All disease processes are the integration of the host and pathogen interacting with the environment. Because fishes are in many ways environmental conformers, i.e., their metabolic rates and physiologic processes are closely tied to environmental perturbations, it often becomes difficult to separate infectious from environmental problems. This is because poor water quality, while being important in and of itself, it may also potentiate infectious processes by suppressing the fish's innate resistance to an infectious agent. For example, we have seen how the ability of fish to mount a humoral immune response is highly temperature-dependent. In many instances, the optimal temperature for reproduction of a pathogen is not within the optimal range of the host's immune response to that pathogen.

The importance of environment and water quality management is also reflected in the number of opportunistic facultative pathogens, such as Aeromonas hydrophila and Flexibacter columnaris, and the fact that so many infectious agents discussed today can be present as inapparent carrier infections just waiting for the proper conditions to become a fulminating disease. Many of these pathogens survive and grow best in "polluted" environments.

If you had to rank disease groups in order of prevalence or importance in pet fishes, they would probably be ranked with environmental problems first, followed by parasites, then bacteria, and finally, viruses and nutritional diseases. Host specificity of virtually all of the important infectious diseases of pet fishes is very broad. While the resistance to fish pathogens varies somewhat among phylogenetic groups, in general, diseases are cosmopolitan in nature. This is understandable, since pathogens having a broad host range and simple life cycle are most adapted to typical aquarium systems in which a large number of unrelated species are mixed and in general, there are few intermediate hosts available.

While pathogens affecting both freshwater and marine fishes are cosmopolitan in nature, there are very few pathogens that are important in both marine and freshwater fishes. But the types of diseases that affect both freshwater and marine fishes are surprisingly similar. For example, white spot disease is a skin problem caused by an ectoparasitic ciliate that in freshwater fishes is caused by Ichthyophthirius. A disease having virtually the same pathophysiology and life cycle is Cryptocaryon, which only affects marine fishes. There are numerous other examples.

In terms of drug treatments, we are at a very embryonic state of development. Most drug treatments in pet fish are administered by the water-borne route. This adds a very important complicating factor in the pharmacokinetics of drug use in fish. Not only must one be concerned about the handling of drugs in fish, but one must also be concerned with the interaction of the drugs in the water, because of both the effect of the environment on drug effectiveness and the effect of the drug on the environment.

In summary, managing pet fish disease problems requires an integrated approach that should emphasize disease prevention using adequate quarantine procedures, minimizing environmental stress, and rapidly responding to disease problems before they overwhelm the susceptible population.