Haplosporidium nelsoni (MSX) epizootic in the Piscataqua river estuary (Maine New Hampshire, USA)

A major epizootic of Haplosporidium nelsoni occurred during 1995 in the Piscataqua River Estuary, near the northern distributional limit of the parasite. Prevalence of N. nelsoni in samples of oysters, Crassostrea virginica, collected between 5 September and 18 December 1995 from sites in Maine and New Hampshire ranged from 15 to 81%, with up to 50% systemic infections. Associated proportions of dead oysters ranged from 25 to 83%. The epizootic was most likely triggered by unusually warm and dry climatic conditions in 1995 that resulted in a temperature and salinity regime in the estuary that favored the parasite.