Abstract

Lesions caused by exposure to low salinity water, also referred to as freshwater exposure, have been observed in free-ranging cetaceans across the globe. Appropriate salinity for cetaceans typically ranges from 25–35 ppt; however, those found in bays, sounds, and estuaries can experience salinities lower than 15 ppt during the year. Prolonged exposure to even lower levels causes negative health impacts and often death.¹ Grossly, freshwater lesions appear as multifocal to coalescing irregularly marginated patches that can vary in color and severity.² Histologically, these lesions range in severity from hydropic degeneration to ulceration with bacterial and fungal overgrowth.³ Currently, there is no proposed histopathological scoring system for freshwater lesions. The objectives of our research were to provide a histopathological scoring system for suspect freshwater exposure lesions and to test the scoring system on control and known freshwater exposure cetacean cases.

Histological skin samples were examined from 168 bottlenose dolphins (Tursiops truncatus). Animals used in the study were necropsied between the years 2018 and 2023 and had varying age and sex ranges. Out of 168 animals, 40 animals were diagnosed with freshwater disease based on histopathology results. When evaluating the skin for freshwater exposure, three main variables were utilized in our scoring system—epidermal features, suppurative inflammation, and the presence/absence of infectious organisms. Logistic regression was performed to assess the associations of specified variables with the final freshwater diagnosis. Variables that were compared include year of strand date, animal carcass code (severity of autolysis), salinity at the stranding location, gross diagnoses at the time of stranding, histologic epidermal features, depth of inflammation, presence of organisms, final diagnoses to assigned score (0–10), and final diagnoses to the assigned score level (mild, moderate, severe). Significant associations were found in all variables considered and led to an association of a positive freshwater diagnosis. This research proposes a histopathologic scoring system that supports the diagnosis of freshwater disease in bottlenose dolphins.

Acknowledgements

The authors wish to thank Mississippi State University (MSU) Global Center for Aquatic Health and Food Security (GCAHFS). Tissues collected for this project were part of the Marine Mammal and Turtle Conservation, Recovery, and Monitoring Program, which is funded by the National Fish and Wildlife Foundation under Mississippi Department of Environmental Quality Agreement No. 18-00081.

*Presenting author

Literature Cited

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

2.  Ewing RY, Sutton MN, Herring HM, Schubert MR, Boyd DM, Richardson JL, Rotstein DS. Standardizing gross descriptions of skin lesions in common bottlenose dolphins (Tursiops truncatus) stranded in Southwest Florida, 2015–2019. Front Mar Sci. 2023;7:1–10.

3.  Duignan PJ, Stephens NS, Robb K. Fresh water skin disease in dolphins: a case definition based on pathology and environmental factors in Australia. Sci Rep. 2020;10:1–17.