PROTOZOAN PARASITES IN OYSTERS FROM MAINE: A TWO-YEAR SURVEY

Protozoan parasites, particularly Perkinsus spp. and Haplosporidium nelsoni, are currently recognized as significant threats to farmed oyster populations. On the other hand, shellfish have been identified as a potential source of Toxoplasma gondii responsible for toxoplasmosis and Cryptosporidium parvum, which is one of the major causes of gastroenteritis in the world. Since a die-off in 2010, attributed to H. nelsoni, the oyster industry in Maine has relayed on oyster strains selected for resistant/tolerance to H. nelsoni. Even in the absence of reported die-offs, epizootiological studies are showing that the range of H. nelsoni in Maine continues to expand. Similarly, the prevalence of Perkinsus spp. is at a record high. In 2014, we reported for the first time in Maine, oyster's PCR-positive for T. gondii and C. parvum. Here we performed an exhaustive qPCR-based survey for P. marinus, P. chesapeaki, H. nelsoni, T. gondii, and C. parvum during 2016 and 2017 in eastern oysters (Crassostrea virginica) (n=1,440) from six Maine (USA) locations including Bagaduce River, Weskeag River, Jack's Point and Prentiss Island in the Damariscotta River Estuary, New Meadows River, and Webhannet River. Pooled samples (mantle, gills, and rectum) from individual oysters were used for DNA extraction and qPCR. Our results confirmed the continuous presence during most of the year of the three oyster protozoan parasites in the six sampled sites. Interestingly, we also detected positive samples for T. gondii and C. parvum, especially in 2017. Our study represents a broad snapshot for these protozoan parasites in bivalves from Maine; at this point, the abilities of the oocysts to excyts and origin (fecal contamination from wildlife or runoff from farmed land) are unknown. The actual public health threat posed by both T. gondii and C. parvum via consumption of raw oysters remains unresolved; nevertheless, it has been suggested to include in the analysis of oysters when outbreaks occur, and oysters have been consumed. Our findings also indicate that establishing a baseline of waterborne parasites in commercial shellfisheries together with defining high versus low-risk areas would contribute to the development of sustainable and safe aquaculture.