Toxoplasma gondii Survival in Eastern Oysters

Recent research by Drs. Marina Collins and David Lindsay has expanded the understanding of the interaction between Toxoplasma gondii and oysters in the marine environment. T. gondii is prevalent in the marine environment, and it has previously been determined that Eastern oysters (Crassostrea virginica) can remove sporulated T. gondii oocysts from seawater and that the oocysts retain their infectivity for some time. This study examined the long term survival of T. gondii oocysts in oysters and examined how efficient oysters were at removing oocysts from seawater.

Oysters were exposed to 1x106 oocysts for 24 hours and examined at intervals up to 85 days postexposure (PE). Using a mouse bioassay, it was determined that ninety percent of these oysters were positive for T. gondii by the end of the first day PE. Tissue cysts were still observed in 1 of 2 mice fed tissue from oysters exposed 21 days previously. Toxoplasma gondii antibodies were found in 2 of 3 mice fed oysters that had been exposed 85 days previously.

In the second part of the study, groups of 10 oysters were exposed to 1x105, 5x104, or 1x104 sporulated T. gondii oocysts for 24 hours, and then processed for bioassay in mice. All oysters in the first group were infected, and 60% of oysters in the second group were positive when fed to mice. The studies with exposure to 1x104 oocysts (the third group) were positive when fed to mice in 10 and 25% of cases. These studies indicate that T. gondii can survive for several months in oysters and that oysters can readily remove T. gondii oocysts from seawater. Infected filter feeders may serve as a source of T. gondii for marine mammals and possibly humans.

Toxoplasmic encephalitis has recently been recognized as a primary disease of sea otters and other coastal-dwelling marine mammals. Previous research indicated that coastal storm runoff could introduce T. gondii into the marine environment. Once in the seawater, the Toxoplasma oocysts can accumulate in filter feeders such as oysters, mussels, and clams, creating the potential for risk to humans who ingest contaminated shellfish without proper cooking.

Dr. Collins, who has now completed her Ph.D., plans to combine her M.D. and Ph.D. training to pursue further research projects in the area of public health medicine.

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Toxoplasma gondii Survival in Eastern Oysters