Genetic biotechnologies are being used to improve fish health through conventional selection for disease resistance and through the use of molecular investigation of pathogens for characterisation and diagnosis. DNA-based technologies are being used now to characterize different species and strains of pathogens. Genetic characterisation of the pathogen may also reveal information about its origin, e.g. DNA analysis revealed two strains of crayfish plague fungus in Sweden: one from the local species and one originating in Turkey. Once the pathogen is characterized, DNA probes can be developed to screen for specific pathogens in tissue, whole animals and even in water and soil samples. These techniques are being used to detect viral diseases of marine shrimp throughout the world and for bacterial and fungal pathogens in fishes in many areas.
Modern biotechnology is also of great value in the field of vaccines and immunostimulants for aquaculture species. These allow preventative measures to be taken to combat disease through vaccination or immunity enhancement. Both can be administered via additives in feeds, immersion or, in the case of the larger culture animals like fish, by injection. Genetically engineered vaccines are also being developed to protect fish against pathogens. Genetic immunisation of rainbow trout with a glycoprotein gene from the virus causing viral haemorrhagic septicaemia has recently been shown to induce high levels of protection against the virus. Work is also underway on immunising carp, salmon and other fishes with genetically engineered vaccines for other diseases. Currently, vaccines are of questionable effectiveness in crustacea and may be difficult or too costly to use in developing countries.
Detecting human pathogens
The type of DNA 'probes' that are used to screen for pathogens that affect fish or shrimp could also be developed to check for pathogens that affect humans. This will become increasingly important in answering concerns in the market place about the safety of aquatic products for human consumption. These new molecular techniques are extremely sensitive and can identify pathogens in fish long before there are any clinical signs of the disease. This has implications for quarantine and the trade of aquatic species, which is currently governed by the World Trade Organisation and the Office International des Epizooties. Trade can be restricted based on the disease status of a product or a region; identification of minute quantities of a pathogen or of a new strain of an existing pathogen could change or influence existing trade patterns.
Farmed aquatic animals are much more sensitive to their immediate environment than land animals. The water they are immersed in, and on which they depend for oxygen and a range of other important chemicals, also takes up their waste products and may carry pollution from the nearby environment. The process of disease in aquaculture species is thus much more strongly connected to environmental factors than would be the case say, with cattle. A further biotechnology field that has developed in aquaculture, because of the nature of this relationship, is that of bio-remediation. This refers to the use of 'friendly bacteria or 'pro-biotics' to treat water or feeds and, by natural processes, discourages the development of 'unfriendly' bacteria that potentially would cause disease. Many such products are currently being marketed' -- while conclusive studies have yet to be carried out to check their effectiveness, some of them do seem to bring production benefits.