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Representatives of many bacterial taxa have, at one time or another, been associated with fish diseases. However, not all of these bacteria constitute primary pathogens. Many should be categorised as opportunistic pathogens, which colonise and cause disease in already damaged hosts. Here, the initial weakening process may involve pollution or a natural physiological state (e.g. during the reproductive phase) in the life cycle of the fish. There remains doubt about whether some bacteria should be considered as fish pathogens. In such cases, the supportive evidence is weak or nonexistent. Possibly, such organisms constitute contaminants or even innocent saprophytes. However, it is readily apparent that there is great confusion about the precise meaning of disease. A definition, from the medical literature, states that:

This definition is certainly complex, and the average reader may be excused for being only a little wiser about its actual meaning. Dictionary definitions of disease are more concise, and include “an unhealthy condition” and “infection with a pathogen [= something that causes a disease]”. One conclusion is that disease is a complex phenomenon, leading to some form of measurable damage to the host. Yet, it is anticipated that there might be profound differences between scientists about just what constitutes a disease.

The first report of botulism as a disease of fish was from Denmark, stemming from the work of Huss and Eskilden (1974). These workers showed that botulism was a chronic disease, termed “bankruptcy disease’”, of farmed trout, with the causal agent recognised as type E. Subsequently, the disease was found on one farm of rainbow trout in Great Britain (Cann and Taylor, 1982, 1984) and was similarly identified among farmed coho salmon in the U.S.A. (Eklund et al., 1982). Characteristic disease symptoms seemed to be very vague, but fish have been observed to exhibit sluggish, erratic swimming, appeared to be listless, and may alternately float and sink, before showing temporary rejuvenation. This pattern was repeated until death eventually ensued (Cann and Taylor, 1982).

Taxonomists rarely consider the ecological relevance of their findings. Similarly, ecologists ignore taxonomy, which is perceived to be old-fashioned and/or boring.

Some Gram-negative bacteria have been described as fish pathogens with little if any supportive evidence. Consequently, a question mark hangs over whether or not such organisms should be regarded as pathogens. In the following text, it will become apparent that varying amounts of information are available for the Gram-negative bacterial fish pathogens. For some, e.g. , a wealth of information is available. Unfortunately, for others, such as , barely enough information is available to enable scientists to recognise fresh isolates.

There is no single technique suitable for the recovery of all known bacterial fish pathogens. Scientists need to use a combination of methods and incubation conditions to achieve pure cultures.

Historically, scientists have seemed loath to make rapid diagnoses, preferring to adopt laborious testing regimes. Why do scientists bother to identify the precise cause of a disease, when the information is often not useful for control purposes? Yet, there have been dramatic improvements in diagnostics, encompassing recent developments in molecular biology.

The reservoir of many Gram-positive bacterial fish pathogens is unknown. Whereas some groups, e.g. streptococci, occur in polluted waters, other organisms, e.g. , seem to be restricted to fish. How do such organisms spread between separate fish populations?

The value of survival experiments based on laboratory cultures is questionable. Cultures grown on laboratory media tend to be larger and less aggressive than their “natural” counterparts. So, when laboratory cultures are added to experimental microcosms, the outcome need not represent the true fate of the organism in the aquatic environment.

Many publications about pathogenicity mechanisms have resulted from the examination of single isolates, often of questionable authenticity. The usefulness of such approaches to the understanding of pathogenicity of bacterial species is doubtful. Also, the value of studies involving bacterial subcellular components produced on agar plates or in broth cultures at explaining disease mechanisms is unclear. Nevertheless, an interesting development concerns the potential role of quorum-sensing signal molecules (= acylated homoserine lactones [AHLs]) in the regulation of some virulence factors, with work revealing that AHLs are produced by some Gram-negative bacterial fish pathogens, notably and , but not in or (Bruhn , 2005).

It is worth remembering the age-old adage that “prevention is better than cure”, and certainly it is possible to devote more attention to preventing the occurrence of disease in fish. This is especially true for farmed fish, which tend to be at the mercy of all the extremes which their owners are capable of devising. Principally, in the industrialised nations farmed fish are subjected to questionable water quality and high stocking regimes. These are among the known prerequisites for the onset of disease cycles. Yet, the owners are among the first to seek help if anything adverse happens to the valuable stock. Fish may be reared under ideal conditions, in which case, the stock are inevitably in excellent condition without signs of disease. Such sites, for example located in Venezuela and the former Yugoslavia, are usually supplied by fast-flowing, clear river water. Careful feeding regimes are adopted, and the stocking levels are comparatively low. The latter point would make the enterprise unacceptable in the more industrialised nations of Western Europe. Therefore, much attention has been devoted to control measures. These have been categorised in Table 10.1. Although most emphasis has been placed on aquaculture, some effort has gone towards considering disease in wild fish stocks.