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强烈的季节性模式，但支撑年内感染周期的核心机制仍不明晰，在热带地区这一问题尤为突出。我们提出假说：宿主免疫功能存在季节性波动，这会驱动热带传染病系统中感染模式的周期性振荡。为验证该假说，我们针对两栖动物抵御致死性病原蛙壶菌（Batrachochytrium dendrobatidis，简称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，对巴拿马丛蛙（Colostethus panamansis）的皮肤分泌物——这一关键宿主免疫机制——实施了扰动操作。研究发现，在野生蛙类种群中，采用削减皮肤分泌物的免疫抑制手段后，Bd的感染率与感染强度均有所上升，表明该免疫防御机制可增强野生蛙类对Bd的抵抗力。我们还发现，蛙类皮肤分泌物的化学成分与抗Bd效能随季节发生变化：相较于湿季，旱季对病原的抑制效果更为显著。上述结果提示，该宿主防御机制的效力会随季节发生动态改变，这大概率是致死性传染病系统中季节性感染模式的驱动因素之一。从更广泛的研究视角来看，我们的发现表明，即便在温度相对稳定的热带地区，宿主的免疫防御功能也会出现季节性波动，这一发现深化了我们对传染病动态年内循环机制的理解。