Skip to content

'Heat dome' likely primed B.C. waters for spike in shellfish illnesses: health officials

As waters warm, scientists worry more coastal regions — from Washington to British Columbia to Alaska — could be in for more outbreaks due to shellfish-borne contagions.
Tens of thousands of dead mussels carpet the rocks at Vancouver’s Kitsilano Beach on July 6, just over a week after the peak of B.C.'s heat wave. It's now thought the heat primed coastal waters for an outbreak of vibriosis, an intestinal disease brought on by eating undercooked shellfish.

B.C. health officials are asking residents to avoid eating undercooked shellfish.

The warning follows a spike in intestinal illnesses caused by a sea-borne bacteria.

A specialist with the BC Centre for Disease Control (BCCDC) says 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 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 last week and a half, at least 10 people have fallen ill from the vibriosis illness across 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 days and one week 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 occurs. 

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.

This time around, the link between a spike in shellfish illnesses and warming seas has once again been closely associated with a long-term shift in the planet’s climate. In recent weeks, researchers found the record-breaking heat wave was made 150 times more likely due to human-caused climate change. 

Plumes of the sea-borne bacteria have been recorded marching steadily north in recent decades.

An outbreak of the diarrheal disease 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 self-harvesting shellfish, though McIntyre says some illnesses in the last two weeks have been attributed to swallowing sea water while swimming.

The BCCDC food safety specialist says anyone looking to pick their own oysters and mussels or dig their own clams must ensure they cook them properly.  

“We always say 90 C for 90 seconds. That will also kill noravirus,” says McIntyre.

The BCCDC urges any shellfish harvester with a tidal water licence to check its newly improved status map that tracks 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. Alternatively, they can choose not to harvest.


In the end, McIntyre says protecting the health of both humans and shellfish is going to become increasingly challenging as oceans get warmer and more acidic. 

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 says. “We need to stay vigilant.” 

Stefan Labbé is a solutions journalist. That means he covers how people are responding to problems linked to climate change — from housing to energy and everything in between. Have a story idea? Get in touch. Email [email protected].