First Case of Nocardia seriolae Infection in China Cultured Channel Catfish (Ictalurus punctatus).

Nocardia seriolae is a causative agent for fish skin ulcers and sarcoidosis, resulting in significant losses in both brackish and freshwater fish farming industries. From October to November 2023, channel catfishes (Ictalurus punctatus) cultured in Xiaowan reservoir at Dali city, Yunnan province in China, showed gross pathological changes with skin ulcers, white nodules in the liver and heart, histopathological tests revealed the typical granulomas and chronic hepatitis changes in the liver, and the clumps of filamentous bacteria in the heart and spleen. A Gram-positive acid-fast bacterium (CCF_NS01) was isolated from viscera (liver, spleen and kidney); biochemical tests and 16sRNA-GyrB gene concatenated sequences analysis of this isolated microorganism identified it as N. seriolae. Drug sensitivity testing indicated susceptibility to commonly used antibiotics such as co-trimoxazole, doxycycline, enrofloxacin, florfenicol, amikacin and rifampicin. In isolated CCF_NS01, multiple pathogenic factors corresponding to encoding genes were detected, including espG, mftF, pcaA, fadD32, pks13, narJ, feoB, sodA, katG and mceF. 100% mortality with 1 × 108, 1 × 107 and 1 × 106 cfu/mL in a 35-d challenge test period was observed, and the intraperitoneal injection median lethal dose (LD50) was 1.89 × 104 cfu/mL. This is the first isolation report of N. seriolae infection from cultured channel catfish, aiming to warn against the continuous host-range expansion of this pathogen, which is threatening the inland aquaculture industry.