. These findings were concurred with the results of trend surface analysis of echinococcosis conducted by Qi Yanfeng in Aba Prefecture in 2013 and also with the spatial and temporal distribution characteristics of the new human echinococcosis prevalence in Sichuan province between 2007 and 2017 by He Wei et al. The true explanations why the areas with high prevalence were essentially distributed in the northwest of Sichuan may be as follows. First, the climatic conditions of high altitude and low temperature favored the Echinococcus eggs to survive for a long time to exacerbate the spread of echinococcosis. Second, the existence of a large number of intermediate hosts increased the spread of diseases, developed animal husbandry was brought a large number of intermediate hosts of CE, and grassland was also a favorable environment for the intermediate hosts of AE (small mammals). Previous findings have shown that most of the areas with high prevalence had developed animal husbandry. Third, close contact with dogs, feeding diseased organs to dogs, not washing hands before eating or preparing food, and lack of safe drinking water sources significantly increased the risk of infection [29–34].
According to the results of the 2012 national ecshinococcosis prevalence survey, the number of patients with CE in Sichuan province was distinctively higher than those with AE, while the findings of the 2016–2019 survey showed that AE patients accounted for 52.77% of the total number of patients. The major reasons for this discrepancy may be as follows. First, the infectious source of CE was primarily dogs, and since the implementation of the Echinococcosis Prevention and Control Project, Sichuan province had standardized the management and deworming of dogs, and reduced the number of stray dogs, the infectious source had been effectively controlled, so the number of newly found patients had decreased. Second, the surgical operation difficulty of CE was less compared to that of AE, and more CE patients were effectively cured [35, 36]. Third, the experience and diagnostic techniques of ultrasonographers differ from one region to another, and there may be caused inaccurate diagnosis.
Numerous reports have concluded that the prevalence and transmission of echinococcosis are influenced by natural, biological, and social factors, and were also restricted by the imago in the definitive host segments and eggs in the external environment, larvae in intermediate hosts and stability of parasites, and had spatial autocorrelation [37–42]. According to the literature, spatial autocorrelation can accurately reflect the aggregation degree of an indicator in a spatial unit. For example, if an indicator shows the same distribution between its center and its surroundings in space, it is referred to as a spatial positive correlation, if an indicator is spatially opposite, it is called spatial negative correlation, and ultimately if an indicator is spatially random, it indicates that the spatial correlation is not significant. In addition, spatial autocorrelation is categorized into global and local spatial autocorrelations. Specifically, the global spatial autocorrelation analysis is used to explore the spatial aggregation of an indicator over the entire study area, using a single value to reflect the autocorrelation of the whole region. Subsequently, the common indicators of analysis included Global Moran's I index, Getis-Ord General G coefficient, and the Geary C coefficient. On the other hand, the local spatial autocorrelation analysis is used to investigate the degree of correlation between an indicator in each spatial unit and neighboring units. To test whether its spatial autocorrelation is significant for the whole survey area, thus indicating the spatial distribution pattern of the index with high and low and low, the common analysis indexes include local Moran's I index and local Getis-Ord G coefficient [43, 44]. Therefore, spatial autocorrelation analysis can be employed in medicine and related fields, which can improve the understanding of researchers of the spatial distribution law of disease prevalence, hence providing a reliable basis for formulating disease prevention and control strategies. Here, we employed spatial autocorrelation statistical analysis to explore the spatial distribution characteristics of echinococcosis at the township level in Sichuan province. Through the global autocorrelation analysis of the total prevalence of echinococcosis, and the prevalence of populations CE and AE, we found that the predominance of different types of echinococcosis exhibited a positive spatial correlation, with an aggregated distribution.
Moreover, the results of global spatial autocorrelation analysis showed that the prevalence of echinococcosis was clustered in space with a positive spatial correlation, that is, the prevalence of neighboring areas of the townships with high prevalence was also high, while that of neighboring areas of the townships with low prevalence was also low, which was spatially diffuse. On the other hand, in the local spatial autocorrelation analysis, the LISA aggregation map clearly showed the “high-high” and “low-low” clusters of human echinococcosis prevalence at the township level. Of note, the “high-high” clusters were dominantly distributed in most of the townships of Shiqu, Seda, Ganzi, Dege, and Baiyu counties near the northwest of Sichuan province. The areas with high prevalence might be associated with special natural environmental and socio-geographical factors influenced by the high altitude. Meanwhile, the unhealthy production lifestyle of local Tibetans might have a great similarity due to the close geographical location, thus forming a high-risk behavior of echinococcosis. Additionally, the “low-low” gathering areas were mostly located in some townships of Jiuzhaigou, Songpan, Heishui, Mao, Li, Wenchuan, Xiaojin, Baoxing, Tianquan, Kangding, Luding, Muli, Daocheng, Xiangcheng, and Yuexi county near the southeast in Sichuan province. These areas did not belong to the Qinghai-Tibet plateau, and their altitude was relatively low, and the natural conditions were superior to those of high prevalence areas, production and lifestyle were relatively healthy, thus these risks were relatively small. These findings suggest that we need to strengthen the comprehensive prevention and control of echinococcosis in “high-high” gathering areas, at the same time, we should actively explore the favorable factors for the low prevalence of echinococcosis in “low-low” gathering areas to provide a baseline reference for the prevention and control of echinococcosis in “high-high” gathering areas. In this regard, we thus recommend the following suggestions. First, based on previous screening results, “high-high” gathering areas should carry out herd screening in endemic villages, especially in remote agricultural and pastoral areas, nomadic and migratory groups; “low-low” congregation areas should strengthen screening in pastoral areas, temples, and other key areas. Second, “high-high” gathering townships should perform standardized management on dogs, bring dogs into legal management, establish a standardized management and control mechanism for dogs, manage household registration of domestic dogs, establish a regular deworming system for domestic dogs in combination with local conditions, establish the deworming day and conduct its regularly. Conversely, “low-low” gathering townships should prohibit the import of wild dogs and prevent the introduction of infection sources. Third, according to local conditions, “high-high” clustering townships should explore a centralized slaughter management mechanism for cattle and sheep. All endemic townships should strictly implement the quarantine system of cattle and sheep origin, take harmless treatment of diseased organs, immunize sheep in stock, ensure good immunization registration, and carry out small mammals in settlements and the surrounding 1 km radius in the epidemic areas of alveolar echinococcosis. Fourth, “high-high” gathering townships should actively organize and publicize echinococcosis prevention policies and knowledge, mobilize patients who meet the treatment conditions to carry out the treatment in medical institutions, make full use of various media to disseminate health knowledge, strengthen publicity and education to the masses in townships where echinococcosis was prevalent, guide key populations such as monks and nuns to support participation in echinococcosis prevention and control, and compile echinococcosis prevention and control materials parallel with local customs. Health education materials containing the core information of echinococcosis prevention and control are produced, farmers and herdsmen are guided to develop good health habits, and the knowledge of echinococcosis prevention and control is included in the health education content of schools in epidemic areas. Students are regularly guided to spread the knowledge of echinococcosis prevention and control to parents and society through “small hands pull big hands” activities. Finally, “low-low” gathering townships should strengthen the publicity of the knowledge of prevention and treatment of echinococcosis among the migrant population, in order to prevent infection from outside [45].