Seasonality of host immunity in a tropical disease system

Infectious disease systems frequently exhibit strong seasonal patterns, yet the mechanisms that underpin intra-annual cycles are unclear, particularly in tropical regions. We hypothesized that host immune function fluctuates seasonally, contributing to oscillations in infection patterns in a tropical disease system. To test this hypothesis, we investigated a key host defense of amphibians against a lethal fungal pathogen, Batrachochytrium dendrobatidis (Bd). We integrated two field experiments in which we perturbed amphibian skin secretions, a critical host immune mechanism, in Panamanian rocket frogs (Colostethus panamansis). We found that this immunosuppressive technique of reducing skin secretions in wild frog populations increased Bd prevalence and infection intensity, indicating that this immune defense contributes to resistance to Bd in wild frog populations. We also found that the chemical composition and anti-Bd effectiveness of frog skin secretions varied across seasons, with greater pathogen inhibition during the dry season relative to the wet season. These results suggest that the effectiveness of this host defense mechanism shifts across seasons, likely contributing to seasonal infection patterns in a lethal disease system. More broadly, our findings indicate that host immune defenses can fluctuate across seasons, even in tropical regions where temperatures are relatively stable, which advances our understanding of intra-annual cycles of infectious disease dynamics.传染病系统通常呈现显著的季节性模式，但支撑其年内循环的机制仍不明确，在热带地区尤为如此。我们提出假设：宿主免疫功能存在季节性波动，这会推动热带传染病系统中感染模式的周期性变化。为验证该假设，我们针对两栖动物抵御致死性真菌病原体蛙壶菌（Batrachochytrium dendrobatidis，Bd）的关键宿主防御机制展开研究。我们整合了两项野外实验，在巴拿马箭毒蛙（Colostethus panamansis）中干扰其皮肤分泌物——这是宿主免疫的关键机制。研究发现，在野生蛙类种群中通过抑制皮肤分泌物实现的免疫抑制手段，会升高Bd的感染率与感染强度，表明该免疫防御有助于野生蛙类种群抵御Bd感染。我们还发现，蛙类皮肤分泌物的化学成分与抗Bd效果随季节发生变化：相较于湿季，旱季对病原体的抑制作用更强。上述结果提示，该宿主防御机制的效能随季节发生改变，这很可能是致死性传染病系统中季节性感染模式的成因之一。从更广泛的视角来看，我们的研究表明，即便在温度相对稳定的热带地区，宿主免疫防御也会随季节波动，这一发现加深了我们对传染病动态年内循环机制的理解。