Maritimes outbreak showed listeria can taint food
The Canadian Press
Published Friday, August 22, 2008 5:34PM EDT
Last Updated Friday, May 18, 2012 8:52PM EDT
TORONTO - Canada is no stranger to Listeria. In fact, it was a large outbreak in the Maritimes a quarter-century ago that proved for the first time that the bacterium was a food-borne organism that could cause human disease.
In 1980-81, a cluster of 41 cases of listeriosis occurred in Nova Scotia, New Brunswick and Prince Edward Island, causing 18 deaths.
Dr. Walter Schlech was sent by the U.S. Centers for Disease Control in Atlanta to investigate the outbreak, the largest the world had seen to that date.
"There had been some clues from veterinary medicine, as well as an outbreak in Boston, that Listeria could be a food-borne illness," Schlech, a specialist in infectious diseases at Dalhousie University, said Friday from Halifax.
Laboratory rabbits had been infected by the bacterium and sheep sickened after eating spoiled silage, said Schlech, who moved from the U.S. to Halifax in 1982.
"But that had never been proven in humans before, probably because most outbreaks occur in small clusters and you can't get enough epidemiologic information."
The Maritimes cluster changed all that, allowing disease-tracking investigators to discern that it was the same Listeria strain that had made people sick and where it had come from.
After weeks of interviewing surviving patients and the families of those who had died, Schlech and his team of investigators traced the disease to Listeria-laced commercial coleslaw.
The culprit turned out to be cabbage grown on a farm where contaminated sheep manure had been used to fertilize the crop.
"They were using raw manure on the fields and the guy was raising sheep as well and he also had Listeria in his sheep flock," Schlech recalled. The realization that Listeria could make people sick "came as a complete surprise to him."
While it was known that microbes like salmonella and campylobacter could cause food-borne illness, the connection with Listeria hadn't been made before, he explained.
The outbreak of disease rocked Maritime communities as people fell sick and the number of deaths mounted.
"Yes, there were certainly headlines in the newspaper ... you know, `Baby dies, infection occurring,"' he said, agreeing that there was much uncertainty and a great deal of heartbreak, "particularly for the people losing babies."
Listeria can cause life-threatening illness in the elderly and those with suppressed immune systems. But it also can infect pregnant women and severely affect the fetus.
"The adult cases, a number of those individuals died," Schlech said. "And of course in the pregnancy-related cases, the mothers survived, but they lost their child, often a late-term infant or sometimes an early miscarriage."
"It was a very sad situation."
Yet some good did emerge from the suffering.
The Canadian outbreak gave disease detectives knowledge they had not had before when it came to subsequent outbreaks elsewhere in the world, including two in Switzerland and California traced to unpasteurized cheeses and one in the New England states linked to unpasteurized milk.
"Unfortunately, some of these outbreaks actually inform the public that there is that danger out there," Schlech said. "We don't want to use that as a normal way of informing the public, but that's the way that it sometimes happens."
The 1981 Maritime cluster also led to changes in how food was handled and processed -- and how those operations were monitored.
"It basically said that instead of this being an organism of unknown etiology, or unknown source, that this in fact was a food-borne organism like any other-- like campylobacter or salmonella -- and therefore we need to start paying attention to manufacturing issues, we need to pay attention to what goes on on the abattoir floor."
Listeria monocytogenes, as it came to be known, multiplies in cool, moist conditions like those found in refrigerated units, unlike most other bacteria that thrive in warm temperatures. Indeed, L. monocytogenes can even survive repeated bouts of freezing and thawing.
A listeriosis outbreak that spans central and western Canada has spawned the recall of 23 Maple Leaf Foods deli meats, although a definitive link to the company's shut-down Toronto plant has not been established as the source of infection.
The same Listeria strain has been found in all 17 cases, including three deaths: 13 in Ontario, two in British Columbia, one in Saskatchewan and one in Quebec.
Schlech said he had a case of listeriosis two weeks ago in the Maritimes, but the patient and his wife did not identify cold cuts as part of their diet during his interview with them. The new cluster is making him rethink that case.
"I'm going to go back to them now that we know about the Maple Leaf association," he said. "I have to go back and see if there's a connection."