Abstract

Protozoal encephalitis was first recognized as a significant cause of mortality in Pacific harbor seals (Phoca vitulina richardsi) and Southern sea otters (Enhydra lutris nereis) in California in 1992.^1,3,4 At least two different protozoal parasites have been implicated in these infections, and others are suspected. Both free-living and captive sea otters and seals have been affected.^1,2 Mortalities have been observed predominantly in sub-adult and adult animals, and some seasonal trends are apparent.¹ Affected animals were often found dead, but when observed, clinical signs typically consisted of tremors or seizures, deficits in ambulation or proprioception, pupil dilation and obtundation. Neurologic signs were invariably progressive and ultimately fatal despite aggressive medical intervention. The source of infection for wild and captive sea otters and seals is unknown.

In 1998 we initiated a long-term study on protozoal brain infections in California sea otters and harbor seals. The goals of this study are to: 1) confirm the identity of protozoal parasites infecting harbor seals and sea otters by parasite isolation in tissue culture, followed by amplification and sequence analysis of phylogenetically informative parasite DNA; 2) develop a panel of serodiagnostic tests to investigate the relationship between seropositivity, protozoal infection and protozoal disease; 3) incorporate the geographic, necropsy and parasite isolation information collected from necropsied otters and seals into a database, and analyze this data to assess for risk factors for protozoal exposure and disease; and 4) apply our panel of serodiagnostic tests to live otters and seals, both to expedite clinical therapy and to investigate the dynamics of protozoal infection in wild and captive populations.

To accomplish these tasks, all freshly dead otters and seals submitted for examination receive a complete postmortem examination, including culture of aseptically collected cerebrum, cerebellum, and CSF. Tissues are collected for microscopic examination, additional tissues are banked for toxicologic testing and bacterial and/or fungal cultures are completed. Serum is assessed for presence and relative concentrations of antibodies to recognized pathogenic protozoa, including Toxoplasma gondii, Sarcocystis neurona and Neospora caninum.

To date 25 live isolates of two distinct protozoal types have been isolated from these otters and seals. Isolates were characterized serologically, antigenically, molecularly and ultrastructurally, and were found to be indistinguishable from Toxoplasma gondii and Sarcocystis neurona parasites that are pathogens of terrestrial animals and humans. Pathogenic protozoa have been isolated from both brain tissue and CSF, and in one case both parasites were isolated simultaneously from brain tissue of a harbor seal that died of protozoal encephalitis. We are currently completing the serodiagnostic testing to investigate the association between seropositivity, protozoal infection and protozoal disease. Our database has been constructed, and we have initiated analysis for geographic, seasonal or age-related risk factors for protozoal infection.

Acknowledgments

This research was made possible through the generous support of the Lindbergh Foundation, the Morris Animal Foundation, the California Department of Fish and Game, The UC Davis Wildlife Health Center, The Marine Mammal Center, The San Francisco Foundation, PKD Trust, Friends of the Sea Otter and the Genetic Resource Conservation Foundation, UC Davis.

References

1.  Lapointe JM, PJ Duignan, AE Marsh, FM Gulland, BC Barr, DK Naydan, DP King, CA Farman, KA Burek Huntingdon, LJ Lowenstine. 1998. Meningoencephalitis due to a Sarcocystis neurona-like protozoan in Pacific Harbor Seals (Phoca vitulina richardsi). J. Parasitol. 84:1184-1189.

2.  Rosonke BJ; SR Brown, SJ Tornquist, SP Snyder, MM Garner, LL Blythe. 1999. Encephalomyelitis associated with a Sarcocystis neurona-like organism in a sea otter. J. Am. Vet. Med. Assoc. 215:1839-1842.

3.  Thomas NJ, RA Cole 1996. Risk of disease and threats to wild populations. Endangered Species Update. vol. 13(12):23-27.

4.  Thomas NJ. 1997. An update on the causes of mortality in southern sea otters. Sixth Joint U.S.-Russian Sea Otter Workshop. Forks, Washington. November 1997.