Abstract:
To investigate the epidemiological characteristics of bacterial pathogens in tilapia (
Oreochromis ) aquaculture from 2023 to 2025, we conducted a survey in tilapia-farming regions across southern China, including Guangdong, Hainan, Yunnan, and Fujian Provinces, thus clarifying the prevalence dynamics, antimicrobial resistance profiles and pathogenicity of dominant pathogens, and screening for sensitive antibiotics. The results showed significant regional differences in bacterial diseases:
Lactococcus garvieae is the predominant pathogen in Guangdong and Hainan (Isolation rates: 26.7% to 100%), while
Streptococcus agalactiae isolation rates ranged from 0% to 50%;
S. agalactiae remained dominant in Fujian and Yunnan, where
L. garvieae was not detected. Clinical signs in diseased tilapia from Guangdong and Hainan included exophthalmia, corneal opacity, and hemorrhage in the lower jaw, operculum, and fin bases. Necropsy revealed hepatosplenomegaly and congestion, intestinal inflammation, and cerebral congestion. Morphological, physiological and biochemical, and 16S rRNA gene sequence analysis confirmed the dominant pathogen as
L. garvieae. Antimicrobial susceptibility tests indicated that the isolates exhibited obvious multidrug-resistant phenotypes. They were sensitive to nine antibiotics (e.g. florfenicol, amoxicillin, doxycycline), while the resistance rates to sulfisoxazole, quinolones, and enrofloxacin exceed 60%. Ten representative
L. garvieae strains carried nine virulence genes:
hly,
LPxTG-1–LPxTG-4,
adh,
adhPav,
adhPsaA, and
adhCII. After intraperitoneal injection with
L. garvieae (1×10
7 CFU·tail
–1), challenged tilapia exhibited symptoms similar to natural infections, with cumulative mortality rates of 10% to 50% over 14 d; histopathological observation showed characteristic damage in the liver, spleen, kidney, gills, and brain. This study clarifies the prevalence trends and multidrug-resistant characteristics of
L. garvieae in four major tilapia-farming regions of southern China, analyzes its virulence genes and pathogenicity, and provides a scientific basis for the diagnosis and control.
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