Document Type



Virginia Institute of Marine Science

Publication Date



Marine Fisheries Review





First Page


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Three major diseases of oysters have been monitored in Virginia estuaries for 2-3 decades. Dermocystidium marinum, causing a warm-season wasting disease, was discovered in Virginia in 1950 and continues actively to kill oysters where beds or populations are found in high salinity waters ( >15%). This disease spreads by close proximity of dying oysters to other oysters, hence each isolated bed must be sampled in early fall annually to document activity of the pathogen. Control involves avoiding infected seed oysters, cleaning beds of all oysters after harvest, and isolation of new beds. A new pathogen, Minchinia nelsoni (MSX), caused catastrophic oyster mortalities in 1959-60, and oyster planting ceased thereafter in a large area of high salinity ( >15%o) waters in lower Chesapeake Bay. A third pathogen, Minchinia costalis (Sea Side Organism or SSO), was found almost simultaneously on Seaside of Virginia in high-salinity waters ( >30 %o). Both these haplosporidan parasites kill native susceptible oysters at rates of 20-50 percent annually. Strains resistant to MSX were selected from survivors by laboratory breeding. SSO appears to be an endemic pathogen that causes confined periods of infection and mortality. Sporulation and infection occur regularly each May-June associated with oyster deaths. A long incubation period of 8 months with hidden or subclinical infections characterizes the disease. SSO is confined to high-salinity waters along the seacoast from Cape Henry to Long Island Sound. MSX is a highly infectious pathogen that appears to be new by importation or advent of a virulent strain. Infections occur during 5 warm months (June-October) and deaths occur throughout the year. Direct transmission has not been achieved in the laboratory for either haplosporidan. Transmission of the diseases and life cycles are still important objectives after 18 years of studies.