Freshwater Skin Disease (FWSD) is due to the detrimental effects of freshwater exposure in coastal bottlenose dolphins, and is an emerging cause of morbidity and mortality in many regions. We, for the 1st time, provide a case definition of this disease entity based on two Australian mortality events. The 1st affected Burrunan dolphins (Tursiops australis) in Victoria’s Gippsland Lakes in 2007 (a similar event is currently unfolding - see https://www.abc.net.au/.../burrunan-dolphin.../12900270). The 2nd event occurred in 2009 and affected the resident Indo-Pacific bottlenose dolphins (T. aduncus) in WA's Swan-Canning Riverpark. Simultaneously with the Gippsland Lakes event, the first outbreak of FWSD in US waters was recorded for common bottlenose dolphins (T. truncatus) entrapped in Lake Pontchartrain, Louisiana, USA, following Hurricane Katrina in 2007. However, the dermatopathology was not described for the US event, as post-mortem examinations were not conducted.

By contrast, the Australian events occurred where the resident populations were well documented by long-term and ongoing field ecology, population, and behaviour studies (including Citizen Science projects e.g. WA's River Guardians and Dolphin Watch programs); the waters inhabited by the dolphins were intensively monitored for physical and chemical parameters before, during and after the events; and when mortalities occurred, thorough and systematic post-mortem examinations were carried out. Based on these data, FWSD occurs when there is a sudden (over days) and precipitous (>25ppt to <5ppt) decrease in salinity, persisting for weeks to months. The skin lesions initially appear as patchy pale areas that progress to raised targetoid areas of ulceration and opportunistic colonisation by algae, diatoms, fungi and bacteria. In our paper we detail the gross and histological appearance of the characteristic skin lesions of FWSD. Death may ensue from fluid loss and electrolyte imbalance - because the ulcerative skin lesions are akin to severe third degree burns and often affect a large % of the body surface.

In Australia, the outbreaks followed resumption of seasonal rainfall following a prolonged drought that flooded the Gippsland Lakes (normally brackish to marine saline) with fresh water. In WA, unusually high winter-spring rainfall in the river catchments similarly turned a normally marine/brackish habitat to freshwater. In the Gulf of Mexico, USA, events have followed the heavy rainfall and storm surges in the aftermaths of Hurricanes Katrina and Harvey. Common to all events is a preceding extreme weather event - which are predicted to increase in frequency and severity with Climate Change. Therefore, FWSD is an emerging disease of cetaceans which we are likely to see increasing in frequency in vulnerable estuarine and coastal habitats that continue to be affected by worsening Climate Change.

The research paper is titled ‘Fresh water skin disease in dolphins: a case definition based on pathology and environmental factors in Australia’ published in Nature Scientific Reports. Read the full paper here.

Photo: Case 3 from the paper, a dolphin from Perth’s Swan-Canning Riverpark that died in 2009 (photo credit – Douglas Coughran, AM, DBCA)