Oyster Disease Alarms Maine Industry
by FV Staff
Marine scientists at the University of Maine have documented an alarming regional increase in the prevalence of a largely fatal pathogen in Maine oysters. At this time the problem is confined to the Damariscotta River system where 96% of the oysters tested were infected.
The pathogen enters the gills, goes to the intestines and to a full body infection. There is no human health impact said Marcy Nelson, a Maine DMR biologist.
The testing method is to take a number of same-size plots in an area, then count the number of oysters infected and not, to get a percentage.
Deborah Bouchard from the University of Maine Cooperative Extension Animal Health Lab and the Aquaculture Research Institute, said the presence of the pathogen (haplosperidium) locally known as MSX, is not new, but it is an emerging issue since the incidence has been low but it is now high. She said the oyster industry in Maine is facing a large decline, but added that with pathogens, which are parasites, animals develop a resistance and typically recover.
While that is good news in the long run for the oyster industry, in the near term it could be devastating.
MSX has been known to be in Maine since 1996 when one case was reported. It came to Chesapeake Bay from the west coast of the U.S. in 1957. Eventually the oysters there developed a resistance to it, but not before the industry collapsed.
Bouchard said little is known about this pathogen’s relationship to oysters. She said the pathogen/host relationship is complex and it is not known for certain if MSX is being brought to the Maine oyster by an intermediate host parasite that the MSX pathogen is living on.
Cold water reduces infectious prevalence. The pathogen/host relationship is highly influenced by environmental effects. These environmental effects, water temperature, salinity, feed, etc, can compound the complexity of what is going on in any given oyster population.
The researchers are looking at the problem from both the molecular level and observing what happens from year to year, comparing for example, mortalities. The survivors, however small the percentage, can be studied and cross-bred to help develop a resistant oyster. This resistance develops over many generations which is not a financial solution for Maine oyster growers.
The aquaculture industry would take the hardest hit.
Adam Campbell operates North Haven Oyster Farm. He said he is very concerned about MSX which was first detected on this scale this fall. Campbbell said he hopes his being isolated from the mainland (on North Haven Island) will be to his advantage. Campbell is also a lobster fisherman.
Among things not understood about this parasite is how it spreads. “We don’t know the entire life cycle of this pathogen,” said Bouchard. This puts scientists and growers at a considerable disadvantage. DNA testing is now possible. It was not in 1957, and MSX can be detected in the DNA. Tests show the MSX pathogen new to this region. How it got here is not known.
The Damariscotta River system has been an exceptionally productive area for oysters for hundreds, possibly thousands of years. The middens there indicate native Americans feasted on oysters for centuries leaving deep piles of shells as evidence.
There is no way now to know if MSX was ever a problem in the past. Marcy Nelson, said the Great Salt Bay above Damariscotta has warm waters with high levels of phytoplankton and zooplankton which have made it ideal for the American oyster.
Nelson said the Sequencing in the DNA of the MSX matched the sequencing found in oysters to the south in New Jersey and Massachusetts. Because of the clearly defined margins of the outbreak in the Damariscotta region, “it strongly suggests the MSX was brought here,” Nelson said.
How it spreads after being brought in is the question growers want answered.