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In October 2023, a disease outbreak in pufferfish (Takifugu obscurus) farms in Zhongshan City, Guangdong, China, caused high mortality. Diseased fish (mean length: 15 ± 1 cm) exhibited swimming disorders, fin rot, hemorrhage, and an enlarged spleen. Histopathological observations generally revealed inflammation, necrosis, and congestion in the spleen, kidneys, and brain tissues. The most severe pathological changes included interstitial edema and tubular atrophy in the kidneys, hemosiderin deposition in the spleen, massive red blood cell infiltration, and a decrease in lymphocytes. A single strain of bacteria (Tol-1) was isolated from the diseased pufferfish and identified as a Gram-positive streptococcus strain, exhibiting α-hemolysis on sheep blood agar plates. Through biochemical characterization, 16S rDNA sequencing, morphological analysis, and specific primer-based identification, the Tol-1 strain was identified as Lactococcus garvieae, serotype I. Antimicrobial susceptibility testing indicated that Tol-1 was sensitive to Chloramphenicol, Ampicillin, Cephalexin, and Doxycycline, but resistant to Kanamycin, Gentamicin and Ciprofloxacin. In addition, 15 common virulence factors were detected in the Tol-1 strain, including adhPav, adhPsaA, adhC I–II, adh, and hly 1–3. Pufferfish (mean length: 17 ± 1 cm) subjected to artificial infection via intraperitoneal injection (IP) with the Tol-1 strain exhibited clinical symptoms and histopathological damage similar to those observed in naturally infected fish. An infection dose of 1 × 105 CFU/fish resulted in 80% mortality. The study fulfilled Koch's postulates, indicating that the disease outbreak in pufferfish was caused by L. garvieae, which exhibited a high mortality rate in pufferfish despite the subtle clinical symptoms. These results serve as a warning for pufferfish farming areas and provide a scientific basis for future prevention and control efforts. [ABSTRACT FROM AUTHOR]

Cryptocaryon irritans (Brown, 1951) is an obligate parasite that mainly invades the skin, fins and gills of marine fish and causes marine 'white spot disease'. It is common to aquarium and maricultured fish and causes high mortality due to its widespread geographical distribution, indiscriminate host specificity, short and simple life cycle, and high level of virulence. Numerous physical methods and chemical bath agents are applied to cure this disease in closed, re‐circulating aquaculture systems, but these treatments are not suitable in open environments such as cages and ponds. Fortunately, new measures, such as oral chemical drugs, oral plant components, vaccines and even biological control, are being developed as promising methods to treat and prevent cryptocaryoniasis in open aquaculture systems. Here, we provide a broad overview of C. irritans research, focussing primarily on work conducted in the last decade that may offer new insights into the treatment of disease in a practical setting. [ABSTRACT FROM AUTHOR]

Flavobacterium columnare , which causes columnaris disease, is responsible for significant mortality in grass carp. Vaccination is a safe and effective measure to combat this disease, and this study aimed to investigate the immune protective effects of different treatments using an inactivated F. columnare vaccine. The vaccine was prepared by inactivating the bacteria with 0.05% formaldehyde at 4°C for 24 hours. The experiments involving grass carp were divided into two parts. In Experiment 1, the immune effects of two isolates, JX-01 (genomovar I) and MU-04 (genomovar II), were compared, along with the impact of white oil adjuvant and the number of immunizations. The results showed that when the white oil adjuvant was used as a booster, the relative percent survival (RPS) of the JW2 group and MW2 group after 8 weeks of the first immunization was 34% and 61%, respectively. In Experiment 2, only the MU-04 (genomovar II) isolate was used as an antigen, with the white oil adjuvant as a booster. The effects of different doses (CFU=10 8 , 10 7 , and 10 6 bacteria/mL) on immune responses were compared, and the RPS values in the MW6, MW7, and MW8 groups after 4 weeks of the first immunization were found to be 38%, 57%, and 71%, respectively. Furthermore, in the cross-antigen protection experiment, the MW2 group exhibited an RPS of 55% against the JX-01 isolate, which was significantly higher than the control group (33%). These findings suggest that an inactivated vaccine comprising an appropriate antigen isolate when administered with a white oil adjuvant as a booster, can provide effective protection in grass carp., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Guo, Han, Xu, Lu, Li, Dan and Mo.)

Pearl gentian grouper is a new aquacultural hybrid resulted from breeding of female tiger grouper ( Epinephelus fuscogutattus ) and male giant grouper ( E. lanceolatus ). Our preliminary study found that pearl gentian grouper exhibits less susceptible to the primary infection of Cryptocaryon irritans , which is an important parasitic ciliate in marine aquaculture, indicated that pearl gentian grouper might own a strong innate immune system. Complement system play key roles in innate immunity, whether pearl gentian grouper's complement component contribute for the defensing against the C. irritans infection remain unclear. In the present study, we found that C. irritans can be immobilized by untreated serum but not heat-treated serum from pearl gentian grouper, suggested that the heat-labile components in serum are responsible for the immobilization of C. irritans . Moreover, we cloned and characterized the encoding sequence of pearl gentian grouper complement C3 (PGC3), a key component in complement system. We also found that the expression level of PGC3 was increased in infected grouper serum when compared with that of control grouper. Furthermore, the binding of PGC3 on the surface of C. irritans trophonts located on the grouper skin was detected. These data suggested that pearl gentian grouper's complement system indeed play roles in the immune response against the C. irritans infection., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2021 The Author(s).)