Regardless of the aquatic species an aquaculture venture produces, there are significant risks to success due to the ever-present threat of a disease outbreak. As reported by the World Bank (2014), disease results in an estimated economic impact of $6 billion USD annually for the aquaculture industry on a global scale. Bringing the cost associated with disease outbreaks down is a major challenge, and in most cases, there is no easy solution. Therefore, aquaculture operations must explore solutions and implement various tools (products) that can work under the specific conditions associated with each facility or species they rear. This is where things become confusing and complicated for most aquaculture managers, veterinarians, or aquatic animal health specialists.

A disease can be characterized as infectious or noninfectious, which simply refers to an infection by a pathogenic organism that results in disease or other noninfectious factors such as genetics, environment, or nutrition that cause a disease (or abnormal) condition. The risk of pathogen exposure and disease is especially pronounced in aquatic systems (Oidtmann et al., 2013) where animals are continually exposed to potential pathogens that are endemic or introduced to the water they are reared in. They can also experience sudden water quality shifts that can impact health and disease resistance. In many instances, there is not often just one specific virus, bacteria, parasite, or fungal infection that causes disease, but a combination of co-infections and environmental stressors that contribute to the manifestation of a disease outbreak. Snieszko (1973) indicated that in most cases an infectious disease outbreak occurs when a virulent pathogen, a susceptible host, and an unfavorable environment are all concurrently present. Often, an infectious disease can be avoided if one of these components is missing, which emphasizes the importance of maintaining high biosecurity and optimum husbandry practices to avoid a virulent pathogen or unfavorable environment, respectively. However, even under the best conditions, a highly virulent pathogen may cause disease in a susceptible host.

There are products and strategies that can assist in the prevention or control of infectious diseases in many aquaculture species, but the best approach or products are not always obvious. For some species that are commonly cultured, such as Atlantic salmon, there are multiple vaccines that have been proven to protect against certain diseases. Antibiotics can also be utilized, but overuse can sometimes lead to greater problems. In some parts of the world they are heavily regulated, but in other regions restrictions on their use are less clear. This presents challenges and increases the risk of bacterial pathogens developing resistance to specific antibiotics. Parasites, such as sea lice, have also been shown to develop resistance to chemical treatments when repeatedly administered. This has caused ongoing problems for the salmon industry, which spends billions of dollars annually combating sea lice infestations.

Most would agree that a preventative approach to disease management is far better than being forced to react to an outbreak once it has occurred. Antibiotics or chemicals aimed at removing external pathogens typically represent a reactive approach, whereas vaccines or the feeding of high-quality or functional feeds (discussed below) can promote a preventative approach. An efficacious vaccine that can target a specific disease of concern is often the most effective method for preventing or minimizing disease-related impacts and improving productivity (Hegde et al., 2022). However, the availability of commercial aquaculture vaccines is often limited or a vaccine may be licensed in one country but not another. If a fully licensed vaccine is available, can be administered effectively to animals prior to exposure to a pathogen of concern, and the cost of the vaccine outweighs the cost associated with the disease outbreak, then it should be implemented into a health or disease management plan. The challenge is that not all vaccines can be delivered to the animal prior to the life stage of greatest susceptibility. For example, many fish are susceptible to a specific disease at young stages when they may not be fully immunocompetent; therefore, they are incapable of mounting a protective immune response. In addition, it is not always possible to deliver an injection vaccine to fish at small sizes due to the logistics, costs, and stress of handling fish during these early stages, and many vaccines provide only limited protection when administered by immersion or oral (within feed) delivery. This is an active area of research but remains a significant challenge to implementing solid vaccination programs for many operations. Research on new and improved vaccines is also ongoing, and there are even companies that can produce and provide “autogenous” vaccines from pathogens isolated directly from specific aquaculture facilities for use at those facilities. These can be effective in some cases, but they often lack the rigorous testing of a fully licensed vaccine and there is no guarantee they will work. Most vaccines contain a killed version of the pathogen of concern, which when administered to an animal can stimulate an immune response that provides protection when the same virulent pathogen is encountered. Some of the earliest killed vaccines remain effective for diseases such as Vibrio or enteric redmouth and can be delivered by injection or immersion, but many vaccines derived from “killed” bacterial cells lack the ability to stimulate a strong protective immune response or are only effective when delivered by injection, thus limiting their potential for use in small fish. Recent improvements have been observed by utilizing vaccines that contain live attenuated (non-virulent) versions of the pathogen of concern. If these have undergone the rigorous testing required to show they are safe and will not revert back to virulence, they can be quite effective, even when delivered by immersion for mass vaccination of smaller animals (Sudheesh & Cain, 2017). A range of other less traditional vaccines (such as DNA vaccines) have been tested experimentally and a few are available commercially in some countries. Clearly, there are limitations to vaccines, and they are not available for many major diseases of concern, but if an effective licensed vaccine exists, it often represents the best option when taking a preventative approach to managing the impact of specific disease problems.

Vaccines, antibiotics, and chemical therapeutants that act on pathogens in the water or on the surface of animals are the most common “tools” in the fish health toolbox, but more and more alternative products are being developed or marketed for use in aquaculture. This is when it becomes challenging for the farmer or hatchery manager to sort through the science and determine how to minimize disease-related impacts while maximizing production capacity. In many instances, the key to producing healthy animals is not in managing the disease outbreak, but managing stress, which leaves animals weak, vulnerable, and less able to resist infection and disease. In general, there are acute stresses that involve short-term physiological responses, and chronic stress that may result in depressed immunity and increased pathogen susceptibility (Mauri et al., 2011; Small & Bilodeau, 2005; Tort, 2011; Tort et al., 1996). If optimum environmental conditions can be maintained, it can minimize such stress and should be the highest priority. However, there are products available that can improve overall animal health.

One area of increased emphasis has involved producing “functional feeds” to enhance the animal's immune response and improve resistance to stress and disease. Such products may include various immunostimulants and feed additives advertised for their ability to promote aquatic animal health. Other additives and products reported to have health-enhancing properties include various herbs and spices (or their derivatives), probiotics that can be added to the feed (or water) and minimize bacterial loads or increase host resistance, prebiotics that can be fed to animals to promote healthy microbial flora, and even postbiotics that include antimicrobial peptides or exopolysaccharides that can act against bacterial pathogens. Given that there is a lack of effective vaccines available to prevent specific diseases in many species and an inherent risk of antibiotic and chemical overuse, it is wise to consider alternatives and work to enhance an animal's underlying ability to combat stress.

Beneficial effects of immunostimulants and feed additives such as beta-glucans, nucleotides, and probiotics have been well documented for aquaculture (De et al., 2014; Irianto & Austin, 2002; Li & Gatlin III, 2006). Commercial diets that include beta-glucans have been shown to improve immune status and aid in controlling diseases in aquaculture (De et al., 2014). Other dietary feed additives such as nucleotides, when supplemented in diets can provide growth and health benefits (Li & Gatlin III, 2006). In a study by Sudheesh et al. (2016), rainbow trout fed for a period of 3 weeks with a commercial functional feed, which claimed to contain proprietary functional feed ingredients and immunostimulants, had significantly lower mortality compared with fish fed a basal diet following an acute stress exposure of 0.5 ppm chlorine. This suggests that the inclusion of functional feed ingredients such as immunostimulants, probiotics, and/or prebiotics can provide fish the capacity to tolerate acute stress events in addition to the immune-enhancing and health benefits offered by these feed ingredients. Furthermore, the use of immune nutrients or “prebiotics” that represent nondigestible food ingredients can confer benefits to animals by serving as a substrate or energy source to stimulate the growth of beneficial gut microbes. Although benefits can be realized from these additives, the success and overall effect of these products may be unpredictable and we do not fully understand the role of stress, dose, and feeding strategies in relation to the wide range of aquaculture species. Often, such immune stimulating additives can only elevate an animal's immune system for a short time and continued feeding of such products does not necessarily provide continued benefit. Therefore, strategies that utilize pulse feeding during periods right before potential stress events such as handling or transport or prior to possible pathogen exposure (e.g., moving fish from a bio-secure hatchery to outside ponds) could maximize the benefit and reduce the cost associated with continual feeding.

The challenge is that every facility operates differently, and this must be taken into account when considering disease management strategies. The benefits offered by traditional tools, functional feeds, and alternative products can vary widely depending on the culture conditions, species being reared, and other outside factors. For aquaculture to be sustainable into the future, producers must strive to produce healthy animals by minimizing the risk of major disease outbreaks. Therefore, a big-picture approach that incorporates both traditional and newer tools into an overall health management plan can be highly beneficial.

REFERENCES

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