Abstract
A distinct ulcerative dermatitis known as “freshwater skin disease” is an emerging clinical and pathological presentation in coastal cetaceans worldwide. In Australia, two remarkably similar mortality events enabled the creation of a case definition based on pathology and environmental factors. The first affected a community of endemic Tursiops australis in the Gippsland Lakes, Victoria, while the second occurred among T. aduncus resident in the Swan-Canning River system, Western Australia. The common features of both events were (1) an abrupt and marked decrease in salinity (from > 30ppt to < 5ppt) due to rainfall in the catchments, with hypo-salinity persisting weeks to months, and (2) dermatitis characterized grossly by patchy skin pallor that progressed to variable circular or targetoid, often raised, and centrally ulcerated lesions covering up to 70% of the body surface. The affected skin was often colonized by a variety of fungal, bacterial and algal species that imparted variable yellow, green or orange discoloration. Histologic lesions consisted of epidermal hydropic change leading to vesiculation and erosion; alternately, or in addition, the formation of intra-epithelial pustules resulting in ulceration and hypodermal necrosis. Thus, the environmental factors and characteristic pathologic lesions, are necessary components of the case definition for freshwater skin disease.
Highlights
A distinct ulcerative dermatitis known as “freshwater skin disease” is an emerging clinical and pathological presentation in coastal cetaceans worldwide
Dermatopathology has long been used as a proxy for health in cetaceans with adverse environmental conditions assumed to cause physiological stress and impaired immunological function that can manifest as epidermal disease[1,2,3,4,5,6,7,8,9,10,11,12,13,14] Given the obvious logistical challenge of assessing the health of free-ranging dolphins, their lack of hair or fur is fortunate in that the photographic catalogues widely used in cetacean ecology and ethology research, have found a novel use in recording the prevalence, severity and temporal trends in lesion appearance reducing the need for capture or restraint
For dolphins of the Gippsland Lakes, Victoria, and the Swan-Canning River system, Western Australia, these stressors include the physiological stress of inhabiting a highly-variable environment; the physical trauma of becoming entangled in fishing gear; the immunological challenge of being exposed to natural pathogens and to industrial or agricultural contaminants; the disturbance caused by increasing vessel traffic and anthropogenic noise; and the environmental stress of living in an ecosystem adjacent to a major metropolitan area or that is surrounded by a large agricultural catchment
Summary
A distinct ulcerative dermatitis known as “freshwater skin disease” is an emerging clinical and pathological presentation in coastal cetaceans worldwide. Long-term photo-identification studies of the resident bottlenose dolphins provided an ideal opportunity to document how dolphins evacuated the hyposaline waters of the upper bay and changes in the extent and severity of grossly-visible skin lesions (pallor and ulceration) before, during and after the hurricane While these were the first studies to associate gross skin lesions with prolonged exposure to hypo-saline water, an earlier study by Wilson et al.[2], based on photographs of free-ranging bottlenose dolphins, made the association between dermatopathology and environmental factors. Even if the overall trend is towards reduced runoff, stochastic extremes are likely to result in more devastating flood events for estuaries, lagoons and coastal marshes, and/or more rapid fluctuations in conditions The impact that these environmental changes will have on coastal dolphin communities remains to be determined but continued monitoring for skin lesions is likely to be a useful and non-invasive method for researchers
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