Abstract

From 2005 to 2020, a series of extreme spring flooding events in both south eastern and south western Australia have resulted in mortality events affecting resident coastal bottlenose dolphins. Pathology in affected animals is characterized by a distinct ulcerative dermatitis known as freshwater skin disease. Two remarkably similar mortality events enabled the creation of a case definition based on the gross, histopathologic and environmental factors.¹ The first, in spring 2007, affected a community of endemic Tursiops australis in the Gippsland Lakes, Victoria, while the second, in spring 2009, 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 (natural or anthropogenic) and characteristic pathologic lesions, are necessary components of the case definition for freshwater skin disease. Recently (2019 and 2020), freshwater skin disease outbreaks in the northern Gulf of Mexico and south-eastern Australia have killed hundreds of coastal Tursiops sp. making this disease one of the most significant emerging threats to coastal dolphins.^2,3,4,5

Acknowledgements

We thank the following for assistance with carcass retrieval and necropsy: SRT/DBCA officers, Dr. M. Lynch, A. Monk, D. Donnelly, F. Bedford, A. Howard, J. Weir, C, Bayley, Dr. C. Holyoake, Dr. H. Finn, Dr. C. Salgado, Dr. S. Allen, Dr. D. Chabanne, G. Spoelstra, M. Slaven, and Dr. R. Loh. Dr. J. Wang, AAHL, for cetacean pox and herpes virus PCR; Dr. S Crameri, AAHL, for electron microscopy and Dr. K. Trayler, DBCA (Swan River chemistry data). C Foord for maps; F. Bedford for in situ images (Gippsland Lakes). We acknowledge WA State Government’s Department of Biodiversity, Conservation and Attractions as well as the Animal Health Lab, Western Australia for financial and technical support. Members and citizen scientists in WA’s Dolphin Watch program, the Victorian State Government’s Department of Environment, Land, Water and Planning (DELWP), Zoos Victoria, the Dolphin Research Institute, and members and volunteers at the Marine Mammal Foundation.

Literature Cited

1.  Duignan PJ, Stephens NS, Robb K. 2020. Fresh water skin disease in dolphins: A case definition based on pathology and environmental factors in Australia. Sci Rep. 10, 21979. https://doi.org/10.1038/s41598-020-78858-2.

2.  McClain AM, Daniels R, Gomez FM, Ridgway SH, Takeshita R, Jensen ED, Smith CR. 2020. Physiological Effects of Low Salinity Exposure on Bottlenose Dolphins (Tursiops truncatus). J Zool Bot Gard 1: 61–75.

3.  Ewing RY, Mase-Guthrie B, McFee W, Townsend F, Manire CA, Walsh M, Borkowski R, Bossart GD, Schaefer AM. 2017. Evaluation of serum for pathophysiological effects of prolonged low salinity water exposure in displaced bottlenose dolphins (Tursiops truncatus). Front Vet Sci 4(80): doi: 10.3389/fvets.2017.00080 (2017).

4.  Deming AC, Wingers NL, Moore DP, Rotstein D, Wells RS, Ewing R, Hodanbosi MR, Carmichael RH. 2020. Health impacts and recovery from prolonged freshwater exposure in a common bottlenose dolphin (Tursiops truncatus). Front Vet Sci 7, 235. doi: 10.3389/fvets.2020.00235.

5.  Booth C, Thomas L. 2021. An expert elicitation of the effects of low salinity water exposure on bottlenose dolphins. Oceans 1:179–192. https://doi.org/10.3390/ oceans2010011.