Abstract
An outbreak of Cryptocaryon irritans ( Ci ) occurred during the quarantine of marine teleost fish at a large aquarium facility, leading to 457 mortalities across 16 species. This study investigates the outbreak through a retrospective analysis of clinical and pathological findings, emphasising the role of detailed pathology in outbreak characterisation. Lesion diversity, host‐specific susceptibility and mortality rates were examined alongside molecular diagnostics to confirm Ci presence. Key findings include significant differences in susceptibility and pathological presentations among fish families, with Chaetodontidae and Acanthuridae exhibiting severe gill and skin pathology, whereas Labridae and Scaridae showed enhanced parasite clearance associated with mucus production. A variety of treatment modalities were used, including formalin bath, Leteux–Meyer mixture and chloroquine, with variable treatment responses and species‐specific drug sensitivities. Molecular analysis confirmed Ci with high sequence identity to isolates from the Red Sea and East Asia but highlighted primer limitations for frozen samples. This study outlines the challenges of managing a Ci outbreak in quarantine under unprecedented environmental conditions, underscores the importance of species‐specific management strategies and provides detailed insights into host–pathogen interactions and treatment responses.