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A pathogen that has long plagued oysters in the Mid-Atlantic states caused an outbreak of disease in Maine oyster farms for the first time this summer, threatening a $3 million industry renowned for high quality and taste.

MSX, shorthand for the spore-forming protozoan Haplosporidium nelson, is not harmful to humans, and can be present in small numbers without hurting oysters. In fact, MSX was first detected two decades ago in the Damariscotta River, the heart of Maine's oyster industry. MSX can flourish under certain conditions not fully understood, infecting a large percentage of the oysters in an area.

MSX affects an oyster's feeding and reproduction, and over time the oyster weakens, including the muscle that holds the shell closed, and the oyster dies. Maine's oyster growers are well aware of the potential damage caused by MSX, as they've watched their peers struggle with the pathogen since it was first identified in Delaware Bay in 1957.

Chesapeake Bay oysters have been particularly hard hit, and have never recovered from the large mortality events in the 1960s. During the mid 1990s, oyster growers in Connecticut and throughout Long Island sound faced major MSX disease events. By 2002, the first major outbreak occurred north of Maine, in the Bras d'Or Lakes region in eastern Nova Scotia.

"Throughout July and August, we were doing our normal routine, what we call ‘relaying'. We harvest oysters from our seafloor beds with a small dredge and transport them down the river to our holding raft, which basically serves as an in-water warehouse for the oysters," recounted Chris Davis of Pemaquid Oyster Company, on the Damariscotta River, and director of the Maine Aquaculture Innovation Center. Davis and his partners typically store the oysters in 120- count ‘baskets' in the colder, saltier water of the lower Damariscotta River for a week or two to improve their taste and allow them to naturally de-silt before selling to local restaurants and dealers. "In July, we started to notice a troubling sign. Two or three oysters in each basket were dead when we returned to collect them, their shells cracked," said Davis. The phenomenon continued each week, and by mid-August they were concerned enough to send 60 oysters off to a laboratory for testing. The results showed that MSX was striking Maine? oysters for the first time.

Oyster growers up and down the river acted quickly, testing each growing area for the presence of MSX and working with the Maine Department of Marine Resources (DMR) to implement an immediate quarantine on moving oysters from the Damariscotta River to other water bodies, in an attempt to contain the outbreak.

Meanwhile, Marcy Nelson of the DMR, the oyster growers, Dana Morse of Maine Sea Grant and University of Maine Cooperative Extension, researchers at University of Maine and Rutgers University, and staff at Microtechnologies Inc. in Richmond, Maine, designed surveys to assess the extent of the problem within and immediately surrounding the Damariscotta River. Oyster tissue samples are being analyzed by Debbie Bouchard of University of Maine Cooperative Extension and director of the Animal Aquatic Health Laboratory, who will use the results to create a map of the disease, and help oyster growers minimize the impact of future MSX outbreaks.

MSX-related oyster die-offs usually occur in the spring, perhaps because oysters are at their weakest after using energy to survive through harsh winter conditions. Spring also appears to be when MSX spreads, leading to mortality events in late summer and fall.

Previously, Maine's cold winters were thought to protect oysters from MSX, which thrives in warm, salty water. Some oyster growers and scientists attribute the MSX outbreak in the Damariscotta to the record-breaking warm temperatures throughout winter, spring, and summer 2010 (link to September 2010 fathoming article),. The dry summer also contributed to higher salinities than usual in the river.

It is unclear how MSX spreads, but researchers think the pathogen uses a third-party host to infect oysters, rather than spreading directly between animals. This makes eradication from the local environment impossible. However, MSX can be present and not kill oysters, therefore other options include changing the oysters or changing the conditions.

Natural resistance has developed in some oysters that have lived with high concentrations of MSX for years, including Chesapeake Bay, and researchers also selectively breed oysters for resistance. This year, Damariscotta River oyster growers plan to purchase seed (baby oysters raised in a hatchery) stock that is resistant to MSX. In addition, both Chris Davis and Bill Mook, of Mook Sea Farms, suggested that faster-growing strains might be part of a solution, since they could reach marketable size before the disease caused any real damage.

When it comes to the conditions, growers can't control the temperature or rainfall, but they can hope that this winter brings cold and ice to the Damariscotta River.

This article is made possible in part by funds from Maine Sea Grant and the Oak Foundation.

Heather Deese holds a doctorate in oceanography and is the Island Institute's director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.