Investigation into the processes promoting T. gondii infections in sea otters may be our best opportunity to understand terrestrial parasite flow into the coastal marine system. In the absence of evidence that the high number of sea otters infected off the coast of California can be attributed either to the ingestion of prey containing tissue cysts or vertical transmission, the most likely source of infection is exposure to the environmentally resistant, infective oocysts of T. gondii, which can survive for months or years in contaminated soil, freshwater or seawater. Toxoplasma gondii oocysts may remain infective despite water chlorination or sewage processing... Furthermore, the infectious dose for some species may be as low as one oocyst.
Several studies have demonstrated that oocysts and cysts of pathogenic protozoa are concentrated by clams, mussels and oysters during filter-feeding activity. Thus, raw shellfish could serve as a source of pathogenic protozoal infection for both marine mammals and humans. To maintain normal body weight and meet metabolic demands, sea otters consume approximately 25% of their body weight each day in invertebrate prey, such as mussels. Other possible modes of T. gondii infection could be by the ingestion of oocysts in seawater, by grooming and ingesting oocysts collected on their fur or by ingestion of oocysts that are on or in prey other than bivalves...however, these hypotheses require further investigation!
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