July 24, 2024

Under The Sea: Oysters and Norovirus Outbreaks

Area 23, a shellfish harvesting zone off the Louisiana coast roughly equal in size to the city of New Orleans, was closed this week after health officials linked a norovirus outbreak to its oysters.

Raw Oyster PlateAn investigation into the outbreak that sickened 14 people who ate oysters at a Louisiana restaurant determined that the oysters were tainted before they arrived at the restaurant. Health officials issued a recall of the oysters and the temporary closure of Area 23.

Closing a harvesting zone the size of a major metropolitan area might seem like an indicator of a massive outbreak, but that’s likely not that case, according to Ken Pastorick, spokesman for the Louisiana Department of Health and Hospitals (LHH).

When oysters are harvested, they are put into burlap sacks marked with tags that identify the area. “Restaurants, by law, are required to keep those tags for traceback purposes,” said Pastorick. (In this case, the restaurant did and that’s how health officials determined the source so quickly.)

But within a harvesting area, there can be a number of oyster reefs and fisherman are not required to specify from which reef each bag of oysters was pulled. So, sick oysters from one reef will close the whole area, Pastorick said.

“For an outbreak with 14 illnesses, it’s likely that the oysters, sickened with norovirus, came from one reef,” he said.

How do oysters get sick?

“As immobile filter feeders, oysters get sick from germs in the water that surrounds them,” Pastorick said. “The source of those germs includes everything from agricultural runoff of livestock manure, to a sick fisherman throwing up over the side of the boat. When they get sick, it takes them about three weeks to flush their systems, so that’s the minimum amount of time a harvesting will be closed.”

“For 21 days, we allow the oysters to purge,” said Pastorick. Then, they test the waters. Literally. When the test are all clear, the zone reopens. For some zones, it takes much longer to recover than others, said Pastorick. Zone 8, for example, where the Mississippi River meets the Gulf of Mexico, has been closed since Hurricane Katrina in 2005.



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