Two parasitic diseases, Dermo and MSX, have devastated oyster populations in the Chesapeake Bay, as well as in other parts of the East Coast of the United States. Dermo is caused by the parasite Perkinsus marinus, which was first discovered in the Chesapeake Bay in the mid-1940s. MSX is caused by the parasite Haplosporidium nelsoni, which was introduced from Asia, and began causing outbreaks in the Chesapeake Bay in the late 1950s. These diseases, combined with decades of overharvesting of oysters, have reduced oyster populations in the Bay to approximately 1% of what they were in the 1950s.

The graph below shows oyster landings for Maryland and Virginia since 1880. The years when the oyster diseases MSX and Dermo were found in the Bay are indicated with arrows.

H. nelsoni usually infects and kills oysters earlier than does P. marinus. MSX can kill oysters within the first year that they are exposed to H. nelsoni, whereas it usually takes two to three years for Dermo infections to kill an oyster. Unfortunately, both diseases can kill more than 90% of exposed oysters in two to three years. Both diseases are more virulent in higher salinity waters, so oysters in the southern portion of the Bay have been affected more by MSX and Dermo than those in the northern parts of the Bay or its tributaries. In years of drought, the upper portion of the Bay becomes more saline, increasing the prevalence of these diseases there as well. Warmer winters also allow more of the parasites to survive the season, which increases the rate of infection in the following year.

Certain oysters have shown a greater resistance to MSX and Dermo than others. Both Rutgers University and The Virginia Institute of Marine Science (VIMS) have selective breeding programs that create populations of disease-tolerant oysters, which have been used for restoration and aquaculture. While these oysters may still become infected with the parasites, they are slower to develop disease from the infection. This allows them to live longer so that they may reach market size (at least three inches) and also gives them more time to reproduce and repopulate the local oyster population before they die.

NOAA Sea Grant established the Oyster Disease Research Program (ODRP) in 1990. This long-term, national program supports innovative research to better understand and combat oyster disease in order to improve restoration and stock enhancement. As one part of this program, scientists from academic institutions collaborate to select and breed individual oysters that are better than average at combating disease-causing pathogens. These hardy strains are often developed from native oysters that have demonstrated an ability to survive and grow, even in the presence of disease.