Chesapeake Bay oysters fight back against MSX, Dermo

The bad news: MSX and Dermo, diseases that have ravaged Chesapeake Bay oysters, are more widespread than ever.

The good news: More oysters are overcoming the diseases, particularly MSX.

The mixed bag is the latest analysis from Virginia Institute of Marine Science researchers, who say oysters must continue to fight off the diseases to re-emerge from a decades-long decline.

"We can't really hope for anything more than that. There's no way we're going to eradicate the diseases," said Ryan Carnegie, an associate research scientist at VIMS' shellfish pathology laboratory.

Scientists discovered MSX in Chesapeake oysters in 1959, a decade after they found Dermo. The diseases do not affect people who eat oysters, but their effects have combined with the loss of habitat and overfishing to deplete oysters to 1 percent of their historic population.

The oyster industry reflects the slide. More than 4 million bushels were taken from Virginia waters in 1958-59, but the harvest dropped to 65,000 in 2006-07.

The trend hasn't greatly affected prices because local supplies are supplemented with oysters brought here from the Gulf of Mexico. But watermen are without work and oyster houses have closed, and the decline contributes to the degradation to the bay. Filtering as much as 50 gallons of water per day, oysters reduce excesses of nutrients, such as nitrogen and phosphorus, that create dead zones.

Each May, VIMS researchers gather about 700 oysters from a disease-free area of the bay - the current site being the upper Rappahannock River, where salt levels are low enough to prevent onset of the diseases.

Researchers carry the oysters to Gloucester Point and place them in cages in the much saltier York River. The researchers observe the oysters until October.

When the program began - during the 1960s - about 40 percent of the oysters would contract MSX, Carnegie said. Since 2000, the infection rate - about 90 percent - has more than doubled.

"We put them in the York and they get absolutely creamed by MSX," Carnegie said.

Dermo, rarely encountered in transplanted oysters until the 1980s, also has a significant effect, Carnegie said. He didn't have specific figures but said the disease "continues to be a major killer of oysters."

Despite mounting pressure from MSX and Dermo, fewer oysters are becoming infected by the diseases.

For example, 82 percent of the oysters on Wreck Shoal in the lower James River had MSX in 1996. In recent years, the percentage has dipped to below 50 percent, Carnegie said.

He attributes the decline to oysters developing a resistance to both diseases, as well as heavy rainfall, which lowers the bay's salinity levels.

While the turnaround doesn't mean oysters will make an immediate recovery, it is welcomed news for the troubled bivalve.

Last April, state and federal officials decided against introducing a hardier, Asian oyster into the bay - proponents believe the species is the best hope to revitalize the once dominant fishery.

Since then, scientists have quarreled over ways to revitalize the native species. The state eliminated oyster restoration funding. And watermen complain that a rotational harvest plan does nothing to boost the population.

On a more positive note, heavy rain last year will reduce salinity levels, which will stave off the diseases. But there's a catch: the rain washes pollutants into the water, which curtail oyster growth.

MSX & Dermo
•Most scientists believe the MSX disease originated with the Japanese oyster, which was intentionally introduced into the Delaware Bay. Once an oyster is infected with MSX, the disease can kill the oyster within several weeks.

•Dermo is caused by a parasite that thrives in above-average water temperatures and during droughts. It inhibits growth and reproduction, and causes death in heavily infected oysters.