Seasonal dynamic and individual determinants of Amphibiocystidium sp. infection in the palmate newt during the aquatic breeding season.

Amphibian populations are undergoing global declines, with infectious diseases playing a major role in these losses. While chytridiomycosis has been widely studied, other pathogens, such as mesomycetozoean parasites, are increasingly recognized as potential contributors to amphibian health deterioration. In this study, we investigated the seasonal dynamics of Amphibiocystidium sp. infection in the palmate newt (Lissotriton helveticus) during its aquatic breeding period in western France. From January to July 2023, across 12 sampling sessions and 585 captured individuals, we monitored host morphology, body condition, and infection status. We found strong seasonal variation in host traits, with body mass and condition declining over the breeding season, particularly in females. Infection prevalence and cyst intensity peaked at the end of March, and were not observed after the end of May. Smaller individuals were more likely to be infected, and infected females exhibited higher parasite loads than males. Additionally, infection probability increased with body condition in males only. The temporal restriction of infection suggests that environmental factors and host aggregation during breeding may play a key role in transmission.