Vital Surveillances: Rodent Ecology and Etiological Investigation in China: Results from Vector Biology Surveillance — Shandong Province, China, 2012–2022

Rodent surveillance data from 2012 to 2022 were obtained from surveillance sites in 16 regions of Shandong Province. Survey habitats were classified as urban residential areas, rural residential areas, and key industries (catering, food production and sale, construction sites, slaughterhouses, breweries, etc.). Year-round surveillance was conducted from 2012 to 2015. In 2016, the National Vector Surveillance Implementation Plan was issued, and surveillance was carried out in odd-numbered months from 2016 to 2022.

From 2020 to 2022, rodents captured for pathogenic surveillance were distributed across 12 regions in Shandong Province. The surveying time and habitat were consistent with the ecological surveying of rodents. The methods of nucleic acid extraction and pathogen detection were as previously described (6). Briefly, DNA was extracted from the liver, kidney, and spleen using the QIAGEN RNeasy Mini Kit (QIAGEN, Germany) and analyzed for the detection of Leptospira interrogans (L.interrogans), Rickettsia typhi (R. typhi), Orientia tsutsugamushi (Ot), Anaplasma phagocytophilum (A. phagocytophilum), and Francisella tularensis (F. tularensis) via real-time PCR assay (qPCR). RNA was extracted using the QIAGEN RNeasy Mini Kit (QIAGEN, Germany). RNA extractions from the liver, kidney, and spleen were analyzed for the detection of Dabie bandavirus (DBV), and RNA extracted from the lung was analyzed for the detection of HV via real-time reverse-transcriptase PCR assay (qRT-PCR). The positive status of specimen tests is shown in Supplementary Table S1. Count data were analyzed using the χ² test and Fisher exact tests with R software (version 4.0.2; TUNA Team, Tsinghua University, Beijing, China). A P<0.05 was considered statistically significant.

From 2012 to 2022, a total of 593,563 effective traps (cages) were deployed at rodent surveying sites in Shandong Province, resulting in the capture of 4,145 rodents. The average rodent capture rate was 0.70% [95% confidence interval (CI): 0.68%–0.87%]. The highest average annual rodent capture rate was 1.00% in 2012, while the lowest was 0.51% in 2022. Overall, rodent densities displayed a fluctuating downward trend (Z=−3.581, P<0.001) (Figure 1A). Among the rodent surveying sites, Dongying (1.99%) had the highest rodent capture rate, followed by Heze (1.23%), while Binzhou (0.29%) had the lowest (Supplementary Table S2); this difference was statistically significant (χ²=981.33, P<0.001).

Figure 1. 

Annual trends in rodent capture rate and composition ratio from 2012 to 2022. (A) The trend of rodent capture rates; (B) Annual changes in rodent capture composition.

Note: The error bars represent a 95% confidence interval of the rodent capture rate.

From 2012 to 2022, the capture rate of rodents in rural residential areas and other habitats (farmland and woodland) was relatively high, with Rattus norvegicus (R. norvegicus) as the dominant species (Table 1). In recent years, the proportion of R. norvegicus and Mus musculus (M. musculus) remained relatively high, while the proportion of A. agrarius and other species increased (Figure 1B).

The rodent capture composition differed among the 16 districts in Shandong Province (χ²=1023, P<0.001). Areas with high capture rates of R. norvegicus were mainly distributed in Dongying (0.82%) and Heze City (0.63%). Capture rates of M. musculus were relatively high in Dongying (0.81%), Weihai (0.56%), and Heze (0.56%). Capture rates of A. agrarius were relatively high in Dongying (0.30%) and Qingdao (0.11%). Most R. flavipectus were captured in Tai’an (0.05%) and Zibo (0.03%) cities (Supplementary Table S2).

From 2012 to 2022, a seasonal trend was observed in Shandong Province, with a higher rodent capture rate from March to September (Figure 2). However, some regional differences were noted. The regions with high rodent capture rates in winter and spring were mainly Jinan, Zaozhuang, Yantai, Weifang, Jining, Linyi, Dezhou, and Binzhou cities. The regions with high rodent capture rates in summer and autumn were mainly Zibo, Tai’an, and Weihai cities. The seasonal fluctuation of rodent capture rates in Dongying and Heze cities was not significant.

Figure 2. 

Heat map of seasonal changes in rodent surveillance in Shandong Province, China, from 2012 to 2022.

A total of 2,769 rodents (including shrews) were captured for the detection of rodent-borne pathogens. Among them, 2,748, 2,769, 2,647, 962, 1,042, 624, and 520 rodents were screened for HV, DBV, L. interrogans, R. typhi, Ot, A. phagocytophilum, and F. tularensis, respectively. Of the seven pathogens screened, HV had a positive rate of 2.58%, the highest positive rate in rodents captured in Shandong Province. The annual changes in the detection results of the various pathogens are shown in Supplementary Table S3.

HV was detected in R. norvegicus (4.58%), A. agrarius (2.29%), and R. flavipectus (0.85%). L. interrogans was detected in M. musculus (1.80%) and R. norvegicus (0.90%). R. typhi was detected in M. musculus (2.80%) and R. norvegicus (0.44%). A. phagocytophilum was detected only in S. murinus (20%), and F. tularensis was detected only in R. norvegicus (0.56%). DBV and Ot were not detected in any captured rodents (Table 2).

The rodent-borne pathogens found in rural residential areas were HV (2.22%), L. interrogans (1.22%), R. typhi (1.94%), and F. tularensis (0.41%). Pathogens found in rodents in key industries were HV (0.42%), L. interrogans (1.25%), R. typhi (0.20%), and A. phagocytophilum (0.29%). Pathogens detected in rodents captured in farmland and forestland were HV (9.64%) and L. interrogans (0.53%). None of the above seven pathogens were detected in urban residential areas (Table 1).

The total HV-positive rate was 2.58%, with higher rates found in Weifang (15.53%) and Qingdao (3.50%). The total L. interrogans-positive rate was 1.10%, with higher rates found in Jinan (9.00%) and Dongying (2.10%). The R. typhi-positive rate was only found in Jinan (9.00%). A. phagocytophilum and F. tularensis were found in one rodent from Jinan (1.00% each) (Supplementary Table S2).

In this study, the average rodent capture rate in Shandong Province was 0.70% from 2012 to 2022, relatively higher than in southern China but lower than in northern China (7-8). The overall rodent capture rate is decreasing, potentially due to sanitary city initiatives and enhanced rodent breeding site cleanup in rural residential areas (9). Rodent capture rates varied geographically; higher capture rates were primarily in eastern (Weihai, 1.22%; Qingdao, 0.93%), southwestern (Heze, 1.23%), and northern (Dongying, 1.99%) Shandong Province, largely consistent with the high HFRS prevalence in Shandong Province (5).

In our study, R. norvegicus was the major species, followed by M. musculus, A. agrarius, and R. flavipectus. These results were similar to those of an investigation in Zhejiang Province (7). This finding may be related to the habits and adaptations of various rodent species and suggests that R. norvegicus and M. musculus were the key rodent species for rodent control in human residential areas (10).

A previous study showed that the rodent capture rate increased during the spring breeding season, with two insignificant capture rate peaks in the spring and autumn in Shandong Province (11). In contrast, this study found that rodent seasonal fluctuation increased from winter to spring, peaked in May, July, and September, and did not exhibit two distinct capture rate peaks. A previous study indicated that monthly HFRS outbreaks significantly correlated with the rodent capture rate 2 months earlier (12), which was generally similar to HFRS incidence in Shandong Province (5).

Our results indicate that four of the seven pathogens tested were detected in rodents, with the exception of DBV and Ot. HV and L. interrogans were the most prevalent. R. norvegicus and A. agrarius were the main reservoirs of HV (13). The total HV-positive rate in rodents in our study was 2.58%, with HV circulating mainly in R. norvegicus and A. agrarius, with infection rates of 4.58% and 2.29%, respectively. The proportion of M. musculus carrying HV was very low in this study. Farmland and forestland (9.64%) were the key habitats for the detection of HV in rodents. Five regions had HV-positive results, with Weifang exhibiting a positive rate of over 15%. A previous study also found that farmers account for the largest proportion of all HFRS cases (13). One study showed that L. interrogans had the highest detection rate of 12.28% and tested positive in almost all common rodent species in Zhejiang Province (7). We found that L. interrogans circulated mainly in M. musculus and R. norvegicus, with infection rates of 1.80% and 0.90%, respectively. Rural residential areas and key industries were habitats with a high positive rate of L. interrogans. The northern cities of Shandong (Jinan, Dongying, and Binzhou) were the key regions for the prevention and control of leptospirosis. These findings suggest that farmers and workers in farmland and forests are at high risk of rodent-borne diseases. It is necessary to strengthen surveillance-based integrated rodent control and enhance the personal protection of rural field workers in areas with a high risk of rodent-borne pathogen infection.

R. typhi, A. phagocytophilum, and F. tularensis were found positive only in Jinan. Rodents in rural residential areas tested positive for R. typhi and F. tularensis. R. typhi and A. phagocytophilum were found positive in only one M. musculus and one S. murinus from key industries, respectively. These data indicate that the infection rates of these rodent-borne pathogens were low, but it is essential to strengthen rodent control and personal protection in rural areas and key industries.

Although rodents were suspected to be reservoir hosts of bunyaviruses, and a previous study found a pronounced discrepancy in the prevalence of severe fever with thrombocytopenia syndrome virus (SFTSV) in rodents (14), no positive results were found in our study. This finding was generally consistent with the results of a study conducted in Zhejiang Province, which tested 1,604 rodents for SFTSV (7). Scrub typhus is an emerging infectious disease caused by O. tsutsugamushi. Among rodents captured in autumn, the O. tsutsugamushi infection rate was 8.1%; however, none of the 68 rodents captured during spring, summer, and winter tested positive for O. tsutsugamushi (15). The negative results in our research might be due to low infection rates of DBV and O. tsutsugamushi or seasonal susceptibility in rodents.

This study has three limitations. First, continuous rodent surveillance was not possible due to changes in the rodent survey program. Second, only a selection of zoonotic pathogens in rodents and shrews were screened, rather than all pathogens, which would have required next-generation sequencing. Third, the sample size of rodents and shrews varied by geographic region and species.

Knowledge of rodent capture rates, seasonality, geographic distribution, and zoonotic pathogens is important for clinical recognition and personal protection. Strengthening ecological and pathogenic surveillance of rodents can facilitate the prediction and early warning of related diseases. Maintaining low capture rates of host animals in rural areas remains a vital measure for preventing rodent-borne diseases in China.