A new epizootic of Haplosporidium nelsoni (MSX), a haplosporidian oyster parasite, in Long Island Sound, Connecticut

Epizootic prevalences of MSX, Haplosporidium nelsoni, were detected in east ern oysters (Crassostrea virginica) at several sites in Long Island Sound, Connecticut. Diagnosis was based on histological sections. Twenty-two sampl ing stations representing commercially important oyster grounds were tested . Three sampling stations were negative for MSX, the rest had prevalences f rom 7 to 89% (319 MSX positive specimens out of 653 processed). Two major s eed areas (Housatonic River and Bridgeport Natural Bed) were uninfected, bu t all studied shallow (<15 feet) or deep (>15 feet) water growing areas wer e infected along the whole shoreline. Hatchery-raised seed, uninfected when imported to Connecticut, acquired infection in New Haven and Groton. Sporu lation with acid-fast spores was detected in 1% of MSX-positive specimens. One specimen showed unusual pattern of sporulation with spores inside cilia ted digestive duct cells and vesicular connective tissue in addition to dig estive tubules. The percentage of potentially resistant specimens was low ( <1%). A periodic range extension of an alternate host, usually restricted t o the mid-Atlantic region of the U.S. coast, is suggested as the cause for the recent epizootics in Long Island Sound.