Aquaculture  - Biotechnology topics

Fish and shellfish are subject to diseases that have no implications for human health, but that can have devastating impacts on fish and shellfish health. New biotechnology techniques, such as nucleic acid probes and other gene-based tools, are providing faster and more accurate ways to detect and identify fish and shellfish disease agents. These tools are based on nucleic acid – sequences of DNA or RNA that are characteristic for a particular organism and can be used to identify it. Many nucleic acid probes already exist for fish and shellfish disease agents, and many more are under development.

Effective screening requires standardised, well-established test methods that provide accurate results and can be replicated by different laboratories. Since biomolecular diagnostic techniques are relatively new to aquatic animal health, we currently lack standardized procedures for their application in this area. This means that, although these tools may be more sensitive and specific than traditional techniques, there are problems with the ability to accurately interpret results.

The potential impact of mis-interpretation of results from these techniques is serious – false negative results could mean that pathogens escape detection and get transferred to a new area. Conversely, false positives could result in emergency disease controls being wasted on healthy fish or shellfish. Also, accurate differentiation between benign and infectious agents could help prevent unnecessary restrictions on the movement of aquatic animals. This is especially significant for international trade in live and fresh aquatic animal products. Canada’s ability to sell fish and shellfish products internationally could be hampered if we cannot prove that the products are disease-free and do not represent a threat to cultured and wild fish and shellfish in the importing country.

These are the issues that researchers at Fisheries and Oceans Canada (DFO) are seeking to resolve by establishing ‘validation protocols’ for molecular-based technique being developed to detect and identify fish and shellfish pathogens. Validation protocols are guidelines that clearly define the steps required to ensure that any molecular probe developed meets the needs of the intended user with respect to both sensitivity (ability to detect small traces of infectious agent) and specificity (ability to distinguish significant pathogens from benign relatives). These protocols are essential for the protection of both the probe developers, and their clients, from mis-application and mis-interpretation of the probe results.

Researchers need to know the answers to a number of questions to validate the applicability of molecular probes for disease detection, including:

• is the technique user-friendly and readily transferable from laboratory to laboratory?
• are positive results consistent with positive results obtained from other methods?
• are positive results (from the new technique) that correlate with negative results (obtained using traditional techniques) due to greater sensitivity of the new technique or because of other reasons?
• similarly, what are the reasons for negative results (from the new technique) that correlate with positive results (obtained using traditional methods)?
• do variations in probe preparation and storage and tissue sample preparation and storage affect the consistency of results?

To answer these and other questions, researchers are developing draft validation protocols and are starting trials with existing nucleic acid probes. Once the procedures are refined in the laboratory, field validation will begin. Nucleic acid probes that are currently available will be tested on samples taken from the relevant fish or shellfish species. Some of these samples will be known to contain pathogens and others will be known to be pathogen-free. The samples will be widely varied, taken for example, at different stages of host development, collected during different seasons to see if results differ with age, environmental conditions and other factors. In addition, they will be checked against a variety of unrelated aquatic species. Results will be cross-checked with traditional methods for accuracy and consistency. These tests will result in a validation protocol which will be refined and developed as new probes for additional pathogens become available, and undergo the verification process.

The development of reliable and replicable procedures for using molecular-based tools to screen and diagnose disease in fish and shellfish will bring about a number of benefits. These include:

• more effective disease surveillance and management in wild and cultured fish and shellfish, and, therefore, a healthier resource;
• earlier detection of finfish diseases, and therefore, reduced reliance on drug treatments (there are no therapeutants for use against most shellfish diseases);
• more security for coastal communities that depend on aquatic animals for sustainable food production and income; and,
• the removal of potential disease-related barriers to international trade in aquatic animals and their products.