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Introduction

Fish in culture are susceptible to a wide range of infections and losses through disease currently make a significant impact on the quality and volume of fish produced in Europe. Disease management is therefore a vital component of successful aquaculture. Disease control is especially important during the first stage of the production chain as infective agents can be transmitted vertically from broodstock to eggs.

Detection methods

A variety of sensitive methods (traditional, immunological and molecular) have been developed for the detection and identification of fish and shellfish pathogens in general and such methods have been employed in broodstock testing. Many of the techniques used (e.g. culture, immunohistochemistry, IFAT, ELISA), however, may not sensitive enough to detect the extremely low pathogen levels that can be encountered in broodstock and eggs. In addition, although a number of commercial test kits (immunological and molecular) are available (e.g. Renibacterium salmoninarum, IPNV, VHS) many methods are currently not standardised with regard to reagents, relevant controls, protocols nor interpretation of results.  

Molecular methods offer great potential not only for the detection of pathogens. Modifications of PCR can be utilised to identify to species level, differentiate one species from a closely related species, or to  distinguish live from dead pathogen. Some of the most common PCR variations used in diagnostics are Nested PCR, Random amplified polymorphic DNA (RAPD), Reverse transcriptase-PCR (RT-PCR), and Reverse Cross Blot PCR (rcb-PCR). A variety of novel technologies are currently being developed for use in fish diagnostics, including Real-time PCR, and Nucleic Acid Sequence Based Amplification (NASBA), RT-PCR Enzyme hybridisation Assay. The RT-PCR Enzyme Hybridisation Assay offers the advantage of detecting live pathogens and large sample through-put. These methods all require standardisation and validation before they can be used effectively for broodstock testing.

 Multiplex tests offer vast potential in broodstock screening as this would enable the testing for the presence of a variety of pathogens simultaneously from the same small sample. Both Luminex technology and microarray are currently in their infancy with regard to diagnostics in aquaculture.

 As well as those methodologies to detect pathogens there are also well recognised serological methods for the detection of host antibodies in response to exposure to pathogens. Detection of specific antibodies in the serum of animals is recognised as a useful indicator of previous exposure to pathogens and is regularly used in both clinical and veterinary medicine. Specific serum antibody responses against a pathogen may indicate infection before it is possible to detect the pathogen by culture or using molecular methods such as polymerase chain reaction (PCR).Such methods are presently underused in aquaculture although their potential in disease management in other animals is well proven. Such tests for fish require anti-fish species IgM antibody probes and many such MAbs are now commercially available (e.g. anti trout/salmon, anti-sea bass, anti-sea bream, anti-cod/haddock etc). ELISA provides a convenient method for testing large sample numbers and testing is non-destructive, requiring only a serum sample. Serological testing, however, still requires validation to confirm that specific antibodies can be detected to each pathogen before they can be employed effectively in broodstock testing.

 Professor Alexandra Adams is head of the Aquatic Vaccine Unit at the Institute of Aquaculture. She has worked on the development of fish vaccines and rapid diagnostic tests since 1985, and has over 90 publications in this area, having graduated in Biochemistry at the University of Glasgow (BSc 1979) and University College London (PhD 1983). Professor Adams is also Chief Executive Director of Aquatic Diagnostics Ltd (www.aquaticdiagnostics.com), a spin-out company from the University of Stirling, marketing monoclonal antibodies for the identification of fish pathogens and detection of components of the fish immune system.

Contact details: Alexandra Adams: Institute of Aquaculture, University of Stirling, Stirling FK94LA, Scotland, UK. E-mail: alexandra.adams@stir.ac.uk