MSX Fact Sheet

Scientific Name: Haplosporidium nelsoni
Common Name: MSX (multinucleated sphere unknown)
Taxonomic Affiliation: Phylum = Haplosporidia
Species Affected: Crassostrea virginica (eastern oyster), Crassostrea gigas (pacific oyster)

Geographic Distribution

In the US, MSX disease ranges from the Damariscotta River, Maine to Biscayne Bay, Florida.

MSX is not present in the Gulf of Mexico, but has recently spread to Nova Scotia, Canada. Epizootic mortalities have been limited to Chesapeake and Delaware Bays, and recently Long Island Sound and Bras d'or Lakes, NS. The parasite has also been found in C. gigas from Korea, Japan and France, but seems to cause little mortality.


The disease was first documented in 1957 in Delaware Bay where it caused massive oyster mortalities and two years later it was found in the lower Chesapeake Bay. At that time the disease agent was given the acronym MSX for multinucleated sphere X (unknown). In the 1960s the parasite was found in coastal bays of North Carolina, Virginia, Maryland, Delaware, New Jersey, Connecticut and New York, but associated oyster mortalities did not occur south of Virginia or north of New Jersey. Description of the spore stage of the parasite led to it being named Minchinia nelsoni in 1966 and then renamed Haplosporidium nelsoni in 1980. In the 1980s, an apparent range extension occurred as the parasite was reported as far north as Maine and as far south as Florida; in 2000 MSX was documented in Nova Scotia, Canada where it caused substantial mortality.

Biology & Epizootiology

Haplosporidium nelsoni is a spore-forming protozoan. The predominant stage of the organism in the oyster is a multinucleated plasmodium, which ranges in size from 5-70 µm. The production of spores (sporulation) is rare in adult oysters but has been observed at prevalences as high as 40% in spat. Sporulation occurs in late June through early July and in the autumn. The infective stage to oysters has never been determined, and it is believed that the spore is infective to some as yet unknown intermediate host. The complete life cycle remains unknown. The inability to transmit the parasite combined with the rarity of spore stages, the lack of correlation of the disease with oyster density, and the parasite's ability to spread rapidly over long distances has led to the hypothesis that an intermediate host exists.

In the Chesapeake Bay, oysters become infected from mid-May through October; however, infection pressure during late summer and autumn is quite variable from year to year. Infections develop rapidly in susceptible oysters resulting in mortalities from July through October. Surviving oysters may maintain a high prevalence of the disease through the winter and a second period of mortality may occur in spring. Oysters acquiring infections in late autumn may harbor low-level infections, which intensify the following summer. These infections often proliferate as temperatures warm in June causing early summer mortalities. The disease can affect all ages of oysters, spat to adult. Infections are acquired through gill and mantle tissue, and can rapidly spread throughout the oyster.

Environmental Influences

Temperature and salinity play an important role in regulating MSX. Infections are acquired at temperatures above about 20°C. Three critical temperatures have been proposed for oyster-H. nelsoni interactions. Both parasite and oyster are inactive at temperatures <5°C. At 5-20°C, the parasite proliferates more rapidly than the oyster can control it. Above 20°C, resistant oysters can overcome the parasite while susceptible oysters are killed.

Salinity is important in determining the distribution of the disease within an estuary. A salinity of 15 ppt is required for infection, 20 ppt is required for rapid and high mortality, and 10 ppt or below results expulsion of the parasite at temperatures above 20°C.

Control Measures

Use selectively bred MSX disease resistant oyster strains. Maintain oysters in disease-free areas (low salinity). If oysters must be moved to high salinity areas for growth and conditioning, the move should be timed to avoid the early summer infection period. Low-salinity immersion to expulse MSX infections may be valuable, but critical time-temperature-salinity combinations have not been thoroughly determined. Avoid importation of infected oysters into grow-out area.

Diagnostic Method

Histological examination using light microscopy of paraffin embedded tissue sections is the standard diagnostic technique for MSX, although plasmodia stages of H. nelsoni are not readily distinguished from a related parasite, H. costale or SSO (seaside organism). In the absence of spores, specific diagnosis requires employment of molecular tools including polymerase chain reaction (PCR) and DNA probe assays.