Discussion
We analyzed a wide range of species and environmental reservoirs across different seasons to evaluate the infection patterns of Bsal in relation to geographic and environmental factors in Guangxi region. We did not detect Bsal in any of the samples from wild and captive animals or sampled water bodies. Our species distribution model estimated a small fraction of suitable habitats in this region under the current climatic conditions, but with patchy distributions. These results suggest that climate is a potential barrier, limitingBsal spread into the surveyed sites. However, there is a possibility that it could be present at undetectable levels, or occur in specific microhabitats that were not sampled in our study.
Our niche model predicted that climatically suitable areas forBsal in the Guangxi region are less and fragmented, generally low suitability. The model identified mean diurnal range as the most crucial variable in our models. This corroborates the findings from previousBsal distribution models in its native Asian range (Moubarak et al., 2022). The relatively high variations in mean diurnal range within the study area seem to limit the potential distribution of Bsal , emphasizing that environmental temperature is a primary factor influencing Bsal occurrences (Martel et al., 2013).
Bsal prefer lower thermal conditions for their existence, the temperatures above 22°C adversely affects its growth and diminish its pathogenicity to hosts (Carter et al., 2021; Martel et al., 2013). Previous studies have proposed that temperature is a key factor behind the low prevalence of Bsal in Asian amphibians, as the environmental as well as the water temperatures in these regions are generally considered suboptimal (≥ 15°C) forBsal ’s survival and growth (Laking et al., 2017; Martel et al., 2013; Yuan et al., 2018). It has also been documented that warmer temperatures can potentially reduce intraspecific transmission rates, especially when temperatures surpass 25°C (Blooi et al., 2015) .
Furthermore, the precipitation of the coldest quarter plays a contributory role in modeling predictions for Bsal occurrences in Asia which shows the same pattern as of European Bsal presence which requires moist, relatively cool and stable conditions (Beukema et al., 2021), the high fluctuations of precipitation in cool periods weaken its occurrences.
The absence of Bsal in northern sites in our study might partly result from competition with other pathogens that share a similar climatic niche, such as Bd . It was identified at ten of the surveyed sites (Fig. 1B), displaying a high prevalence in the northern locales of the Guangxi region (Sun et al., 2023). One of 38 species we examined, A. davidianus (positive to Bd in our subsequent detections), have been reported as Bsal -positive species (Fig.1; Laking et al., 2017; Yuan et al., 2018), but it was negative toBsal infection in our present study. These patterns suggest that Asian Bd lineages might outcompete Bsal when site-specific conditions and species susceptibility enhance the pathogenic performance of Bd lineages as pathogenicity of both pathogens varies with species and environmental conditions (Fisher & Garner, 2020; Martel et al., 2014). This competition could occur because Bd infection can negatively impact Bsal infection under certain circumstances (Direnzo et al., 2021).
Considering the possibility that Bsal has not yet been transmitted to the current sampling sites, particularly in highly suitable habitats (e.g., southwestern sites) where Bd is present (Fig. 1), the coexistence of Bd and Bsal in the same microhabitats could result in severe coinfections or reinfections and potentially generate new, highly virulent genotypes within the same host once the pathogen is introduced to these sites (Greenspan et al., 2018). Among the 38 amphibian species tested for Bsal , none were found to be infected. However, 16 of these species tested positive forBd infection (Table 1), highlighting the need for further investigation into the potential co-occurrence of these fungal pathogens. Specifically, we recommend investigating predicted suitable areas for Bsal where Bd is present, in order to better understand the ecological interactions between these pathogens and their impact on amphibian species.
Geographic barriers such as drier plains which prevents the migration of susceptible species could be another potential reasons for absence ofBsal (restricted slowly dispersal) in the region. This may inhibit the host-dependent intraspecific and interspecific as well as human-mediated transmissions of this pathogen within landscapes (Malagon et al., 2020; Spitzen-Van Der Sluijs et al., 2018; Tompros et al., 2021). In addition, the negative result of Bsal is consistent with previous studies of this pathogen in Asia that primarily detected the fungus in anuran species (Wang et al. 2017; Zhu et al. 2014), which was thought to lead to transient persistence for Bsal and decreased transmission rates due to low galactose proportions in host skin (Stegen et al., 2017; Wang et al., 2021). However, it is possible that Bsal could be present sub clinically at low prevalence in these studied amphibians, with fungal loads below the thresholds of detection. As growth of Bsal could also depend on relevant plant materials in water (Kelly et al., 2021), extending the range of sampling efforts in environmental reservoirs can improve our understanding the existence of Bsal in the region.
Seasonality mainly explained by variations in temperature and precipitation, may influence Bsal occurrences and host susceptibility (Bozzuto & Canessa, 2019; Carter et al., 2021). Our study primarily surveyed amphibians that are reproductively active during spring and summer, with a limited number of individuals sampled in autumn. We were unable to conduct any sampling of wild host species during winter, which limits our understanding of Bsal infection across seasons. Notably, autumn and winter conditions in this subtropical region might be more conducive for Bsal prevalence.
It is important to understand the resource partitioning and natural behaviors of amphibians across seasons. One such factor would be overwintering and potential attenuation of defensive mechanisms such as reduced skin bacterial diversity during hibernation (Tong et al., 2019). This would lead the pathogen to cause sub-lethal infections in susceptible amphibians during autumn and winter. Hence, we recommend integrating experimental and field studies in both summer and winter, to enhance our understanding of the effects of Bsal infection on amphibians in specific ecological contexts.
In conclusion, the absence of Bsal in our study should not undermine the importance of proactive management and conservation strategies aimed at minimizing the risk of Bsal transmission in the region. This is due to the limited presence of climatically suitable areas for Bsal in this region and the isolated suitable sites within mountains could provide an important refuge for Bsal(Beukema et al., 2021; Lötters et al., 2020). Implementing stringent biosecurity measures, such as regulating the trade of amphibians can help reduce the chances of pathogen introduction. Regular monitoring and surveillance programs are crucial for early detection of Bsal , which could allow for timely interventions to prevent severe declines in amphibian populations (Thomas et al., 2019). In addition, further research into the ecology, transmission dynamics, and host-pathogen interactions of Bsal is essential to generate knowledge that will inform targeted conservation.