Prevention and control strategies for viral infections in farmed tilapia

Review discusses the occurrence, transmission, pathology, diagnostic methods and control measures to mitigate outbreaks

Despite the general belief that tilapia is disease-resistant, the intensification of culture systems to meet the increased demands for tilapia exerts greater levels of stress on the fish. This in turn increases the risk of infectious disease outbreaks, making infectious disease outbreaks the main factor that impedes the growth of the global tilapia aquaculture industry.

Tilapia are susceptible to various bacterial, viral, fungal and parasitic infections that cause significant production and economic losses. Among the various infectious agents, viruses are undoubtedly the major pathogen that threatens the aquaculture industry due to limited antiviral therapeutics and vaccines.

A comprehensive understanding of the viral infections that affect the tilapia industry is needed so that effective measures for the prevention and control of viral diseases can be implemented. However, currently there is a lack of up-to-date literature that comprehensively covers all viral infections known to affect tilapia.

This article – summarized from the original publication (Clyde, C.W. et al. 2024. Current updates on viral infections affecting tilapia. Aquaculture and Fisheries, Available online 14 July 2024 – discusses prevention and control strategies for viral infections in farmed tilapia. The original publication also covers in detail viral infections in tilapia.

Viral infections affecting tilapia

To date, ten viral infections consisting of seven DNA and three RNA viruses have been reported to infect tilapia, and the timeline of the first emergence of these viral infections and corresponding regions are shown in Fig. 1. As the years advanced from the 1970s to the 2010s, an increasing number of viral diseases have emerged in tilapia, likely attributed to the intensification of aquaculture practices.

The DNA viruses known to infect tilapia include Tilapia Parvovirus (TiPV), Tilapia Larvae Encephalitis Virus (TLEV), Infectious spleen and kidney necrosis virus (ISKNV), Iridovirus-like agents, Novel Megalocytivirus, Bohle iridovirus (BIV) and Lymphocystis Disease Virus (LCDV).

The RNA viruses known to infect tilapia include Nervous Necrosis Virus (NNV), Infectious pancreatic necrosis virus (IPNV) and Tilapia Lake Virus (TiLV). For detailed information on these viral agents and the characteristics of viral diseases and diagnostic methods in tilapia, refer to the original publication.

Husbandry, biosecurity and surveillance

Considering that tilapia is a commercially important protein source, losses in production due to viral outbreaks may have significant negative socio-economic impacts and compromise food security. To mitigate these impacts, effective strategies for preventing and controlling viral disease epidemics need to be developed and implemented in aquaculture farms.

One such strategy involves the use of antiviral therapeutics or prophylactics. However, presently, there is a lack of commercially available therapeutics to treat viral diseases in tilapia aquaculture. Although past studies have developed therapeutics for some viruses, none have been certified for commercial use. Furthermore, these therapeutics have only been tested experimental conditions, leaving uncertainty regarding their performance in field applications. Notably, tilapia was not the model fish used in these studies, posing a challenge, as certain therapeutics derived from other fish species may require specific receptors to be effective, potentially rendering them ineffective in tilapia. Due to these limitations, strategies such as good management practices (GMPs), biosecurity measures and surveillance programs remain the primary methods for preventing and controlling viral outbreaks.

Impact of stress

Stress induced by inadequate culture conditions or during fish transportation often increases the vulnerability of fish to viral pathogens, emphasizing the importance of GMPs to mitigate stress and reduce viral outbreak incidences. Maintaining optimal water quality, appropriate fish stocking densities, effective sanitization practices and providing adequate nutrition collectively contribute to minimizing stress levels in cultivated fish.

Biosecurity measures, in conjunction with screening and surveillance programs, play a crucial role in controlling disease spread. These measures encompass stringent quarantine protocols, prompt disposal of moribund fish, and the establishment of different zones within aquaculture facilities. Biosecurity also entails rigorous sanitation and disinfection practices at all farming stages to prevent viral transmission.

Several disinfectants have demonstrated efficacy in reducing viruses like TiLV, ISKNV, IPNV, NNV and LCDV in in vitro or in vivo experiments, suggesting their potential application in aquaculture settings. However, further investigations are necessary to assess the efficacy of these disinfectants under field conditions and their potential impact on cultivated fish and the environment.

Use specific pathogen-free fish

Given the propensity of viral diseases affecting tilapia for both horizontal and vertical transmission, it is imperative for tilapia farmers to procure specific pathogen-free (SPF) stocks to minimize the introduction of viral pathogens into their farms. Previous studies have demonstrated that different tilapia strains exhibit varying degrees of resistance to viral diseases such as TiLV. Moreover, resistance to TiLV infection has been established as heritable, indicating the feasibility of selective breeding to enhance resistance to TiLV and other viral diseases, thereby mitigating the risk of epidemics.

Development of disease-resistant strains

To expedite the development of disease-resistant strains, molecular selection techniques can be employed to identify quantitative trait loci (QTL; a section of DNA that correlates with variation of a quantitative trait in the phenotype of a population of organisms, and often an early step in identifying the actual genes that cause the trait variation) associated with disease resistance. Recently, a QTL linked to TiLV resistance in tilapia was pinpointed, offering a promising avenue for selecting TiLV-resistant tilapia for breeding programs.

Meanwhile, early detection of viruses in aquaculture can facilitate the implementation of timely control measures to contain the spread of disease. Both biosecurity and surveillance in aquaculture are bolstered by the utilization of various diagnostic tools, including viral isolation, histopathology, immunoassays, and molecular-based methods. The accurate diagnosis of viral diseases in tilapia is particularly crucial, as many of these diseases exhibit overlapping clinical signs, making it challenging to differentiate between different viral infections based solely on gross lesions. Among the available diagnostic tools, molecular-based methods such as PCR are frequently employed for their speed, sensitivity, and specificity.

However, the use of molecular-based diagnostics often requires well-equipped laboratories and trained personnel, resources that may not be readily available in every country. For instance, several African countries lack the necessary laboratory infrastructure and skilled personnel, leading to delays in diagnosing TiLV and implementing control measures. To address this challenge, the development of rapid diagnostic kits capable of on-site testing without the need for sophisticated equipment or expertise is essential for the timely detection of current viral infections affecting tilapia.

Sizing up TiLV and its potential impact on tilapia production

Vaccines

Alternatively, vaccines represent another prophylactic strategy, wherein their administration in fish stimulates an immune response against specific pathogens. Several vaccines against TiLV and ISKNV have been developed for tilapia, with only one vaccine for ISKNV (AQUAVAC® IridoV) currently commercialized. Additionally, vaccines for IPNV, NNV and LCDV have been developed for other fish species, with commercialized vaccines available for IPNV and NNV. However, further research is needed on the development of IPNV, NNV and LCDV vaccines specifically tailored for tilapia, as existing vaccines may not perform optimally due to differences in adjuvants, dosage, delivery route and required vaccination frequency.

Although vaccines have the potential to reduce losses caused by viral diseases, several factors must be considered. One such factor is the cost-effectiveness of vaccines, which should be ensured to promote their widespread use in tilapia farms worldwide. It is especially important to ensure affordability for small-scale tilapia farmers. Nonetheless, vaccination cannot be applied to tilapia larvae or fry due to their underdeveloped immune systems. This limitation is significant as many viral diseases – such as TiLV, TLEV, NNV and ISKNV – infect these early life stages of tilapia.

Immunostimulants

Instead, immunostimulants could potentially be used to enhance the innate immune responses of young tilapia against viral infections. For example, Elkatatny et al. demonstrated upregulation of various immune-related genes when Nile tilapia fry were administered an immunostimulant consisting of an amino acid mixture. Furthermore, the administration of immunostimulants – including yeast, extract of custard apple leaf, garlic and echinacea – has been shown to improve the immune response of adult or juvenile tilapia under experimental conditions.

Similarly, enhanced immune responses were observed after incorporating various probiotics. For example, diets supplemented with Bacillus spp. resulted in lower mortality rates and upregulation of immune-associated genes during experimental TiLV infection in red hybrid tilapia. Additional studies also reported reduced mortality rates caused by NNV, LCDV and iridovirus under experimental conditions in other fish species fed with probiotics. Thus, immunostimulants and probiotics show promise in limiting viral diseases. Nevertheless, field application studies and cost-benefit analyses should be conducted to determine the feasibility of this strategy.

Overall, viral disease management in tilapia aquaculture requires the implementation of various strategies, as no single strategy is sufficient, and these strategies must be sustainable, practical and cost-effective for tilapia farmers to effectively reduce viral infections.

Perspectives

The emergence of viral infections in tilapia aquaculture is inevitable due to the widespread intensification of production. Among viral diseases, TiLV has received extensive research attention due to its current global impact on tilapia aquaculture. In contrast, limited follow-up research has been conducted on other viral infections affecting tilapia. Consequently, the genetic information, pathogenesis, epidemiology and distribution of these viral diseases are yet to be fully determined, leaving the true impact of these diseases on tilapia aquaculture uncertain.

All life stages of tilapia are susceptible to varying viral diseases, posing significant risks to all stages of tilapia production. However, the incidence of viral outbreaks can be reduced by acquiring sufficient knowledge of viral diseases to implement effective prevention and control measures. Biosecurity and surveillance programs are the most effective methods to curb viral diseases, but efforts should be placed on the development of vaccines and therapeutics to further mitigate the losses. Furthermore, future studies should also prioritize investigating the lesser-known viral diseases, as the lack of awareness and study of these diseases in tilapia may inadvertently lead to epidemics in tilapia production.

@GAA_Advocate