Skin ulcers in fish: Pfiesteria and other etiologies

Skin ulcers on fish are one of the most well-recognized indicators of polluted or otherwise stressed aquatic environments. In recent years, skin ulcer epidemics have been either experimentally or epidemiologically linked to exposure to a number of xenobiotic chemicals as well as to biotoxins. Some of these agents, such as toxins produced by the dinoflagellate alga Pfiesteria, have led to serious concerns about the health of aquatic ecosystems, such as estuaries along the east coast of the United States. However, a number of other risk factors besides Pfiesteria have been shown to damage epithelium and may also play important roles in skin ulcer pathogenesis. In addition, increasing evidence indicates that not only may skin damage occur via direct contact with toxins, but it may also be induced indirectly from physiological changes that result from exposure not only to toxins but also to other environmental stressors, such as pH and temperature extremes. The multifactorial pathways that operate at both the ecological and the organismal levels as well as the nonspecific response of the skin to insults make it very challenging to link epidemic skin ulcers to any single cause in natural aquatic populations. Consequently, using pathology to unequivocally identify the specific cause of a lesion (eg. Pfiesteria exposure) is not a valid approach. Only with an increased understanding of the basic mechanisms leading to skin damage (including development of specific biomarkers for specific toxins), along with a better understanding of ecological processes operating in these environments, will we be able to discern the relative importance of various risk factors in skin ulcer development.