Human cases of LB have been confirmed in almost every province found on mainland China including Yunnan Province. However, most of patients only had serological evidence and were not confirmed for specific genotypes. BBSL has been reported in small mammals trapped in the provinces Qinghai, Hunan, Shanxi, Liaoning, Sichuan, Fujian, Zhejiang, Gansu, Guangdong, Jilin and Yunnan [8–16], suggesting that small mammals are likely the main reservoir hosts in China. This study presents a large sample size extending over a wide geographic area, which provides insight into the prevalence, spatial distribution and genetic diversity of BBSL in small mammals collected in Sino-Burmese border area, Yunnan Province.
We documented BBSL infection in 30 species of small mammal, among which, 20 species had not been previously documented. These species may be infected occasionally, whether they serve as reservoir hosts need a further study. The Rattus tanezumi (573/3659, 15.66%) was the predominant species trapped in residential area in Yunnan. Apodemus draco (579/3659, 15.82%) and Apodemus chevrieri (402/3659, 10.99%) were the predominant hosts species in Yunnan, which was consistent with results from Europe where Apodemus are considered a major reservoir of Borrelia . BBSL was detected in Apodemus draco and in Apodemus chevrieri in Yunnan, with Apodemus draco capable of carrying four Borrelia spp. The Ochotona gloveri, Soriculus leucops and Rattus tuekkestanicus also had a much higher prevalence (> 14%) with much larger sample sizes in this study than in other provinces in China [12; 18; 19; 20; 21; 22]. Rattus norvegicus is the prominent household species in Yunnan, which had a high prevalence (12.50%) and was detected positive for pathogenic genotypes (B. afzelii and B. burgdorferi sensu stricto). We also found that the uncommon species Sorex cylindricauda in this study tested positive for B. burgdorferi s.l DNA, requiring further investigation to fully understand their role in maintaining or amplifying infections in nature.
Our findings indicated that prevalence rates in rodents are ranked highest to lowest by landscape type as follows: forest landscape > agricultural landscape > residential landscape (Table 2), which is likely related to tick vector density and preferred habitat. This reiterates the need for individuals traveling into potential tick habitats, like the forest, to take proper protective measures to limit tick bite exposure. Sampling locations in this survey contained a broad range of altitudes from 500 meters to 4500 meters. Among the three altitude classes, small mammals with the highest prevalence of BBSL were found above 2500 m. It was reported that Ixodes ricinus distribution in Sumava National Park extended toward higher altitudes, probably in relation to warming climates . 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 except for B. japonica, have previously been associated with LB [24–25]. B. afzelii was the prominent genotype (83/146, 56.84%) in this study, which 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 sequence analysis carried out in this study, most of the B. afzelii sequences shared 99% identity with clinical isolates from patients in northeastern China . Most of the B. burgdorferi sensu stricto sequences were identical to the sequence from a human case reported in France (KY594010.1). At this time, there have been no confirmed patients with registered sequence of Lyme disease spirochetes 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 within the sequence from Guizhou and Zhejiang province, the other three 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. B. japonica have only been found in Yunlong county, with this representing the first report documenting B. japonica in Apodemus draco and Niviventer excelsior in China. B. garinii is the most common genospecies in China, followed by B. afzelii . However, B. garinii was detected from different species of small mammals only in Deqin County. We found that B. afzelii was the main genospecies detected in Yunnan, which is consistent with previous reports . B. burgdorferi sensu stricto has been detected in Sika deer from Jilin province and in Caprolagu ssinensis 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 (Fig. 1). These findings reflect that Yunnan Province is of particular interest given its diverse topographic range and high level of biodiversity in small mammals that are potential reservoirs for BBSL.
In conclusion, Yunnan Province is an important natural foci of Borrelia burgdorferi sensu lato in China, and given the absence of reported human cases within this region, efforts to expand clinical surveillance are urgently needed.