First Report of Natural Co-Infection by Gyrodactylus lenoki and Lactococcus garvieae in Farmed Rainbow Trout (Oncorhynchus mykiss): Field and Experimental Evidence for Parasite-Enhanced Mortality, With a COI-Morphology Species Discordance.

It is well established that ectoparasite infection can increase the pathogenicity of, and host susceptibility to, bacterial infections in fish, including rainbow trout; however, no interaction between the ectoparasite Gyrodactylus and the bacterium Lactococcus garvieae has previously been documented. This study presents the first report of a severe natural co-infection of Gyrodactylus lenoki and Lactococcus garvieae in farmed rainbow trout in Vietnam, with mortality rates ranging from 25% to 43% within 4 to 5 days of the first recognised pathogen-associated fish mortality. The identity of both pathogens was established using morphological characterisation, sequencing, and PCR assays, providing clear evidence of dual infection under natural conditions. Molecular (COI) data initially suggested homology with Gyrodactylus salaris, a notifiable pathogen of major regulatory concern that has not previously been recorded outside Europe, but size-invariant morphological analysis of the marginal hook sickles confirmed the parasite as G. lenoki, demonstrating that COI barcode data alone can be misleading for regulatory-critical species identification within this genus. Experimental challenge trials demonstrated that while G. lenoki exhibits low virulence toward rainbow trout on its own, it significantly exacerbates the severity of L. garvieae infection, leading to higher mortality (approximately 25% mortality in fish infected with L. garvieae alone, rising to up to 45% in co-infected fish, with mortality occurring earlier in co-infected groups). Notably, the population dynamics of the ectoparasite varied across experimental groups, with a strong correlation (r = 0.97) observed between host mortality and G. lenoki infection levels. These findings underscore the critical role of ectoparasitic infections in increasing rainbow trout susceptibility to bacterial invasion and highlight the diagnostic risks of relying on molecular barcoding alone when regulatory-notifiable pathogens are suspected.