Study and analysis of pathogen co-infection in grouper culture

Co-infection is a common phenomenon in nature. The large-scale morbidity of many livestock farms is caused by a variety of pathogenic factors, among which pathogen co-infection is one of the main causes. Co-infection causes difficulties in clinical diagnosis and treatment. However, most studies only focus on the pathogenic characteristics and pathogenesis of a single pathogen, and many prevention and control methods and drug treatment strategies are also limited to a single pathogen, resulting in weak or even ineffective effects of these prevention and control methods when encountering multiple pathogens co-infection. In fact, the symptoms and damage caused by pathogens co-infection on the host are significantly different, or even more serious when compared with single pathogen infection. Besides, researchers have found that the interaction between co-infected pathogens has several types, including synergism, antagonism, and non-interference. However, it has been confirmed that co-infected pathogens may always change the pathogenic mechanism of the original single pathogen. Therefore, it is of great significance to develop a rapid, sensitive, and efficient detection method for multiple pathogens and explore the pathogenesis of co-infection for the prevention and treatment of co-infection. At present, pathogen co-infection has been studied extensively and deeply in the field of livestock and poultry, including pathogen detection technology and pathogenic mechanism of virus and virus co-infection, virus and bacteria co-infection, and bacteria and bacteria co-infection, but few studies on the co-infection in aquaculture have been reported, and the research of co-infection in aquaculture is still in the initial stage of exploration. Grouper is a kind of precious marine fish with meat delicate and delicious, it is rich in nutrition and has high economic value. With the continuous expansion of the aquaculture scale and the acceleration of industrialization and urbanization, the deterioration trend of the offshore grouper aquaculture environment has intensified, following by the increasingly serious problem of grouper aquaculture diseases. In order to know about the characteristics and patterns of common disease pathogens infection in cultured grouper, this study used PCR method to investigate the viral and bacterial pathogens of diseased grouper collected from grouper farms in Guangxi (Beihai, Qinzhou, Fangchenggang City) for the first time. The pathogens in the diseased fish were isolated through cell specific isolation of toxic pathogens, and bacterial pathogens were separated using plate (LB and TCBS) line separation method. Then, the isolated pathogen was used to infect with grouper cells to determine the infectivity. The results showed that Singapore grouper iridovirus (SGIV), nervous necrosis virus (NNV), Vibrio harveyi, V. vulnificus, V. alginolyticus, V. parahaemolyticus, A. eromonas hydrophila and Photobacterium damselaesingle existed in the diseased grouper in a single form or multiple forms. V. harveyi, V. vulnificus, V. alginolyticus and V. parahaemolyticus was the main bacterial pathogen that infected cultured grouper in Guangxi, and these bacteria also have interspecies and intraspecies co-infection. The detection frequency of V. harveyi was the highest, indicating that the main pathogen causing Vibrio disease in cultured grouper in Guangxi was V. harveyi. The results also showed that the detection rates of grouper virus pathogens and bacterial pathogens were very from different seasons. The detection rate of NNV was 13% in spring (from March to May), which was higher than that of SGIV. In summer (from June to August), the emergence frequency of SGIV was 19%, higher than that of NNV. The highest detection rate of SGIV and NNV among the year was in autumn (from September to November), with 58% and 40% detection rate respectively, and the lowest detection rate of SGIV and NNV among the year was in in winter (from December to February), with 4% and 6% detection rate respectively. At the same time, we found that autumn is the season with the highest detection rate of bacterial disease pathogens. V. harveyi, V. vulnificus, V. alginolyticus, V. parahaemolyticus, A. hydrophila and P. mermaid could be detected in the diseased grouper, and with the detection rates of 32%, 6%, 9%, 15%, 6% and 15% respectively. Among these bacterial species, V. harveyi could be detected in diseased grouper in four seasons. On the other hand, V. alginolyticus had high detection rate in summer, autumn and winter, V. vulnificus was mainly detected in spring and autumn, and the positive rate of V. parahaemolyticus and P. mermaid existed in summer and autumn. In summary, the disease of cultured grouper in Guangxi is accompanied by co-infection of multiple pathogens, and the types of co-infection are complex and diverse. Our results provide a direction for exploring the pathogenic mechanism of grouper pathogen co-infection, offering a theoretical basis for the prevention and control of the grouper epidemic, and shedding new insights for aquatic pathogen co-infection. Future research will focus on the detection of multiple pathogens and the pathogenesis of co-infection, which will be meaningful research work.