measures particularly those involving the legislative machinery for movement

Measures particularly those involving the legislative

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measures, particularly those involving the legislative machinery for movement restric- tion policies. PATHOGENICITY MECHANISMS There has been commendable progress with some pathogens, notably Aer. salmonicida, Ren. salmoninarum and V. anguillarum. However, the research input needs to continue with attention being diverted to other pathogens. Information about pathogenicity mechanisms is especially relevant for vaccine development programmes.
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Conclusions 407 CONTROL MEASURES Although conventional vaccine development programmes seem likely to continue, further progress on difficult candidates, such as Rickettsia and Ren. salmoninarum, necessitate other approaches, such as offered by genetic engineering. Also, the current interest in non-specific immunostimulants, probiotics and medicinal plants/herbs seems likely to continue for the foreseeable future. Antimicrobial compounds con- tinue to be used extensively in many parts of the world, but there is concern about tissue residues and the development and spread of resistance. Therefore, curbs on the non-medical use of antimicrobial compounds are Hkely to be imposed in some countries. It is to be envisaged that there will be ever-tighter regulations on the use of antimicrobial compounds in aquaculture. Attention should also be focused on stock improvement programmes, which may highlight genetically determined disease-resistant fish strains. THE EFFECTS OF POLLUTION There is increasing concern about the possible role of pollution in disease, particu- larly of wild fish stocks (see also Pippy and Hare, 1969; Mahoney et ai, 1973; Robohm et ai, 1979). As this is a politically emotive issue, there is Hkely to be an increase in monies available and, thus, a stimulation of research interest. At present, there is considerable confusion over the precise role of pollution and fish health (Bucke, 1991, 1997). Nevertheless, there are good data that long-term exposure to pollutants has adversely affected the health of fish, especially in the North Sea and Great Lakes. However, mortahties among fish populations do not necessarily imply disease. Furthermore, disease may develop long after the pollutant has been effec- tively removed from the aquatic environment. Much of the work attempting to correlate fish disease with aquatic pollution has resulted from surveys, many of which have been carried out in the North Sea (e.g. Dethlefsen and Watermann, 1980; Dethlefsen et aL, 1987, 2000; McVicar et aL, 1988; Vethaak and ap Rheinallt, 1992). Briefly, fish are caught with nets, and the relative incidence of disease deter- mined. One conclusion from these surveys is that larger numbers of diseased fish occur generally in the polluted compared with clean/unpolluted locations (Dethlefsen et ai, 2000). However, the distinction between polluted and clean sites is imprecise.
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