Efficacy of Haplosporidium nelsoni testing protocols for the American oyster (Crassostrea virginica)

Abstract

Haplosporidium nelsoni, more commonly known as Multi-nucleated Spherical X-unknown (MSX), is a protozoan parasite that has caused catastrophic American oyster (Crassostrea virginica) losses along the mid-Atlantic coast of the United States since the 1950s. In 2002, an outbreak was first detected in Canada in the Bras d’Or Lake in Cape Breton, Nova Scotia where mortalities on oyster leases and natural oyster beds exceeded 90%. The oyster industry in the Bras d’Or Lake, Cape Breton, which previously accounted for approximately 80% of oyster production in Nova Scotia, has yet to recover. Current detection protocols include testing a single gill section for parasite presence using DNA analysis followed by confirmation of infection through histological examination. The presence of localized infections may go undetected using standard DNA analysis protocols resulting in false negative test results. The testing of additional samples could allow for the detection of localized infections, but would require more time, labour, and expenses, thus this approach would only be worth implementing if it detects a greater number of infections than the current protocol. This study involved using two different methods for analysis of tissues: (i) analysis of four additional samples from oysters previously tested using standard protocols, and (ii) analysis of samples from homogenized oysters previously tested using standard protocols. The results of this study suggest that the standard protocol does not detect all infections and a more thorough sampling/testing protocol may be more reliable for detecting the presence of the parasite. For surveillance purposes where early detection is critical to assess and plan for disease presence and spread, analysis of additional samples may prove beneficial moving forward.