We documented BBSL infection in 30 species of small mammal, among which, 20 species had not been previously documented. These species may be incidental hosts that are infected occasionally, further study is required to determine what, if any, role the play as reservoir hosts. The Rattus tanezumi (15.66%) was the predominant species trapped in residential areas in Yunnan. Apodemus spp. (31.35%) were the predominant hosts captured in Yunnan, which was consistent with results from Europe where Apodemus are considered a major reservoir of Borrelia [17]. BBSL was detected in A. draco and in A. chevrieri in Yunnan, with A. draco capable of carrying four pathogenic Borrelia spp. Soriculus leucops had a much higher prevalence (>14%) with a larger sample sizes in this study than in other provinces in China [12; 18-22]. Rattus norvegicus, the prominent household species in Yunnan, had a relatively high prevalence (2/16, 12.50%), although only 16 small mammals were captured, and tested positive for pathogenic genotypes B. afzelii and B. burgdorferi sensu stricto. We also found that Sorex cylindricauda tested positive for BBSL DNA (14.28%), requiring further investigation to fully understand their role in maintaining or amplifying infections in nature.
Our findings indicated that detection rates in small mammals were ranked highest to lowest by landscape type as follows: forest landscape > agricultural landscape > residential landscape, which is likely related to tick vector density and preferred habitat. Sampling locations in this survey contained a broad range of altitudes from 500 meters to 4500 meters. Among the three altitude classes, the highest prevalence of BBSL was found above 2500m. In the Czech Republic, the distribution of Ixodes ricinus, a known vector of LB, extended toward higher altitudes, probably in relation to warming climates [23]. The roles temperature and humidity play in tick reproduction and reservoir preferences requires further investigation within these altitude ranges. Additionally, there are no reported human cases at these heights, which might reflect lower populations living in these areas.
Our study found five genospecies of BBSL in small mammals in Yunnan Province, four of them, excluding B. japonica, have previously been associated with LB [24-25]. Borrelia afzelii was the prominent genotype (56.84%) in this study, which was detected in 24 species. There exists a wide distribution and genetic diversity of BBSL in Yunnan, compared to only 1-2 genospecies of BBSL in most provinces in China, such as Qinghai, Zhejiang, Guizhou and Guangxi. According to the sequences detected in this study, most B. afzelii shared 99% identity with clinical isolates from patients in northeastern China [26]. The B. burgdorferi sensu stricto sequences were identical to the sequence from a human case reported in France. At this time, there have been no confirmed patients with PCR confirmed LB in Yunnan province, requiring further investigation in the near future. The sequence of B. valaisiana obtained from small mammals cluster into two clades, one cluster fell within the sequence identities from Guizhou and Zhejiang province, the other cluster fell within close proximity to sequences from Europe. Birds are major reservoirs for B. valaisiana in Europe, however the transmission cycle maintaining B. valaisiana in Yunan may be different from other areas. Borrelia japonica was only detected in Yunlong county, with this representing the first report documentation of B. japonica in Apodemus draco and Niviventer excelsior in China. Borrelia garinii is the most common genospecies in China [27], but was only detected in Deqin County in this study. We found that B. afzelii was the main genospecies detected in Yunnan, which is consistent with previous reports [4]. Borrelia burgdorferi sensu stricto has been detected in Sika deer from Jilin province and in Caprolagus sinensis from Hunan province, and detected in small mammals in Yunnan province within the more populated counties of Gongshan, Deqin, and Weixi (S1, S2, S5) found in northwestern Yunnan (Figure 1). These findings reflect a high level of Borrelia genetic diversity found in a wide range of small mammals, many of which are likely reservoirs for BBSL in Yunnan.
In conclusion, Yunnan Province is an important natural focus of BBSL in China. Given the absence of reported human cases within this region, efforts to expand clinical surveillance are urgently needed.