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Volume 5, Number 2
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The Culture of Disease

By Jack Greer and Michael W. Fincham

Plot showing dramatic deline in oyster harvest from 1950 to present day
Dermo arrived early in the Chesapeake, but its effect in Maryland wasn't fully felt until years after MSX hit, when drought caused both diseases, which thrive in higher salinities, to spread. Together, these diseases decimated what was once the Chesapeake's most valuable fishery. Source: Graph, NOAA Fisheries Annual Commercial Landings Statistics Database.

Dermo, not MSX, is now the dominant oyster killer in Chesapeake Bay. A protozoan parasite, Dermo (known to scientists as Perkinsus marinus) was first documented at low levels in the Bay in 1949, a decade before MSX showed up. It may have been here forever, scientists say, or it may have been carried in from southern U.S. waters where it had been killing oysters for years.

In the mid-1980s, a series of warm winters and dry summers unleashed a Dermo epidemic in Chesapeake Bay, and by 1990, Dermo had exploded in Delaware Bay also. A remnant population left over in low levels from the 1950s had apparently endured, moving from oyster to oyster and surviving cold winters in small numbers, suggests Susan Ford, a researcher at Rutgers University's Haskin Shellfish Laboratory in Bivalve, New Jersey.

In Delaware Bay, Dermo has so displaced MSX as Parasite Enemy Number One, that in Ford's words, "If we didn't have Dermo we wouldn't have a disease problem."

In Virginia, VIMS researcher Gene Burreson notes that when Dermo moved into the rich seed areas of the James River it brought the oyster industry to its knees. Back in the 1950s, he says, some 75 percent of the harvest came from leased bottom stocked with James River seed oysters — 3 to 4 million bushels a year. First MSX arrived, then after the 1980s drought, Dermo followed saltier water up into the rich seedbeds and hit the oysters — and the oystermen — hard.

Dermo zoospore sketch
Zoospore drawing from The Eastern Oyster: Crassostrea virginica, by Kennedy, Newell, and Eble.

"They just lost everything," he says.

What have we learned about this other oyster killer? How does it survive? How does it grow? Does it have an Achilles Heel?

To attack these questions, scientists sought to study the parasite under controlled conditions in the laboratory, but as the 1980s gave way to the 1990s, they did not yet know how to culture Dermo and had to rely on samples gathered from the wild.

Culturing an organism in the laboratory is a key step in biological research. Cultures provide controlled populations for ongoing studies of all kinds. But coaxing a microorganism to grow outside of its native habitat can be tricky — after a half-century of research, researchers still cannot culture the elusive parasite MSX.

The effort to develop a reliable method for culturing Dermo became priority number one for researchers like Gerardo Vasta at the University of Maryland's Center of Marine Biotechnology (COMB) in Baltimore.

Vasta was well equipped for the task. He had two Ph.D.'s — one in biochemistry and one in zoology, both from his home country of Argentina. Vasta had come to the U.S. in 1979 on an international scholarship, and he soon discovered that oysters and their immune systems provided excellent biochemical models.

In 1989, only four years after the creation of COMB, Vasta joined the new faculty. Right away he focused on the oyster's defense mechanisms, especially its defenses against Dermo. The lack of a reliable means to culture Dermo in the laboratory proved a fundamental barrier in pursuing this research, and so he set his sights on that.

Vasta and his colleague Julie Gauthier proceeded to test one growth medium after another, Vasta drawing on his extensive knowledge of mammalian systems and applying it to this marine organism. During 1992 and 1993, their breakthrough came, and they announced their development of a novel means for culturing this parasite that was ravaging the Chesapeake's oyster bars.

It was a spectacular moment for oyster research. With federal funds and scientific competitiveness at a peak, other researchers also devised methods for culturing Dermo at about the same time. Jerome La Peyre at VIMS and Stephen J. Kleinschuster at the Haskin Shellfish Laboratory at Rutgers University used different methods, but all three teams soon reported their findings in the scientific literature.

The breakthroughs did not stop there.

Vasta's lab went on to produce molecular probes for Dermo, with work done by Adam Marsh, who later joined the faculty at the University of Delaware. Now, within a matter of months, Vasta says that his lab, working with the Institute for Genomic Research (TIGR), will complete the sequencing of Dermo's entire genome.

According to Kennedy Paynter, a longtime oyster researcher at the University of Maryland College Park and the UM Center for Environmental Science, the ability to culture Dermo has allowed researchers to ask and answer important questions, to test the parasite's response to salinity and temperature ranges. "We can see which conditions spur the parasite's growth and which hinder it," he says.

The molecular probe has also allowed scientists and research managers to pinpoint the presence of Dermo, whether in an oyster or in the environment.

Surveys conducted by Vasta and his colleagues from New England to the Chesapeake turned up Dermo everywhere. It used to be that oysters from Maine were uninfected, he says, but in a recent batch of Maine oysters Vasta found that 30 percent were infected with the disease.

The future will not be easy for oysters facing this persistent parasite, but Vasta remains optimistic. He envisions finding Dermo's weak spots — in its need for iron to fuel its metabolism, for example — and using selective breeding or genetic manipulation to tilt this tough biological battle in favor of the native oyster.

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