A protozoal-associated epizootic impacting marine wildlife: mass-mortality of southern sea otters (Enhydra lutris nereis) due to Sarcocystis neurona infection

Abstract

During April 2004, 40 sick and dead southern sea otters (Enhydra lutris nereis) were recovered over 18km of coastline near Morro Bay, California. This event represented the single largest monthly spike in mortality ever recorded during 30 years of southern sea otter stranding data collection. Because of the point-source nature of the event and clinical signs consistent with severe, acute neurological disease, exposure to a chemical or marine toxin was initially considered. However, detailed postmortem examinations revealed lesions consistent with an infectious etiology, and further investigation confirmed the protozoan parasite Sarcocystis neurona as the underlying cause. Tissues from 94% of examined otters were PCR-positive for S. neurona, based on DNA amplification and sequencing at the ITS-1 locus, and 100% of tested animals (n=14) had elevated IgM and IgG titers to S. neurona. Evidence to support the point-source character of this event include the striking spatial and temporal clustering of cases and detection of high concentrations of anti-S. neurona IgM in serum of stranded animals. Concurrent exposure to the marine biotoxin domoic acid may have enhanced susceptibility of affected otters to S. neurona and exacerbated the neurological signs exhibited by stranded animals. Other factors that may have contributed to the severity of this epizootic include a large rainstorm that preceded the event and an abundance of razor clams near local beaches, attracting numerous otters close to shore within the affected area. This is the first report of a localized epizootic in marine wildlife caused by apicomplexan protozoa.

Figure 1

A: Number of southern sea otters stranding rangewide by month. 1B: Number of sea otters stranding between Cayucos and Point Sal, California from March through May. 1C: Locations of Estero Bay and Morro Bay along the central coast of California.

Figure 2

A: Grossly normal (control) sea otter heart with moderate coronary groove fat. 2B: Heart from a sea otter that stranded during the epizootic. There is mild, diffuse cardiomegaly, mottling and discoloration of the ventricular myocardium and dilation and hyperemia of the epicardial vasculature.

Figure 3

A: Normal (control) sea otter liver, characterized by diffuse, pale tan-brown color and sharp lobular margins. 3B: Liver from an otter that died during the epizootic, showing severe, diffuse passive vascular congestion and rounded lobular margins, presumably due to cardiac insufficiency as a result of the myocardial lesions depicted in Figure 2 above. 3C: Diffuse chemosis affecting the ocular conjunctiva of an otter that stranded during the epizootic. 3D: Multifocal petechiation and mild serous atrophy of adipose of the ventral abdominal subcutis of an otter that died during the epizootic.

Figure 4

Photomicrographs of paraffin-embedded, H&E-stained tissues from otters dying during the epizootic. 4A: Skeletal muscle containing a single smooth-walled, immature-appearing sarcocyst encompassing numerous zoites (Bar = 30 µm). 4B: Two intramuscular sarcocysts that are larger and more fully developed, with a thick cyst wall and prominent surface villi (Bar = 25 µm). 4C: Neuropil containing numerous S. neurona schizonts, including 2 rosette-form schizonts, characterized by a distinctive radial arrangement of budding merozoites (arrows) (Bar = 20µm).

Figure 5

Paraffin-embedded brain from a sea otter dying from disseminated Sarcocystis neurona infection during the epizootic. 5A: Hematoxylin and eosin-stained cerebrum, demonstrating a marked diffuse and perivascular infiltrate of lymphocytes and plasma cells, with fewer macrophages and neutrophils. Protozoal schizonts of varying stages of development are apparent in the cytoplasm of neurons and glial cells arrows) and there is moderate rarefaction of adjacent neuropil (Bar = 40µm). 5B: Paraffin-embedded cerebrum from the same otter labeled with antibodies to S. neurona using an immunoperoxidase method. Numerous protozoal schizonts (slender arrow) and both free and intracytoplasmic merozoites are apparent (thicker arrow) (Bar = 20µm).

Figure 6

Timeline of major local events preceding and including the epizootic that could have enhanced exposure of susceptible sea otters to Sarcocystis neurona.