Molecular diagnosis of fish and shellfish diseases: present status and potential use in disease control

Abstract

Developments in molecular biology have led to a rapid growth in new methods for diagnosing fish diseases. Techniques of major significance include polymerase chain reaction (PCR) amplification of nucleic acids, restriction enzyme digestion, probe hybridisation and nucleotide sequencing. These are briefly described, as is the requirement for thorough validation of new methods before application and some potential problems associated with the use of molecular diagnostics. Many molecular techniques are potentially faster or more sensitive than methods such as culture, serology and histology that are traditionally used to identify fish diseases. In addition, the use of molecular biology is often able to readily detect genetic variations that denote subspecies or strains. There are now many publications describing the development of molecular techniques for the examination and identification of fish and shellfish pathogens. These include diseases of bacterial, viral, parasitic and fungal origin. This review summarises the published methods for molecular diagnosis of diseases listed under European Commission (EC) fish health regulations. Molecular research has demonstrated not only the ability to identify pathogenic species, but also to discriminate below the level of species and identify strains. In some cases, it may be appropriate to control only the pathogenic strains or subspecies, rather than an entire group of bacteria or viruses. Molecular techniques have proved especially suited to the discrimination of these strains. Before new diagnostic tests are applied, available methods should be reviewed, the extent of validation or tests assessed, and a programme of validation and confirmation trials established for each disease. By this means, it can be demonstrated that new methods are as robust as other techniques and an established method can then be disseminated to other laboratories. At present, there are very few descriptions of processes carried out to validate any of these new methods and validation studies are urgently required. New developments in molecular biology methods and hardware are also considered, as it is likely these will have a significant impact on the application of new diagnostic tests.