MSX/Dermo

A major challenge to oyster restoration in the Bay is overcoming the effects of disease. Two single-celled oyster parasites—Haplosporidium nelsoni, which causes MSX, and Perkinsus marinus, which causes Dermo—currently are major sources of oyster mortality in the Bay. While MSX and Dermo are not harmful to humans, they can kill more than 90 percent of exposed oysters within two to three years.

MSX

MSX is a parasitic disease that can affect oysters of all ages. Once an oyster is infected with the parasite, MSX develops rapidly and kills the oyster within several weeks. H. nelsoni thrives in warm, high-salinity waters, which is why MSX is restricted to the middle and lower Bay. There is good evidence that the disease came from the Japanese oyster, Crassostrea gigas, which was intentionally introduced into Delaware Bay.

MSX was first found in the Bay in 1959, two years after it was first documented in Delaware Bay. MSX-resistant populations have begun to evolve in oysters in both Chesapeake and Delaware bays, raising hope for the long-term oyster restoration potential.

Dermo

Dermo is a parasitic disease that, like H. nelsoni, spreads quickly in warm, high-salinity waters. Since the mid-1980s, several consecutive years of drought conditions, combined with above average winter temperatures and poor management practices, have caused P. marinus to spread to parts of the upper Bay and tributaries. Because of its increased range and tolerance of lower salinities, Dermo is more damaging than MSX to oyster populations throughout the Bay.

Dermo was first discovered in the Bay in 1949 and has consistently been present ever since. The disease spreads easily because infected oysters actually release P. marinus into the water, which is then taken up by healthy oysters as they feed. Normally, oysters less than one year old do not become infected with Dermo, but many contract the disease in their second year.