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November 1, 2021

Case History: An adult male 36.80 kg Northern sea otter (Enhydra lutris kenyoni) was found beached in a state park in Oregon, U.S.A., and died within an hour of the initial sighting. 

Gross Findings: Externally, the sea otter had three deep penetrating wounds on the left lateral side of the abdomen, causing multiple sections of the intestine to be expelled (Fig.1A). The wounds were parallel to each other and formed a dorsoventrally oriented line, which extended to a linear transverse depression in the fur of the dorsal abdomen (Fig. 1B).  Each wound had ragged edges that were roughly diamond-shaped and that measured between 1.0 to 2.4 cm wide and 2.8 to 3.5 cm long. On reflecting the skin and subcutis over the abdominal dorsum, a deep transverse and lateral tear in the muscles of the back and the left side was seen; this tear effectively severed the left dorsal half of the body at the level of the fourth lumbar vertebra (Fig. 1C.). Radiographs revealed a fracture of the vertebral column between the fourth and fifth lumbar vertebrae, with left lateral displacement of the fourth vertebra (Fig. 1D). On internal examination, the otter was found to be in excellent nutritional condition. Blood was present in the abdomen, and the large tear in the muscles of the abdominal back and side could be seen to extend through the dorsal abdominal wall.

Photographs and a radiograph from a deceased Northern sea otter with a shark bite.
Figure 1. Photographs and a radiograph from a Northern sea otter (Enhydra lutris kenyoni) from Oregon, U.S.A. (A) The deceased sea otter on a beach in Oregon, U.S.A. (Photo courtesy of Hylah Furnish, Oregon State Parks, Oregon, U.S.A.) (B) Puncture wounds are present in a rough line from the left ventrolateral abdomen to the dorsum, where there is a transverse linear depression in the fur. (C) A deep linear tear in the muscles of the dorsum and left lateral body wall. (D) An abdominal radiograph shows a transverse fracture of the vertebral column, with left lateral displacement of the fourth lumbar vertebra (arrow).

 

Morphologic Diagnosis/es:

  1. Multifocal, severe, acute, puncture wounds extending into the body cavity.
  2. Locally extensive, severe, acute, traumatic bisection of the muscles of the left dorsal and left dorsolateral abdominal region.
  3. Focal acute fracture of the vertebral column at L4-L5, with left lateral displacement of the fourth lumbar vertebra.

Histopathologic Diagnosis/es: N/A

Etiology: Shark bite

Distribution: Occurs wherever the populations of sharks and northern sea otters overlap. The current range of the northern sea otter extends from coastal Oregon and Washington State north to British Columbia, Southcentral and Southeast Alaska, and the Aleutian Islands.

Seasonality: Data regarding shark bite seasonality for northern sea otters is lacking; however, the incidence of shark bites in the Southern sea otter (Enhydra lutris nereis) was reported to peak in late summer.

Clinical signs: Sea otters observed alive after a shark bite will exhibit clinical signs of trauma referable to the anatomical location of the bite wounds.

Pathology: Traumatic wounds from shark bites resemble stab wounds, with ragged edges that are consistent with the shape of shark teeth. These wounds are often, but not always, present in a curved pattern. In this sea otter, there was also a deep linear tear in the musculature of the back and side, consistent with the application of a tremendous lateral force, such as that caused by a shark thrashing while biting into its prey.

Diagnosis: Shark bite wounds may be considered antemortem when accompanied by contusions in the surrounding tissues. Hemorrhaging may not be evident if the carcass has been in the water, as the blood may be washed away. Shark bite wounds must be distinguished from propellor strike wounds, which typically have smooth rather than ragged edges, that are parallel and evenly spaced.

Wildlife population impacts: Impacts of shark bites on sea otter populations vary and may be significant, especially where populations are struggling to recover from historical depletion.

Management: None currently proposed.

References:

  • Kreuder C, Miller MA, Jessup DA, Lowenstine LA, Harris MD, Ames JA, Carpenter TE, Conrad PA, Mazet JAK. 2003. Patterns of Mortality in Southern Sea Otters (Enhydra lutris nereis) from 1998-2001.  J  Wildl Dis. 39(3):495–509. https://doi.org/10.7589/0090-3558-39.3.495
  • Moxley JH, Nicholson TE, Van Houtan KS, Jorgensen SJ. 2019. Non-trophic impacts from white sharks complicate population recovery for sea otters. Ecol Evol. 30;9(11):6378-6388. https://doi.org/10.1002/ece3.5209
  • White CL, Lankau EW, Lynch D, Knowles S, Schuler KL, Dubey JP, Shearn-Bochsler VI, Isidoro-Ayza M, Thomas NJ.  2018. Mortality trends in Northern Sea Otters (Enhydra lutris kenyoni) collected from the coasts of Washington and Oregon, USA (2002-15). J Wildl Dis. 2018 Apr;54(2):238-247. https://doi.org/10.7589/2017-05-122

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