Shellfish warning issued after spike in illnesses linked to B.C. heat wave

Photo: CHRIS HARLEY, UNIVERSITY OF BRITISH COLUMBIA

Tens of thousands of dead mussels carpet the rocks at Vancouver's Kitsilano Beach on July 6, 2021

Health officials are warning against eating under-cooked shellfish following a spike in intestinal illnesses caused by a sea-borne bacteria likely linked to B.C.’s recent heat wave.

A specialist with the B.C. Centre for Disease Control said June’s “heat dome” contributed to soaring sea surface temperatures — perfect conditions to allow the natural occurring vibrio bacteria to contaminate shellfish like clams, oysters and mussels.

“Normally, we start seeing illnesses when [sea surface] temperatures go over 14 C,” says BCCDC food safety specialist Lorraine McIntyre. “In some areas, it was over 20 C during that heat dome.”

Over the past week and a half, at least 10 people have fallen ill from vibriosis Vancouver Island, the Gulf Islands, Lower Mainland and Sunshine Coast. In Washington state, health officials say an outbreak has led to more than 50 cases, the most ever recorded in the month of July.

Symptoms last between three and seven days and can include diarrhea, abdominal pain, nausea, vomiting, fever, headache and bloody stool, according to the BCCDC. Infections have been known to spread to other parts of the body and death can — but rarely does — occur.

The BCCDC has been tracking the rise of vibrio in local waters since 2015, when 62 people became ill. That outbreak was associated with “the blob,” a 1,600-kilometre-wide mass of warmer-than-normal water that appears when long-lasting high-pressure ridges sit over the Pacific Ocean.

Plumes of the sea-borne bacteria have been recorded farther north in recent decades. An outbreak in Alaska in 2004 expanded the range of confirmed human infections north of 60 degrees latitude, more than 1,000 kilometres past its previous record.

Since then, researchers have described the vibrio as a “climate change indicator” after isolating the bacteria in stranded sea otters, a harbour porpoise and a beluga whale on Alaska’s coast.

“The evidence is strong that ongoing climate change is influencing outbreaks of vibrio infections on a worldwide scale,” note the authors of one 2016 study.

Most of the illnesses have been isolated in people harvesting shellfish themselves, though McIntyre said some recent illnesses have been attributed to swallowing sea water while swimming.

Anyone harvesting their own shellfish must ensure they are cooked properly, she said. “We always say 90 C for 90 seconds. That will also kill noravirus.”

The BCCDC urges any shellfish harvester with a tidal water licence to check its status map tracking harmful biotoxins in coastal waters.

When it comes to commercial shellfish harvesters, they have more options, like dropping oyster lines deeper into the water column where temperatures are cooler or passing them through tanks to flush out the filter feeders.

Protecting the health of both humans and shellfish is going to become increasingly challenging as oceans get warmer and more acidic, McIntyre said.

Since the start of the Industrial Revolution, humans have put enough carbon dioxide into the atmosphere to cause 1.2 C of warming. At sea, CO2 absorbed from the atmosphere has raised ocean acidity by 30 per cent, dampening the ability of clams, mussels and oysters to form their shells.

With the recent heat wave estimated to have killed over a billion sea creatures, McIntyre’s message to shellfish harvesters is simple: “Respect the habitat. Respect the site.”

“At these northern latitudes, we’re expecting to see rapid temperature rise,” she said. “We need to stay vigilant.”