Abstract

During routine necropsy investigation of marine animal mortality on the California coast in 1995-1997, multiple cases of protozoal encephalitis were diagnosed at the School of Veterinary Medicine, University of California, Davis or the Armed Forces Institute of Pathology, Washington, DC. Cases were observed in several harbor seals (Phoca vitulina), two Southern sea otters (Enhydra lutris), a common dolphin (Delphinus delphis) and an elephant seal (Mirounga angustirostris). The harbor seals were found dead during an unusual mortality event in the area of Point Reyes, Marin County, California. The juvenile male sea otter was found near Pismo Beach in San Luis Obispo County, while the adult female was recovered from Aptos Beach in Santa Cruz County. Clinical signs in affected sea otters ranged from acute death with no premonitory signs in the juvenile male otter, to ambulatory deficits, ataxia and tremors in the pregnant adult female. The elephant seal stranded on Solano Beach in San Diego County, and no neurolologic signs were noted. The common dolphin stranded on a Los Angeles Beach. Seizures were noted prior to euthanasia.

No gross lesions were observed in the brains of all animals. Histopathologic lesions consisted of nonsuppurative encephalitis with intralesional protozoal tachyzoites and tissue cysts. Immunoperoxidase stains were performed, using antibodies specific for Neospora caninum, Toxoplasma gondii and Sarcocystis neurona. Protozoa in brain tissue from sea otters stained strongly positive using antibodies raised against T. gondii, moderately positive for antibodies against S. neurona, and weakly positive using antibodies raised against N. caninum. Protozoa in harbor seals stained strongly positive using antibodies raised against N. caninum, and negative for S. neurona and T. gondii. Protozoal cysts in brain tissue from the elephant seal and common dolphin were reported as morphologically consistent with T. gondii, and results of immunocytochemistry are pending or not available.

Lesions in otters and harbor seals resemble those previously described in harbor seals examined at The Marine Mammal Center and U.C. Davis, and in sea otters examined at the National Wildlife Health Laboratory in Madison, Wisconsin. Research is planned to further characterize these organisms, develop antemortem diagnostic procedures, and to gain information on prevalence and mode of infection.

Acknowledgements

The authors wish to thank J.M. LaPointe, Padraig Duignan and other collaborators for contributing cases to the collective study.