Others have been associated only with clinically diseased or covertly infected

Others have been associated only with clinically

This preview shows page 27 - 30 out of 594 pages.

of water and/or fish. Others have been associated only with clinically diseased or covertly infected (asymptomatic) fish. Examples of these "obligate" pathogens include Aer. salmonicida and Ren. salmoninarum, the causal agents of furunculosis and bacterial kidney disease (BKD), respectively. In later chapters, it will be questioned whether or not bacteria should be considered as obligate pathogens of fish, at all. It is a personal view that the inability to isolate an organism from the aquatic environment may well reflect inadequate recovery procedures. Could the organism be dormant/damaged/senescent in the aquatic eco- system; a concept which has been put forward for other water-borne organisms (Stevenson, 1978)? It is undesirable that any commercially important species should suffer the prob- lems of disease. Unfortunately, the aetiology of bacterial diseases in the wild is often improperly understood. Moreover, it seems that little if anything may be done to aid wild fish stocks, except, perhaps, by controlling pollution of the rivers and seas, assuming that when environmental quality deteriorates this influences disease cycles. In contrast, much effort has been devoted to controlHng diseases of farmed fish. Conclusion The Ust offish pathogens has extended substantially since 1980. Current interest focuses on the vibrios, CLBs, mycobacteria and streptococci-lactococci. A question mark hangs over the significance of some organisms to fish pathol- ogy—are they truly pathogens or chance contaminants? • There has been considerable improvement in the taxonomy of some groups (e.g., vibrios). There has been a shift from emphasis on culture-dependent to culture-indepen- dent techniques. Molecular methods have become commonplace in laboratories involved in the study of fish diseases.
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