Haplosporidium nelsoni

Haplosporidium nelsoni is a pathogen of oysters that originally caused oyster populations to experience high mortality rates in the 1950s, and still is quite prevalent today. 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.

MSX was first described in 1957, when it caused serious mortalities in Delaware Bay. 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.