Skin Ulceration in Farmed Leopard Coral Grouper (Plectropomus leopardus) Was Associated with Vibrio spp. and Photobacterium damselae in China

The leopard coral grouper (Plectropomus leopardus), an economically important marine fish, is widely farmed in Hainan Province, China. However, intensive farming has led to frequent disease outbreaks, including viral nervous necrosis, vibriosis, and enteritis, with skin ulceration a widespread and significant concern. To identify the pathogen causing skin ulceration, we collected diseased fish from Hainan and conducted histopathological and ultrastructural analyses, parasite examination, viral metagenomic sequencing, and bacterial isolation and identification. The histopathological analysis and ultrastructural observation showed that the diseased P. leopardus displayed marked pathological alterations, with significant hepatic degeneration and necrosis and splenic structural disorganization. Parasites and viruses were both excluded from being the primary pathogen. A total of 79 bacterial strains were isolated from diseased P. leopardus liver tissue, including Vibrio harveyi (38 strains, 48.1%), Vibrio owensii (17 strains, 21.5%), Vibrio rotiferianus (16 strains, 20.3%), and Photobacterium damselae subsp. damselae (8 strains, 10.1%). Infection experiments suggested that all four bacterial species could induce skin ulceration. This study demonstrates that skin ulceration in P. leopardus is associated with multiple bacterial pathogens, refining the previous view that attributed the disease solely to V. harveyi. These findings provide a foundation for developing vaccines and disease-resistant breeding strategies.