The fungal pathogen Batrachochytrium salamandrivorans is not detected in wild and captive amphibians from Mexico.

The recent emergence of the pathogen Batrachochytrium salamandrivorans (Bsal) is associated with rapid population declines of salamanders in Europe and its arrival to new areas could cause dramatic negative effects on other amphibian populations and species. Amphibian species, present in areas with high amphibian diversity such as Mexico, could be highly threatened due to the arrival of Bsal, particularly salamander species which are more vulnerable to chytridiomycosis caused by this pathogen. Thus, immediate surveillance is needed as a strategy to efficiently contend with this emerging infectious disease. In this study, we analyzed 490 wild and captive amphibians from 48 species across 76 sites in the North, Central, and South of Mexico to evaluate the presence of Bsal. Amphibians were sampled in sites with variable degrees of amphibian richness and suitability for Bsal according to previous studies. From the 76 sampling sites, 10 of them were located in areas with high amphibian richness and potential moderate to high Bsal habitat suitability. We did not detect Bsal in any of the samples, and no signs of the disease were observed in any individual at the time of sampling. Our results suggest that Bsal has not yet arrived at the sampled sites or could be at low prevalence within populations with low occurrence probability. This is the first study that evaluates the presence of Bsal in different regions and amphibian species in Mexico, which is the second most diverse country in salamander species in the world. We highlight the risk and the importance of continuing surveillance of Bsal in Mexico and discuss control strategies to avoid the introduction and spread of Bsal in the country.

Comparative Analysis of Skin Bacterial Diversity and Its Potential Antifungal Function Between Desert and Pine Forest Populations of Boreal Toads Anaxyrus boreas.

The skin microbiome in amphibians has gained a lot of attention as some of its members play a protective role against pathogens such as the fungus Batrachochytrium dendrobatidis (Bd). The composition of skin bacterial communities has been suggested as one of the factors explaining differences in susceptibility to Bd among amphibian species and populations. The boreal toad Anaxyrus boreas is known to be susceptible to Bd, and severe population declines in its southeastern range have been documented. However, throughout A. boreas distribution, populations present differences in susceptibility to Bd infections which may be associated with differences in skin microbial diversity. This study compared the skin bacterial diversity and Bd infection levels of A. boreas in one desert population and one pine forest population from Baja California, Mexico. We found that desert and pine forest toad populations exhibit differences in skin bacterial community structure but show similar Bd infection levels. Using a predictive method, we found that the abundance of bacteria with potential Bd-inhibitory properties differed between uninfected and infected individuals but not between populations. Our data suggest that several bacteria in the skin community may be offering protection from Bd infections in these A. boreas populations. This study provides foundational evidence for future studies seeking to understand the skin-microbial variation among boreal toads' populations and its relation with Bd susceptibility.

Early presence of Batrachochytrium dendrobatidis in Mexico with a contemporary dominance of the global panzootic lineage.

Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd), is a devastating infectious disease of amphibians. Retrospective studies using museum vouchers and genetic samples supported the hypothesis that Bd colonized Mexico from North America and then continued to spread into Central and South America, where it led to dramatic losses in tropical amphibian biodiversity (the epizootic wave hypothesis). While these studies suggest that Bd has been in Mexico since the 1970s, information regarding the historical and contemporary occurrence of different pathogen genetic lineages across the country is limited. In the current study, we investigated the historical and contemporary patterns of Bd in Mexico. We combined the swabbing of historical museum vouchers and sampling of wild amphibians with a custom Bd genotyping assay to assess the presence, prevalence, and genetic diversity of Bd over time in Mexico. We found Bd-positive museum specimens from the late 1800s, far earlier than previous records and well before recent amphibian declines. With Bd genotypes from samples collected between 1975-2019, we observed a contemporary dominance of the global panzootic lineage in Mexico and report four genetic subpopulations and potential for admixture among these populations. The observed genetic variation did not have a clear geographic signature or provide clear support for the epizootic wave hypothesis. These results provide a framework for testing new questions regarding Bd invasions and their temporal relationship to observed amphibian declines in the Americas.

Potential risk of Batrachochytrium salamandrivorans in Mexico.

The recent decline in populations of European salamanders caused by the chytrid fungus Batrachochytrium salamandrivorans (Bsal) has generated worldwide concern, as it is a major threat to amphibians. Evaluation of the areas most suitable for the establishment of Bsal combined with analysis of the distribution of salamander species could be used to generate and implement biosecurity measures and protect biodiversity at sites with high salamander diversity. In this study, we identified the areas most suitable for the establishment of Bsal in Mexico. Mexico has the second-highest salamander species diversity in the world; thus, we identified areas moderately to highly suitable for the establishment of Bsal with high salamander diversity as potential hotspots for surveillance. Central and Southern Mexico were identified as high-risk zones, with 13 hotspots where 30% of Mexican salamander species occur, including range-restricted species and endangered species. We propose that these hotspots should be thoroughly monitored for the presence of Bsal to prevent the spread of the pathogen if it is introduced to the country.