What is MSX?: the parasite infecting oysters in P.E.I.

A parasite has been confirmed to be infecting some oysters on Prince Edward Island, and many are left wondering if they should wait until they shuck.

Ryan Carnegie with the Virginia Institute of Marine Science said while multinucleate sphere X, known as MSX, is dangerous to oysters, there’s no real threat to humans.

“MSX is a protozoan parasite, so this is a small, microscopic organism that is parasitic of our oysters, that lives within them in their soft tissues, that proliferates in those tissue spaces in the organs of the animal beyond the abilities of the animal to control it, and when that happens it can kill them causing substantial mortality,” said Carnegie in an interview with CTV’s Stephanie Tsicos.

“I do need to say that this is not a pathogen that affects humans in any way other than the mortality it causes in the oysters they’re growing.”

MSX is a threat for the oyster industry as the parasite can quickly infect a large portion of the population with a high mortality rate.

“In places where it occurs more or less naturally these days it can be highly prevalent, 60 or 70 per cent of oysters in a population may be affected,” he said.

Carnegie says oyster lovers don’t have to worry, as lightly infected ones will have no affects if eaten.

“We are still able to eat them. The very heavily-affected animals that are unfortunately near their death, they may not look very appetizing because of the affects of the parasite, those would be oysters that you wouldn’t be served or wouldn’t want to eat anyway, but the oysters that might be lightly infected, they’re fine to eat and the parasite is not going to cause any adverse affects on human consumers whatsoever,” said Carnegie.

The parasite can be found widely along the East Coast all the way from Florida to the Maritimes as long as the environment is suitable, but it’s still a mystery on how exactly it spreads.

“We don’t know how it spreads. It does not spread directly from oyster-to-oyster in a population unlike most of the other diseases of shellfish. It transmits to oysters through an intermediate host or hosts that have never been identified. When it is not infecting oysters in our coastal systems, it’s living in one of these other organisms. It could be some small invertebrate, and we just don’t know what that is unfortunately,” he said.

“It’s found in whatever this intermediate host or hosts are, it is not found in any of our commercially important species, lobsters or clams or crabs, or any of these other commercially important resource species. If it was infecting them we would’ve known of that long ago.”

Carnegie says once the parasite has been found in oysters it cannot be treated.

“It doesn’t go away once it appears, and that’s the problem,” he said.

He recommends that rather than fight the disease, harvesters should instead try to manage it.

“A lot of aquaculture in the United States is based on hatchery production of disease resistant oyster seed that can be planted in areas where MSX disease and one of our other major diseases are widely distributed, and those oysters are capable of growing to market size without being significantly affected by these major diseases like MSX,” said Carnegie.

“That is one of the major avenues that a newly-affected area like Prince Edward Island would really want to consider pursuing to try and manage this disease.”

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