Increased virulence in an introduced pathogen: Haplosporidium nelsoni (MSX) in the eastern oyster Crassostrea virginica

The protistan parasite Haplosporidium nelsoni has caused extensive mortalit y in the eastern oyster Crassistrea virginica along the mid-Atlantic coast of the United States since 1957. The origin of H. nelsoni has remained unre solved. Molecular diagnostic tools were used to examine the hypothesis that a haplosporidian parasite in the Pacific oyster C. gigas is H. nelsoni. A DNA probe specific for H. nelsoni reacted positively in in situ hybridizati ons with haplosporidian plasmodia from C. gigas collected in Korea, Japan, and California. Primers that specifically amplify H. nelsoni DNA in the pol ymerase chain reaction amplified product from Californian C. gigas infected with the haplosporidian parasite. The DNA sequence of the 565-base pair am plified product was identical to the H nelsoni sequence except for a single nucleotide transition, a similarity of 99.8%. These results are conclusive evidence that the parasite in C. gigas is H. nelsoni and strongly support previous speculation that the parasite was introduced into Californian popu lations of C. gigas from Japan. Results also support previous speculation t hat H. nelsoni was introduced from the Pacific Ocean to C. virginica on the East Coast of the United States, likely with known importations of C. giga s. These results document greatly increased virulence in a naive host-paras ite association and reinforce potential dangers of intentional, but imprope r, introductions of exotic marine organisms for aquaculture or resource res toration.