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Salmonella-tainted cantaloupe is killing Canadians. How scientists track the outbreak

This story is part of CBC Health’s Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven’t subscribed yet, you can do that by clicking here.

At least five people in Canada have died in a salmonella outbreak linked to contaminated cantaloupe that scientists have traced to a rare strain of bacteria. 

Federal health and food officials say Malichita and Rudy brand cantaloupes have been identified as the likely source of the outbreak in Canada. Other potentially contaminated brands have been sold in the U.S.

As of Thursday, at least 129 have been sickened in Canada with Salmonella Soahanina, Sundsvall and Oranienburg illness on top of 230 people in the U.S., which has reported three deaths. The true number of sick people is likely much higher since many recover without seeking medical care.

Both the U.S. Centers for Disease Control and Prevention (CDC) and the Public Health Agency of Canada (PHAC) say they’re concerned about the extent of hospitalizations. Authorities in both countries are trying to prevent anyone else from eating contaminated fruit, such as diced products from the freezer. 

“There’s definitely a strain of salmonella that’s causing severe illness,” said April Hexemer, an epidemiologist and director of outbreak management with PHAC in Guelph, Ont. 

The level of illness severity is higher than expected for salmonella and more could still be reported, Hexemer said.

“I think the cantaloupe were very highly contaminated,” she added.

WATCH | What to do after warning about salmonella in cantaloupes: 

What to do after warning about salmonella in cantaloupes

Dr. Isaac Bogoch, an infectious disease specialist, urged people who have recalled cantaloupes in their home to discard them and get checked if they have symptoms.

When an outbreak like this happens, food inspectors race to track down the culprit. In the case of the contaminated cantaloupe, they were able to trace the salmonella back to produce from a few specific companies by checking invoices from Mexico, the U.S. and Canada. The recall process is crucial for stopping the spread of foodborne illness, health officials said.

The Canadian Food Inspection Agency (CFIA) discovered the contamination at the end of October, Hexemer said, and issued its food recall warnings on Nov. 1. The exact cause of contamination is still under investigation.

‘A very rare strain of salmonella’

Children and people aged 65 and older have faced the majority of illnesses in both countries.

“One possible reason behind the high hospitalization rate is that many sick people in this outbreak are in groups that are at higher risk for severe illness,” Katia Martinez, a health communication specialist at CDC’s division of foodborne, waterborne, and environmental diseases, said in an email.

“At this time, we do not know much about the virulence of the outbreak strains because the strain causing most of the illness in this outbreak is a very rare strain of salmonella in the U.S.”

Cantaloupes are seen for sale at a local Farmers Market in Annandale, Virginia.
The net-like texture of cantaloupe skin presents a perfect place for germs to tuck into tiny crevices. (Paul J. Richards/AFP/Getty)

Rare, too, in Canada.

“It’s never been seen before in B.C. and so it popped out right away,” said Dr. Jennifer Grant, who heads the bacterial and fungal department at the BC Centre for Disease Control in Vancouver.

Until an all clear is provided by public health officials, food science Prof. Keith Warriner at the University of Guelph suggests those who are at high risk such as babies, people with weakened immune systems, pregnant individuals or adults older than 65 consider choosing other fruit to eat.

Male physician headshot without lab coat.
Dr. Mayank Singal is a physician epidemiologist with the BC Centre for Disease Control. (Submitted by Dr. Mayank Singal)

Salmonella can be transmitted to other people if consumers don’t wash their hands properly, although Hexemer said there’s no evidence of that in this investigation so far.

Dr. Isaac Bogoch, an infectious diseases physician at Toronto General Hospital, said if salmonella gets into the bloodstream, it can be dangerous. 

“We need to have better communication in terms of ensuring people know what the affected products are so that they don’t consume them,” Bogoch said.

Officials at the CFIA have encouraged consumers to sign up for food recall notifications. When cantaloupes are under recall, that’s when Warriner suggested avoiding that melon.

Hexemer noted that public health notices were posted online last month and sent directly to daycare and long-term care associations to try to get the word out about the recalls and illnesses. 

Long-term complications of salmonella may include severe arthritis, PHAC said.  

Slicing contamination

As for why cantaloupe is prone to salmonella, Warriner pointed to the surface of the fruit. The net-like texture of cantaloupe skin presents a perfect place for germs to tuck into tiny crevices and resist being washed off, compared with smoother-skinned fruit.

Salmonella bacteria transfer from the skin to the inside flesh when the fruit is cut by a knife.

“If you brought contaminated salmonella into a processing plant, like on a melon or fresh produce even, the risk of transfer of salmonella is fairly high,” Warriner said. That’s one reason why the recall expanded from cantaloupe to fruit platters.

Warriner’s educated guess is manure contaminated the cantaloupe in this outbreak. His hunch is based on the high-profile Jensen Farms listeriosis outbreak in Colorado in 2011, when he said non-composted manure was placed on ground where cantaloupes grew without chlorinated water.

“We import a lot of fruits and vegetables and we’re just hoping that the producing nation has done the right thing, which isn’t always the case,” Warriner said.

In some outbreaks, the source of food contamination is never identified, in part because produce is perishable, and there may not be a sample to test.

When a contaminated food source is identified, such as during the current outbreak, if somebody sees a physician because of salmonella symptoms, the health-care provider can ask: Did you happen to have cantaloupe? 

Salmonella growing a petri dish.
A doctor points out salmonella growing in a petri dish. (Elaine Thompson/The Associated Press)

A foodborne illness like salmonella typically causes symptoms like diarrhea and vomiting.

Given that, teams of scientists compare the genetic sequence of the bacteria from the contaminated cantaloupe to those from stool samples provided by infected patients.

“The smoking gun is when we test the potential [food] sources and find that organism that’s closely genetically related,” to salmonella from the patient, Grant said. 

In the case of the cantaloupe, the two matched exactly.

When the same genetic strain pops up in two or more illnesses, epidemiologists say it’s strong evidence that the patients faced the same source of contamination.

To be thorough, investigators continue to ask those sickened a long list of questions about what they ate, where they travelled and if they were near animals, said Dr. Mayank Singal, a physician epidemiologist at BCCDC.

The outer end of incubation period for this germ is six days. It can take up to four weeks to tell if a sick person is part of an outbreak, Hexemer said.

The last recalled cantaloupe was sold in Canada on Nov. 24. Other brands were sold south of the border.

WATCH | Ways you can protect yourself from salmonella:

Killer cantaloupes: How to protect yourself from salmonella

At least five deaths and dozens of illnesses have been linked to contaminated cantaloupes sold in Canada. Food science expert Keith Warriner breaks down what you need to know about potentially deadly bacteria and how to reduce your risk of getting sick.

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