A significant outbreak of trypanosomosis occurred in farmed large yellow croaker (Larimichthys crocea) in Southeast China resulting in daily mortality rates, over 70 days, of up to 1 % with cumulative losses nearing 60 %. Despite the severity of the outbreak, details of the specific trypanosomes infecting large yellow croaker remain unclear. In this study, samples of fish were collected from the fish farmer during the outbreak. A high prevalence of trypanosomes, confirmed by microscopy and PCR analysis, was associated with pathology. Sequence identification and phylogenetic analysis indicated that the isolated trypanosome had 99.27 % sequence identity with Trypanosoma carassii MARV, a typical freshwater fish trypanosome, and clustered within the T. carassii complex C group. Morphological characterization, using Giemsa staining, further supported the similarity between the trypanosomes infecting large yellow croaker and the T. carassii complex C group. Additionally, a broader host range was established as it also infected Nile tilapia (Oreochromis niloticus), koi (Cyprinus carpio) and largemouth bass (Micropterus salmoides), leading to its designation as a subspecies within the T. carassii complex, named T. carassii spectrum. This represents the first documented instance of T. carassii, a typical freshwater fish trypanosome, infecting a marine fish whilst highlighting its severe pathogenicity and the urgent need for effective control measures. Furthermore, in vitro cultivation was successfully established, providing a foundation for future research on prevention and management strategies.
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