Tilapia culture is an important source of income and nutrition to many rural families. Since 2000, the production of tilapia increased and reached domestic and global markets. Major farmed species is Nile tilapia (Oreochromis niloticus), in earthen ponds and cage cultures. Intensification contributed to global tilapia disease outbreaks, with bacterial infections causing mortalities and morbidities, threatening sustainable production. At tilapia farms, high nutrient concentrations, water temperature and fish densities enhance bacterial growth including virulent bacterial clones and potential zoonotic bacteria. Global warming favours this. This review respectively provides a comprehensive overview of the most common and emerging bacterial pathogens, diseases, clinical presentations and diagnostics of tilapia, including bacteria and diseases with zoonotic potential. First, common bacterial disease outbreaks, including streptococcosis, motileAeromonassepticaemia, francisellosis, columnaris disease and vibriosis are described. Then, information on emerging bacterial infections of concern for tilapia, like edwardsiellosis throughEdwardsiella ictaluriandE.tarda, as well asAeromonas schubertiiis provided. Reports of infectious bacterial tilapia disease outbreaks from other bacteria, includingLactococcus garvieae,Aerococcus viridans,Pseudomonasspp.,Mycobacterium marinumandChlamydiaspp., and others are reviewed. Furthermore, bacteria with zoonotic potential, likeStreptococcus agalactiaeST283,S.iniae,Aeromonassp.,E.tarda,Vibrio vulnificuspathovar (pv)piscisandM.marinumare included in the review, to provide the most current overview of the disease risks affecting production and post‐harvest stages. Additionally, the status and risks of antimicrobial resistance in bacteria from tilapia and other cultured fish through imprudent use of antibiotics, and its future at a global level are provided.

Largemouth bass is an important commercially farmed fish in China, but the rapid expansion of its breeding has resulted in increased incidence of diseases caused by bacteria, viruses and parasites. In this study, moribund largemouth bass containing ulcer foci on body surfaces indicated the most likely pathogens were iridovirus and rhabdovirus members and this was confirmed using a combination of immunohistochemistry, cell culture, electron microscopy and conserved gene sequence analysis. We identified that these fish had been co-infected with these viruses. We observed bullet-shaped virions (100–140 nm long and 50–100 nm in diameter) along with hexagonal virions with 140 nm diameters in cell culture inoculated with tissue homogenates. The viruses were plaque purified and a comparison of the highly conserved regions of the genome of these viruses indicated that they are most similar to largemouth bass virus (LMBV) and hybrid snakehead rhabdovirus (HSHRV), respectively. Regression infection experiments indicated fish mortalities for LMBV-FS2021 and HSHRV-MS2021 were 86.7 and 11.1%, respectively. While co-infection resulted in 93.3% mortality that was significantly (p < 0.05) higher than the single infections even though the viral loads differed by >100-fold. Overall, we simultaneously isolated and identified LMBV and a HSHRV-like virus from diseased largemouth bass, and our results can provide novel ideas for the prevention and treatment of combined virus infection especially in largemouth bass.

Tilapia lake virus (TiLV) is an emerging disease threatening tilapia culture in many parts of the world. A cell line from the brain of tilapia, which was named TiB, was established, characterized and subcultured with more than 100 passages. The TiB cell line was optimally maintained at 27°C using medium 199 (M199) supplemented with 10% foetal bovine serum (FBS). Chromosome analysis revealed that 60% of TiB cells at passage 5 maintained the modal chromosome number 2n = 44, while at passage 60, there were 43% of TiB cells with the diploid chromosome number 2n = 50. A significant cytopathic effect was observed in TiB cells after infection with tilapia lake virus (TiLV‐2017A), and the viral replication in the cells was confirmed by transmission electron microscopy, immunofluorescence assays and viral titres, indicating the susceptibility of TiB cells to TiLV‐2017A. The viral titres of TiLV‐2017A in TiB cells reached 10^7.43 TCID₅₀/ml within 10 days. The stable growth and susceptibility to fish viruses make TiB cells a useful tool for fish virus–host cell interaction and for immune response of fish.

Tilapia lake virus (TiLV) is a newly emerging pathogen responsible for high mortality and economic losses in the global tilapia industry. Currently, no antiviral therapy or vaccines are available for the control of this disease. The goal of the present study was to evaluate the immunological effects and protective efficacy of formaldehyde- and β-propiolactone-inactivated vaccines against TiLV in the presence and absence of the Montanide IMS 1312 VG adjuvant in tilapia. We found that β-propiolactone inactivation of viral particles generated a vaccine with a higher protection efficacy against virus challenge than did formaldehyde. The relative percent survivals of vaccinated fish at doses of 10⁸, 10⁷, and 10⁶ 50% tissue culture infectious dose (TCID₅₀)/mL were 42.9%, 28.5%, and 14.3% in the absence of the adjuvant and 85.7%, 64.3%, and 32.1% in its presence, respectively. The vaccine generated specific IgM and neutralizing antibodies against TiLV at 3 weeks following immunization that were significantly increased after a second booster immunization. The steady state mRNA levels of the genes tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interferon γ (IFN-γ), cluster of differentiation 4 (CD4), major histocompatibility complex (MHC)-Ia, and MHC-II were all increased and indicated successful immune stimulation against TiLV. The vaccine also significantly lowered the viral loads and resulted in significant increases in survival, indicating that the vaccine may also inhibit viral proliferation as well as stimulate a protective antibody response. The β-propiolactone-inactivated TiLV vaccine coupled with the adjuvant Montanide IMS 1312 VG and booster immunizations can provide a high level of protection from virus challenge in tilapia.

Grass carp reovirus (GCRV) is the causative agent of grass carp hemorrhage and causes significant loss of fingerlings. However, little is known about how the virus is distributed in organs and tissues. The aim of the present study was to investigate the distribution of different GCRV stains in tissues and organs of grass carp. The pathogenicity and tissue distribution of GCRV were monitored after intraperitoneal administration. The study showed a distribution of GCRV in different tissues and organs, particularly in the liver, spleen, kidney, intestine, and muscle, which had a higher number of viral RNA copies during the sixth to ninth days. The kidney had the highest numbers of viral RNA copies, as high as 24000 copies. Until the fourteenth day, nearly no viral RNA copies could be detected. This study defined the virus distribution in different tissues of grass carp inoculated by i.p. and supplied clues for the pathogenesis of GCRV.

Recently, substantial mortality of farmed and wild tilapia caused by tilapia lake virus (TiLV) infection has been observed worldwide. However, sensitive and reliable diagnostic method is limited. A reverse transcription–loopmediated isothermal amplification (RT‐LAMP) assay has been applied for the detection of TiLV nucleotide sequence. Six primers targeting two locations on the target gene based on a highly conserved sequence in the segment 1 (S1) region of the TiLV genome have been designed. The optimized RT‐LAMP reaction was maintained at the isothermal condition of 63°C for 45 min. And the amplifications could be verified by turbidity or a colour change with the addition of SYBR Green I. Subsequently, RT‐LAMP products could be observed by a ladder pattern following gel electrophoresis. The species‐specific assay showed that the method was sensitive enough to detect as low as 1.6 copies of viral particle, and the assay was highly specific because no cross‐reactivity was observed with other pathogens, and had a diagnostic sensitivity and specificity of 100% when TiLV‐positive samples and non‐target virus were tested. In summary, all the results demonstrate that this RT‐LAMP is a rapid, effective and sensitive method for TiLV detection in tilapia aquaculture.