SU You-lu1，2, LIU Chan1,3, DENG Yi-qin1, GUO Zhi-xun1, FENG Juan1*
1.Key Laboratory of South China Sea Fishery Resources Exploitation & Utilization, Ministry of Agriculture and Rural Affairs, South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Guangzhou 510300, China; 2.College of Animal Sciences and Technology, Guangdong Provincial Water Environment and Aquatic Products Security Engineering Technology Research Center, Zhongkai University of Agriculture and Engineering, Guangzhou 510225,China; 3.College of Fisheries, Tianjin Agricultural University, Tianjin 300384, China
Abstract: Streptococcusagalactiae as one pathogen in tilapia has brought huge economic losses to the tilapia breeding industry due to strong contagiousness and high mortality by the pathogen. This pathogen infects tilapia by different pathways, including cohabitation, oral route, immersion, intraperitoneal and intramuscular inoculation, and is capable of replicating in tilapia bloodstream, penetrating the blood-brain barrier into brain, resulting in meningitis, and passing through the mucus layer of both the stomach and intestine and finally adhered, colonized and invaded the gastrointestinal epithelium, leading to typical histopathological changes, including meningitis, epicarditis, hemorrhage in renal interstitium, multifocal necrotizing splenitis and septicemia in tilapia. The most frequent clinical symptoms of affected tilapia are erratic swimming, exophthalmia, hemorrhage at the operculum, pectoral, and ventral areas and rapidly progressing mortality. Traditional diagnosis methods for the detection of S.agalactiae in the host tissues are based on bacterial culture, and DNA-based methods including PCR and loop-mediated isothermal amplification are increasingly used. Molecular typing of S.agalactiae strains has been performed by different methods, such as molecular serotyping, amplified fragment length polymorphism, multiple-locus variant-repeat assay, and pulsed field gel electrophoresis, multilocus sequence typing. Virulence genes have faster mutation rates and higher polymorphism than housekeeping genes, and the multi-virulence locus sequencing typing method can be widely used to evaluate genetic variation in S.agalactiae. At present, aquaculture depends on antibiotics to treat or prevent S.agalactiae disease in tilapia. Although these antimicrobials have some benefits, the abuse of antibiotics occurs during tilapia production, leading to the development of antimicrobial-resistant strains. Although vaccination is an alternative method to control streptococcal diseases, few commercial vaccines are currently available for treatment of S.agalactiae disease in a large scale tilapia production. This paper comprehensively summarizes the latest research progress and development trends in etiology, epidemiology, pathology, pathogenic mechanism, detection methods, molecular typing, antibiotic resistance and vaccines of S.agalactiae in tilapia. The current problems that need to be solved are discussed and valuable measurements are suggested for the prevention and control of S.agalactiae disease in tilapia.
2019, Vol. 34 
