The disease caused by H. nelsoni is also known as MSX (multinucleated unknown or multinuclear sphere X).

MSX is thought to have been introduced by experimental transfers of the Pacific oyster (Crassostrea gigas), which is resistant to this disease.

Through extensive studies, an estimated 50% of eastern oyster (C. virginica) mortalities in lower Delaware Bay since the 1960s could be attributed to MSX infections, a devastating loss to the area.

[2] The presence of H. nelsoni in the Chesapeake Bay has hindered attempts to return eastern oyster harvests to historic levels.

After more than 50 years of selection pressure, some populations of C. virginica have become resistant to the MSX.