Tilapia

In common with all other animal farming systems in which animals are raised in greater numbers than they would be found in nature, the farming of tilapia can potentially increase the risk of disease outbreaks due to the number of individual animals living in close proximity to each other. Aquaculture can potentially introduce diseases with movements of stock that have previously been exotic to a region or can enhance the levels of naturally occurring diseases within a given area. Disease can reduce profit for farmers and potentially impact on indigenous species. Thus, it is essential that good husbandry and a pro-active approach to health management is adopted at each farm location in order to minimise and mitigate these risks.

Tilapia can be affected by a range of diseases which can have adverse economic and welfare effects. Common bacterial pathogens and diseases of tilapia include Aeromonas hydrophila¹, Streptococcus iniae² and Streptococcus agalactieae³, columnaris disease (caused by Flavobacterium columnaris⁴) and Francisellosis⁵. A new orthomyxovirus-like virus, tilapia lake virus (TiLV), has recently emerged in S. America and Israel⁶. Other viruses including megalocytiviruses, and betanodaviruses are also reported^7, 8. Tilapia are also affected by a range of myxozoan and other parasites^{6, 9}.

The first line of defence in disease and pathogen management is effective biosecurity management and health plans at individual farm and area level to minimise disease and its spread. The key elements of biosecurity include; practical and appropriate legislative controls, adequate diagnostic and detection methods for infectious diseases, disinfection and pathogen eradication methods, reliable high-quality sources of stock; and best management practices^{10, 11}.

This includes continued maintenance of optimal health conditions and ensuring maximum stocking densities are not exceeded at various stages of the production cycle. Monitoring and achieving target growth affirms that the stock is performing well, and recording of annual average farm survival rate indicates the health of the systems. Certification schemes set targets for maximum average real percentage mortality rates. Regular health checks and screening allows for rapid action to be taken if problems begin to develop. Maintenance of good daily records of mortalities facilitates future management by highlighting when in the production cycle disease problems are likely to occur. The development of a written health plan updated annually and approved by an aquatic animal health specialist is recommended and often part of certification requirements. Along with better regulation, there has been increasing uptake of Best Management Practices (BMPs), codes of conduct, and certification schemes in the Asia-Pacific region and in tilapia aquaculture which help in tackling disease issues.

The farmer should follow the instructions of aquatic animal health specialists about who to inform and how to stop the spread of the disease. When required, veterinary medicines and chemicals (including antibiotics) can play an important role in maintaining tilapia health, but incorrect use can have environmental as well as human health impacts.

Overuse of antibiotics in farming or for human medical treatment speeds up the development of antibiotic resistance, which is when bacteria change and become resistant to the antibiotics used to treat them¹².

Tilapia farms should only use veterinary medicines and chemicals that are approved by national authorities and these should be prescribed by an aquatic animal health specialist. Farmers should follow the instructions of the aquatic animal health specialist regarding storage, and medicines or chemicals should be used as per directions. Stock should not be harvested before completion of the withdrawal period specified for the medicine. Records of medicine and chemical stocks and their usage should be kept and made available for inspection or audit.

The following veterinary medicines should not be used^{13, 14}:

• Antibiotics critical for human medicine, as categorised by the World Health Organisation¹⁵
• Veterinary medicines (excluding vaccines) used prophylactically prior to evidence of a specific disease problem
• Veterinary medicines (excluding vaccines) to serve as growth promoters

Vaccination can play an important role in preventing disease problems in tilapia farming. There are licensed vaccines available to control some of the diseases of tilapia, in particular, the main streptococcal diseases¹⁸. Similarly, selection for increased resistance to bacterial diseases has been started in commercial breeding programmes and can be expected to make an impact in the near future¹⁹.

Functional aquafeeds for farmed fish are developing. These feeds include a range of additives used to improve growth and feed utilization but also to support the health and stress resistance. Additives such as probiotics, prebiotics, phytogenics, and immune-stimulants may help improve disease resistance and reduce disease incidences^{20, 21}.