58 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG cell lines to profile them and to develop control strategies. Pathogenicity, potential host range, and viral replication inhibition must be determined when establishing management strategies, including vaccines and antiviral agents. Since treatment options are limited for many viral diseases, early disease diagnosis and proactive measures are essential for successful fish health management. Detailed information on infectious agents will facilitate better management strategies for intensive fish production systems to improve animal health, enhance production efficiency and thereby impact the profitability of the aquaculture industry. In this context, suitable FCLs for fish virus research are limited and some of the FCLs are even used to study viruses isolated from crustaceans. Replication of virus in suitable FCLs results in cytopathic effects (CPEs) (Figures 2 and 3). Some fish herpesviruses are highly host-specific and might not replicate in nonspecific cell lines (Hanson et al. 2011). Species-specificity of viruses necessitates the development of cell lines from appropriate hosts. Development of Vaccines: Proactive management strategies such as vaccines are very effective in controlling infectious fish viruses and are crucial for successful hatchery and farm operations. FCLs allow for large-scale production of vaccines. Attenuated viral vaccines can be developed by repeatedly passing the wildtype virus through appropriate FCLs, which has proven to weaken the virus (Noga and Hartman 1981, Ronen et al. 2003, Perelberg et al. 2005) and avoids reversion to wild type pathogenic capacity as evidenced in previously published studies (Dishon 2009). Antiviral Agent Studies: Cell lines are frequently used for screening antiviral compounds. Acyclovir (antiviral agent against human herpesvirus) inhibited channel catfish virus and cyprinid herpesvirus-3 in FCLs (Troszok et al. 2018, Hao et al. 2021). Several FCLs were employed to study the antiviral efficacies of different compounds against nodavirus, grouper nervous necrosis virus, Koi herpesvirus, IPNV and IHNV (Huang and Han 2010, Balmer et al. 2017, Reichert et al. 2017). Toxicology: Being relevant representatives of the aquatic environment, FCLs are routinely used to evaluate the effects of drugs and chemicals to optimize dosing, study modes of action and characterize cellular damages (Babich et al. 1991, Segner 1998, Fent 2001). FCLs are used to establish the potential toxicity of heavy metals, aflatoxins, pesticides and other toxicants (Bailey By providing appropriate culture conditions, cells can be grown in vessels (Carrel and Burrows 1911). Cultured cells mimic the host animal and hence can substitute animals used in research, taking into account the three ‘R’s- Replacement, Reduction, and Refinement. Cell cultures avoid the social and ethical concerns of animal use in research. Ever since the use of the Rainbow trout gonad (RTG2) cell line in virus studies (Wolf and Quimby 1962), several fish cell lines (FCLs) have been established from different species and utilized in diverse research fields including virology, toxicology, immunology, genetics and pharmacology (Figure 1). In addition to being an efficient alternative to the use of fish in research, FCLs avail ease of handling, rapid test results, reproducibility, and economical feasibility (Wolf and Ahne 1982; Babich et al. 1991; Bols and Lee 1991; Verma et al. 2020). The acceptance of FCLs in various research fields is due to their ease of generation, immortal nature, and potential for genetic manipulation and cryopreservation for future applications. Fishderived cells can be cultured in a wide range of environmental conditions (Wolf and Quimby 1976a, b). Though there are >30,000 species of fish, only around 300 established FCLs are available globally to researchers, indicating the untapped potential of this field. Some significant applications of FCLs are discussed below. Fish Cell Lines as Model Systems: Cell lines simulate the host animal and hence are used to study cell biology, physiology, effects of drugs, and other topics (Hightower and Renfro 1988; Higaki et al. 2013). Zebrafish cell lines are excellent models to study diseases and host-pathogen interactions due to the ease of genetic manipulation and similarities with human genes. FCLs are considered perfect research models as these are not as subject to interference from environmental disturbances compared to animals. Virus Research: Since cells are essential for the replication and propagation of viruses, cell lines are considered ‘the gold standard’ in virology. Cell lines are required for viral disease diagnosis and confirmation. It might not be practical to rely on infected animals to procure viruses for research purposes. However, cell cultures are reliable sources of viruses (Bowser and Plumb 1980). Virus studies require permissible cell lines as evidenced in the case of fish pandemics caused by infectious pancreatic necrosis (IPN), infectious hematopoietic necrosis (IHN), and viral hemorrhagic septicemia virus (VHSV) (Kelly et al. 1978). Emerging viruses with limited information warrant susceptible The Diverse Research Applications of Fish Cell Lines Suja Aarattuthodi FIGURE 1. The diverse research applications of fish cell lines (FCLs).