Farming-associated variation in chytrid infection and shedding dynamics in invasive American bullfrogs

Abstract The global amphibian decline caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd) threatens biodiversity. Some species, such as the bullfrog ( Aquarana catesbeiana ), can persist with infection without overt disease, yet the processes underlying this response remain unclear. Because bullfrogs are farmed worldwide under high-density conditions, understanding how production environments influence infection dynamics and immune response is essential. We assessed Bd dynamics in farmed and wild bullfrogs, focusing on immunity and environmental zoospore release. Individuals were exposed to two Bd lineages, and we compared infection loads, immune profiles, zoospore shedding, infection progression, and survival. Farmed bullfrogs consistently exhibited higher Bd loads and released more zoospores than wild individuals. Mortality did not differ between groups, suggesting the capacity to sustain high infection loads without immediate survival costs. These findings indicate that farming environments may increase pathogen amplification and environmental zoospore availability. The observed combination of high infection loads, elevated zoospore shedding, and sustained host survival suggests increased transmission risk under farming conditions. Given that bullfrog production systems may operate with variable biosafety standards, strengthened surveillance and containment practices could help reduce pathogen dissemination risks. This study provides insights into host–pathogen interactions in intensively farmed amphibians, with implications for conservation and disease management.