The oyster parasite Haplosporidium nelsoni, popularly called MSX, has been a major cause of mortalities of the eastern oyster,Crassostrea virginica, for nearly 40 years. Despite much knowledge on the basic biology of the parasite, the life cycle stage infective to oysters and the source of that stage have remained a mystery. A major question to be resolved is whether H. nelsoni transmission occurs directly between oysters or via an intermediate host. Newly developed biotechnology methods may help us discover the answer.

   Laboratories at Rutgers and the Virginia Institute of Marine Sciences (VIMS) have recently developed gene primers that bind very specifically to the DNA of H. nelsoni. In a project sponsored by the Sea Grant Oyster Disease Research Program, we are now collaborating in the search for a potential intermediate host using the primers in combination with a technique called polymerase chain reaction (PCR). Genetic material is amplified exponentially by PCR, allowing us to detect minute quantities of H. nelsoni DNA.

   Environmental samples, including sediments, plankton, and benthic organisms that feed on dead and dying oysters, are being collected on a regular basis in areas of Delaware and Chesapeake Bays where H. nelsoni is present. Total DNA is isolated and subjected to PCR amplification using the primers specific to H. nelsoni. If the parasite is present in a sample, its DNA will be selectively amplified.

The bright bands are positive reactions with the MSX primers

Gel A shows the results for MSX-infected oysters (lanes 1-5).

Gel B shows a single positive polychaete worm (lane 3) and 4 negative sediment and worm samples (lanes 1,2,4,5).

In each gel, lane 6 is the negative control, lane 7 is the positive control, and lane 8 is a molecular weight marker ladder.

Positive samples have been found in both locations, although they have been more frequent at the Chesapeake Bay site, where H. nelsoni infection pressure has been heaviest. In addition to continued screening of environmental samples, in situ hybridization is being pursued at VIMS on samples positive for H. nelsoni to identify the host species and to visualize parasite stages that react with the primer.