Human Health Risks Past, But Shellfish Closures Likely to Linger

Oct 20, 2016

Massachusetts shellfish growers and oyster aficionados have suffered a string of unusual closures in recent weeks. Large swaths of New England waters have been closed to shellfishing because of a harmful algal bloom that could cause amnesic shellfish poisoning, while officials closed Wellfleet Harbor after an outbreak of the stomach bug, norovirus. While the two events are unrelated, they have one thing in common – the closures are likely to last weeks.

In the case of Wellfleet Harbor, the minimum required closure for norovirus is twenty one days.

“Viral particles – particularly norovirus - live a long time,” explains Mike Hickey. “A small amount of sewerage can have several million particles, and it takes about ten virus particles to make somebody sick. It doesn’t take much.”

The silver lining is that norovirus outbreaks tend to be one-off events, often traceable to a single individual. As to how norovirus – the most common cause of stomach flu – got into oysters in Wellfleet Harbor, Hickey says it could be as simple as someone vomiting over the side of a boat, or a boat releasing sewage (which is illegal). 

The three-week closure is designed to give officials time to confirm that the norovirus has disappeared, and to figure out where it came from. Hickey says they’ll get around to that, but that officials have been distracted by the much larger algal bloom and concern about amnesic shellfish poisoning.

Ironically, Cape Cod and Massachusetts Bays are some of the only areas in new England that haven’t been affected by the bloom of algae known as Pseudo-nitczschia. The phytoplankton produces domoic acid, a toxin that can cause gastrointestinal illness, and – in some cases – neurological symptoms ranging from permanent loss of short-term memory, to coma, and even death.

The presence of Pseudo-nitzschia and domoic acid has forced closures in Maine,  from Stonington to Machiasport, and in southern New England, from Narragansett Bay all the way to Nantucket Sound.

Pseudo-nitzschia was introduced to New England and has been on the rise over past twenty years. Environmental changes could be part of the cause. However, Pseudo-nitzschia prefers cooler water temperatures, so this is exactly the time of year a bloom might be expected.

What’s unexpected about this year’s bloom is the geographic range and the sheer amount of the algae that’s being detected.

“We’re looking at cell numbers that, I think, are unprecedented,” says Hickey. “We’ve seen cell counts in Rhode Island of over a million cells per liter. That’s a really high amount, believe me.”

Numbers are slightly lower in Massachusetts and Maine, where the amount of the harmful algae has been declining in recent days. Still, Hickey warns the closures could still last weeks longer because this time of algal bloom becomes more toxic as it’s dissipating. Whether due to changing water temperatures or lack of nutrients, stress causes Pseudo-nitzschia to produce more of the domoic acid that makes people sick.

Not all shellfish are affected, though. Scallops are considered safe, as long as only the adductor muscle (round meat) is taken. Nantucket had closed their bay scallop fishery, pre-emptively, but reversed that decision after consulting state officials.

Hickey stresses that human health concerns are under control at this point. Fish flesh and lobster meat are both safe, although he recommends avoiding lobster tamales – the liver equivalent – because that’s where toxins accumulate. He also assures consumers that all shellfish currently on the market is safe. Of course, if you’re heading out to do some clamming, please check with state officials for the latest closings