Asymmetric cross-strain protection for amphibians exposed to a fungal-metabolite prophylactic treatment

Chytridiomycosis, an infectious disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), poses an imminent conservation threat. The global spread of Bd has led to mass mortality events in many amphibian species, resulting in at least 90 species' extinctions to date. Exposure to Bd metabolites (i.e., non-infectious antigenic chemicals released by Bd) partially protects frogs during subsequent challenges with live Bd, suggesting its use as a prophylactic treatment and potential vaccine. However, we do not know whether Bd metabolite exposure protects against strains beyond the one used for treatment. To address this knowledge gap, we conducted a 3x2 experiment where we exposed adult Cuban treefrogs, Osteopilus septentrionalis, to one of three treatments (Bd metabolites from California-isolated strain JEL-270, Panamá-isolated strain JEL-419, or an artificial spring water control) and then challenged individuals with live Bd from either strain. We found that exposure to Bd metabolites from the California-isolated strain significantly reduced Bd loads of frogs challenged with the live Panamá-isolated strain, but no other treatments were found to confer protective effects. These findings demonstrate asymmetric cross-protection of a Bd metabolite prophylaxis and suggests that work investigating multiple, diverse strains is urgently needed.

壶菌病（Chytridiomycosis）是由真菌病原体蛙壶菌（Batrachochytrium dendrobatidis, Bd）引发的两栖动物传染病，对两栖动物保护工作构成迫在眉睫的威胁。Bd的全球传播已导致诸多两栖动物物种发生大规模死亡事件，迄今至少造成90个物种灭绝。研究表明，暴露于Bd代谢物（即Bd释放的非感染性抗原类化学物质）可在后续活Bd攻毒过程中为青蛙提供部分保护，这提示Bd代谢物可作为预防性治疗手段与潜在疫苗。然而，目前我们尚不明确Bd代谢物暴露能否对治疗所用菌株之外的其他菌株产生保护作用。为填补这一认知空白，本研究开展了一项3×2实验：将成年古巴树蛙（Osteopilus septentrionalis）分别暴露于三种处理之一，即来自加利福尼亚分离菌株JEL-270的Bd代谢物、来自巴拿马分离菌株JEL-419的Bd代谢物，或人工泉水对照；随后用对应菌株的活Bd对受试个体进行攻毒。实验结果显示，暴露于加利福尼亚分离菌株的Bd代谢物，可显著降低感染巴拿马分离活菌株的蛙体内的Bd载量，但其余处理均未表现出保护效果。本研究证实了Bd代谢物预防性疗法的不对称交叉保护作用，并提示亟需开展涵盖多种不同菌株的相关研究。