Tilapia is currently the second-most produced fish in the world and production of this species is increasing. In fact, the popularity of tilapia is skyrocketing. By 2010, Intrafish predicts that the global value of tilapia will be US $4 billion.
There are several reasons for increased production. Tilapia can be produced in versatile locations, water systems, temperatures and salinities. They have good performance characteristics such as fast growth, high fillet yield and a low feed-conversion ratio as well as a firm, white fillet that makes them easy to market.
In many areas, tilapia production is already extensive and likely to intensify. Intensified production, however, will undoubtedly lead to challenges, including sourcing quality seed, maintaining fish quality, controlling disease, ensuring food safety and effective marketing.
Although each of these subjects is huge, this paper will focus on control of disease. When farming of any animal is intensified, disease is virtually inevitable, in part because the farming environment is artificial and stressful compared to a natural habitat. In general, there are six to 10 specific diseases for each animal species. In new farming systems, we have found that infections usually require two to five years to become fully established.
Effective prevention and control of disease requires an integrated approach to health management, which takes into account all aspects of fish farming that impact the health of the fish population (Figure 1).
Factors affecting disease expression
The expression of disease on any fish farm involves four controllable factors: the species, farm management, the environment and the pathogens present. No one factor dominates, but the type and severity of the disease will depend on a complex relationship among them all.
Poor farm management, a suboptimal environment and/or the presence of pathogens result in fish stress. Stress is a product of trauma that fish suffer and the length of time that fish are exposed to the trauma. Disease develops when a combination of stressful factors meet and the stress level of the fish population has reached a point that is detrimental to the immune system.
As mentioned before, there is great diversity in the husbandry systems used by tilapia farms around the globe. There is also a correspondingly large variation in tilapia strains and hybrids, allowing farmers to select for different traits to suit local conditions. This ensures good growth in systems ranging from clear, open-water cages or green-water ponds to closed, recirculated raceways and tanks. However, no matter the system employed, there are important issues to address on the farm before its full production potential can be realised.
Farm management
A primary concern is identifying a consistent source of good-quality seed stock. Hatchery-produced fish are recommended because they are often genetically selected or improved, monitored for diseases and given an optimal diet that results in a more uniform starting point for the farmer, which results in more reliable and reproducible results.
Suboptimal feeding or other environmental stressors, such as poor water quality, will result in poor-quality, weak fish and increased susceptibility to opportunistic pathogens. The smaller fish are, the lower the reserves they have and stress will put more strain on their tolerance limit.
With consistent, disease-free quality seed stock, emphasis should be placed on biosecurity to prevent or limit the transfer of pathogens onto the site and between locations. This requires minimising the transfer of vectors, which include humans, animals, equipment, water and machinery. Defined or physical barriers established to minimise the spread of disease will play an ever-increasingly important role in aquaculture. These restrictions should encompass everything from the impact of regional trade on live production to inter-production-unit movements on the farm. The faster the biosecurity process evolves, the more sustainable the industry will be.
To some extent, vector movement between production units within a farm is unavoidable, but correct sanitary measures are a necessity. Indeed, overlooking general hygiene measures is a common problem on farms and is often found to be the root cause of a disease.
There are always three important steps involved in the sanitary process for any situation. The first is “cleaning,” which entails the removal of unwanted substrate; the next is “disinfection,” designed to eliminate the organism with the appropriate choice of products and methodology; and finally “rinsing” to remove remnants of potentially toxic disinfectant chemicals. To control the transmission of fish pathogens, it is vital that this sequence be followed for any surfaces that may have been directly or indirectly in contact with fish. Techniques applied must ensure the safety of fish, humans, equipment and the environment.
When adhered to, good sanitation can control or even eliminate disease outbreaks, but implementation is often a limiting factor. To carry out important routine sanitary measures, employees must be productive, proactive, experienced and, above all, proud and enthusiastic. Clear goals and welldefined protocols are necessary. The aim is to ensure that fish welfare is the top priority and that any adverse or unusual event is reported swiftly and dealt with quickly. Employees can be encouraged to cooperate with sanitation procedures if a bonus or some other type of reward is provided as an incentive for reporting events that may be detrimental to fish health.
Another factor that correlates farm management to disease outbreaks is production recording. If recorded data is relevant, clear and precise, it can help in the early recognition of disease. Disease triggers are multi-factorial and often complex; however, time-data records enable comparisons of historical situations and will often reveal patterns or trends pertaining to the same disease outbreak. Typical patterns may include fluctuations in environmental parameters, changing characteristics in fish behavior and/or the severity and onset of mortality within a population. Recognising these triggers through good productiondata records can go a long way in improving disease management.
The environment
The environment plays a crucial role in the expression of disease. Production recording and pathogen surveillance demonstrate that most diseases in tilapia thrive in certain temperature and salinity ranges. These findings add to the complexity of tilapia farming but may prove to be a useful tool in disease control since, in many cases, temperature and salinity manipulation is possible.
Greenhouses, for example, are now a common phenomenon throughout aquaculture in China and help prevent suboptimal growth and coldwater diseases. In addition, saltwater use, if readily available, can help to control parasitic or salinedependent bacterial outbreaks.
Pathogens
Tilapia are susceptible to both infectious and non-infectious diseases. An infectious disease occurs when a pathogen is present. Simply identifying a pathogen is a good start but does not prove that the pathogen identified is the cause of reduced performance, morbidity or mortality. To accurately diagnose the cause of a disease, field sampling and diagnostic techniques must be performed. It is crucial that the correct samples be sent for analysis; moribund fish should be sampled since common environmental bacteria will contaminate dead fish quickly and mask identification of the pathogens that cause the disease. Furthermore, it is of no value to investigate a fish with clinical signs that do not represent those of the greater diseased population.
To really understand the cause of a disease, it is essential to implement long-term, routine field sampling and disease epidemiology. Correct sampling techniques will identify the specific pathogens present and screen for anything new entering the system. Once established in the farm management system, this information, combined with knowledge about disease triggers, will allow for effective identification and resolution of the problem.
For the last eight years, Intervet/Schering-Plough Animal Health has conducted extensive sampling and epidemiological investigations throughout Asia-Pacific, Africa and Latin America. Four major bacterial diseases have been found—Streptococcus agalactiae, Streptococcus iniae, Flavobacterium columnare and RLO, which stands for a “rickettsia-like-organism” and was recently identified as Francisella spp. We also found one viral disease (iridovirus) and several important external protozoal and monogenetic parasitic infections including Trichodina spp. and Gyrodactylus spp. Non-infectious diseases generally result from non-living causes; the most common are water toxicity and feed toxicity, although biological factors such as genetics, age, diet and stress may also contribute to non-infectious disease.
In short, disease epidemiology and health monitoring are crucial to the development of a worthwhile integrated health management strategy. Prerequisites for disease prevention are the identification of the etiological agents and understanding the epidemiological factors that trigger and aggravate diseases on the aquaculture farm.
Treatment and prevention
* "Effective prevention and control of disease requires an integrated approach to health management, which takes into account all aspects of farming that impact the health of the fish population." |
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Neil Wendover, Technical Services Manager, Intervet/Schering-Plough Animal Health
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An integrated health management plan encompasses all factors that may affect fish health, as shown in Figure 1. Key components of the plan are two complementary strategies aimed at dealing with infectious diseases. One is a reactive pathogen exclusion strategy, and the other is a proactive pathogen prevention strategy. Given time and understanding, the reactive strategy may indeed develop into an optimised metaphilactic treatment given just before the fish get sick.
Therapeutic medicines used in food animals, particularly antibiotics, have resulted in a lot of controversial discussion in the press recently, including aquaculture and the tilapia industry. Environmentalists want assurance that natural ecosystems won’t be damaged, and consumers want to know that any antibiotic treatment used will not contribute to antibiotic resistance in people and that the foods they eat do not contain antibiotic residues. There has also been discussion about the presence of prohibited antibiotics in food animals.
In light of these pressures and to ensure the sustainability of the aquaculture industry, it is imperative that reactive, therapeutic disease management be conducted responsibly. The first step is correct identification of the etiological agent causing a disease outbreak so that appropriate treatment can be determined. For instance, an antibiotic may be indicated for an outbreak of a bacterial infection but cannot be used to treat a parasitic outbreak.
It is fundamental that any chemical or antibiotic administered has regulatory approval for use in fish in the local country. Malachite green, for example, is a banned substance and cannot be used. To achieve effective disease control, the agent causing disease must be sensitive to the chosen therapeutic treatment. In addition, the therapeutic drug must be administered in strict compliance with the manufacturer’s label recommendation— the correct dosage must be administered as directed for the recommended treatment duration and withdrawn at the proper time.
Following the manufacturer’s directions in many cases can help ensure food safety as well as effective disease control and also helps prevent the development of resistance to therapeutic drugs.
Treatment of a disease outbreak should be supervised by a fish health professional to ensure that fish receive appropriate, correctly administered therapy, but it is the fish farmer’s responsibility to adhere to the specified withdrawal periods of antibiotics. Antibiotics should be purchased from trustworthy sources that provide products from plants that utilise Good Manufacturing Practices and that ensure quality, purity and the desired concentration of the active ingredient.
History has taught us that there will always be new diseases. Responsible therapeutic management is an important tool that can minimise the short-term impact of disease and help allay fears among environmentalists and consumers. However, drug treatment should never be considered a long-term solution since the root of the disease problem must be identified and a proactive, preventive strategy devised as part of an integrated health management plan.
The preventive disease strategy should be aimed at reducing stress in fish through good husbandry practices, the use of immunomodulators to boost the immune system and the use of vaccines. Remember that vaccines target a specific disease and cross-protection rarely occurs. For instance, a vaccine to protect fish against S. iniae protects only against S. iniae infection and not against S. agalactiae. In addition, if the environment is suboptimal, a vaccine may still perform, but the fish may die of another cause.
Conclusions
Diversity in production techniques and environments means each aquaculture operation is different. A universal trend, however, is culture intensification, which will undoubtedly lead to disease challenges. The prevention and control of disease on any fish farm depends on integrated health management—a delicate and balanced plan that takes into account all factors that may affect fish health, including the fish species, the environment, farm management and the pathogens challenging the farm. Integrated health management incorporates preventive techniques such as the use of immunomodulators and vaccines, which are likely to gain increased emphasis in the future, as well as the responsible use of antibiotics when bacterial disease outbreaks occur.
Health monitoring and data management systems are integral components of integrated health management; they can serve as the basis for the development of basic guidelines, enable the recognition of disease triggers and, when outbreaks occur, make it easier to devise tailored solutions that foster speedy diagnosis and rapid, appropriate treatment.
Integrated health management can not only help tilapia farmers ensure food safety, it will also improve survival rates, production forecasting and consistency.
