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Rise in Shellfish-Related Food Poisoning Is Likely Linked to Extreme Heat in the Pacific Northwest

Vibriosis cases are the highest ever recorded for the month of July, according to the Washington State health department

Oysters exposed at low tide in a muddy oyster bed.
Many oysters and other shellfish exposed to extreme heat at low tide earlier this summer cooked right on the shore.

According to the Washington State Department of Health (DOH), a recent outbreak of vibriosis — an illness associated with eating raw or undercooked shellfish — is likely linked to the heat dome that enveloped the Pacific Northwest in late June.

The department says the extreme heat combined with low tides to produce conditions where more vibrio bacteria can thrive — and the number of lab-reported vibriosis cases for the month of July, 52, is the highest ever recorded for the month. For comparison’s sake, in July 2020 there were five cases total — the highest in the past five years has been 25.

Vibriosis symptoms include diarrhea, abdominal cramps, nausea, vomiting, headache, fever, and chills, and the illness usually occurs anywhere from four hours to several days after eating contaminated shellfish, with mild or moderate symptoms that typically run their course in two to three days. Most people who contract the illness get sick within one day of consuming raw or undercooked shellfish. “Some of the people who became ill did eat oysters at restaurants,” DOH spokesperson Teresa McCallion tells Eater Seattle. “However, typically DOH doesn’t close restaurants due to this type of an outbreak because it usually isn’t a handling issue. The issue tends to be the oysters.”

Those who harvest recreationally or cook at home should take measures to avoid getting sick, such as making sure that the shellfish remains chilled prior to consumption, consulting the DOH’s shellfish safety map, and cooking the product at 145 degrees for at least 15 seconds. Meanwhile, the local shellfish industry has strict protocols in place that take into account unexpected weather events. “Here in Washington state there [are] actually vibrio control rules that dictate how we can harvest and handle our shellfish when we get these warmer temperatures to try to minimize any risk,” says Bill Dewey, media relations for Taylor Shellfish, adding that the company invested $5 million in a new building and a recirculating refrigerated seawater system that can help eliminate bacteria. “But the more episodes we have like this, the more prevalent vibrio will be and the more of an issue it will be.”

Recent headlines make clear that the impact of extreme temperatures on the Washington shellfish industry goes beyond the increased risk of vibriosis. When temperatures climbed above 100 degrees across the Pacific Northwest in late June, some researchers estimated that around 1 billion sea creatures perished along the Salish Sea coastline, with many mussels, clams, and oysters split open and cooked right on the shore. Smaller shellfish farmers were hit particularly hard, with one Lilliwaup-based purveyor reporting that it lost 50,000 oysters and 10,000 clams (a quarter of its entire stock) to the heat wave. Larger companies, such as Taylor, reported losses as well but may be more able to withstand longer-term economic impacts due to their size and the diversity of their sourcing.

Dewey says that the biggest danger to the seafood industry as a whole, though, may not necessarily be rising temperature and the increasing frequency of record heat waves, but the way the ocean’s chemistry is changing as a whole due to climate change-related pollutants. When more CO2 is absorbed by the ocean, the water gets more acidic and inhibits mussels, urchins, corals, and other sea life from forming shells or other calcium-based structures that they need to survive. According to a 2014 study, oceans are acidifying 10 times faster than at any time in the past 50 million years, and the rate of change is even higher than that in West Coast waters, including the Puget Sound.

Taylor Shellfish and other Pacific Northwest purveyors are trying to adapt to the new reality and mitigate its effect on shellfish harvests. Such efforts include developing more CO2-absorbing seaweed through a process known as co-culturing, and possibly even breeding shellfish in the future that can tolerate changing water chemistries. While these innovations may provide some resiliency for the shellfish industry as a whole, the devastating climate change impacts are no less alarming.

“Even if we can successfully impact the policy world and convince people to stop burning fossil fuels today, because of what’s already been absorbed by the Pacific Ocean, the waters are going to get more acidic for 30 to 50 years because of what’s already absorbed by the Pacific Ocean,” says Dewey. “So our fate, at least for our generation, is pretty well sealed.”