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Science Behind Cantaloupe ‘Alert’

Jim Prevor’s Perishable Pundit, March 28, 2008

When the FDA was unable to answer many questions regarding the science that surrounds this “import alert” and the general issue of salmonella and cantaloupes, it was suggested we talk to Professor Michael Doyle who has done some research in this area. We asked Pundit Investigator and Special Projects Editor Mira Slott to help us learn more:

Michael Doyle Dr. Michael Doyle
Regents Professor of Food Microbiology
Director Center for Food Safety
Dept. of Food Science & Technology
University of Georgia

Q: Could you shed light on the science of salmonella contamination with cantaloupes?

A: We know melons in general and cantaloupe particularly are a highly nutritious medium for growing bacteria if it gets to the meat. The pH in melons is closer to neutral than many fruits, so there are not a lot of microbial barriers to prevent pathogens from growing. E. coli and Salmonella will grow very well in cantaloupe if left at room temperature. The other problem is that bacteria can get trapped in the grooves and crevices of the uneven surface of the cantaloupe and is difficult to remove.

Q: Could you point to studies that have analyzed the impact of foodborne contamination in cantaloupes?

A: Scientists at Texas A&M conducted studies looking at the irrigation water used for cantaloupes grown in Mexico and the U.S. near the Rio Grande River and found substantial contamination in the water. One particular serotype of Salmonella has been a nemesis for the cantaloupe industry. A report in the Journal of Food Protection a few years ago detected Salmonella Poona in irrigation water used for a cantaloupe farm in Mexico implicated in Salmonella cases in the past. Irrigation water is certainly a possibility for the source of the recent cantaloupe outbreak.

Q: What is your take on this cantaloupe outbreak, which the FDA linked back to a Honduras grower?

A: I know the government of Honduras and the grower are questioning the FDA’s actions and I read something about a lawsuit.

My experience with CDC is that they are adept at identifying these outbreaks with their epidemiological data. Depending on what the risk factor is, if it’s above two or three, usually that’s pretty strong statistical evidence that whatever they’ve identified as the source is right. If the odds ratio is above three, it’s a strong ratio.

Based on my past associations with CDC outbreak investigations, there’s often a cry by those identified as the source that they haven’t isolated the agent from their product. The reality is that the epidemiology being used by FDA to advise recalls is valid. I’ve been around this long enough to know that the epidemiological data is very compelling.

Q: Another issue being raised is the timing of the FDA import alert and advisory recall. FDA says the outbreak started beginning of January and the last case was reported on March 5, with the cases following a bell-shaped curve, yet the FDA didn’t issue its alert until March 22. Based on incubation periods for Salmonella, wouldn’t the danger be over by then?

A: The incubation period for Salmonellas is usually in the range of 12 hours to several days, some suggest a week. But who knows where cantaloupe could be; my wife sometimes freezes it. Most perishable product outbreaks have this bell shape curve. Perishables in general are consumed quickly, not like peanut butter, which is stable over several years.

With cantaloupe, it’s difficult to disinfect if its gets in the grooves and crevices of the irregular surfaces. Another point needs to be considered. The FDA wants to find out what caused the contamination, and it needs to do an investigation not just of the field, but the processing plant and other potential problem areas to make sure good agricultural practices are being applied.

Think back in the late 90’s and the problems with raspberries grown in Guatemala; you had to cut imports off. They did that with Mexican cantaloupe some time ago too. We can’t sell more until we determine the problem. It’s prudent of the FDA to do this because we don’t know if the grower or processor has solved the problem. If there’s a reoccurring problem, the FDA would be found at fault if more sicknesses occurred. The most prudent thing to do is stop imports of this product to see if an investigation can determine where the problem originated.

Professor Doyle is respected for his work. He is, however, extremely risk-averse when it comes to food. For example, USA Today interviewed him in the midst of the spinach crisis of 2006 in a piece mostly about Fresh Express. Professor Doyle’s position on personally consuming packaged salads was explained this way:

Doyle won’t eat packaged salads. He says the process, including the mixing of leafy greens inside a processing plant, increases risk of contamination.

Now, of course, everyone makes choices in life and far be it from us to tell the good professor what to eat and what not to. However, it is very clear that few consumers are so risk-averse as to accept this kind of restriction on their own activities.

The Professor’s explanation of the science of salmonella and cantaloupe is intriguing. But on the points specific to this import alert, they still leave many questions unanswered:

1. It is one thing to say that CDC has competent epidemiologists. It is another thing entirely to make a judgment about this specific case. Professor Doyle gives some specific criteria: “Depending on what the risk factor is, if it’s above two or three, usually that’s pretty strong statistical evidence that whatever they’ve identified as the source is right. If the odds ratio is above three it’s a strong ratio.” The CDC, however, has released no information to allow us to check against Professor Doyle’s criteria.

2. Professor Doyle does make the important point that it is unreasonable to expect to find the food item with DNA on it that proves the tie to the food safety outbreak. Although this may be the gold standard, in the vast majority of cases, it is an unreasonable expectation. In the spinach outbreak of 2006, we were able to get this “hard evidence” because bags of spinach are consumed in servings and held — so they were available to be tested. Most bulk produce is eaten and disposed of long before anyone is thinking about testing it.

The industry may not like this but circumstantial evidence is still evidence and people go to jail every day based on it. Just as we don’t need a smoking gun to convict a person for murder, we also don’t need physical evidence to determine the cause of a food safety outbreak.

This being said, it must be recognized that the CDC and FDA can make errors. Some people might have ulterior motives, and when we act without hard evidence, a high degree of transparency is essential to prevent the abuse of process or a mistake in the process.

3. The suggestion that there was a public health risk because “…who knows where cantaloupe could be; my wife sometimes freezes it” is not a justification for such broad action. If the concern is that some infinitesimal number of consumers have frozen cantaloupe in their freezers or have made cantaloupe marmalade, a notice advising consumers to dispose of any products made from cantaloupes imported in January and February would have sufficed.

4. Whatever happened in the past with other countries, there is no reason whatsoever why shipments have to be banned and product thrown in the garbage before we start an investigation. In this case where nobody has gotten sick for weeks, we could have called up this company and asked for permission to send an FDA inspector. They would have cooperated. If not, maybe that would be a reason to consider a ban.

5. The notion that “We can’t sell more until we determine the problem” is simply not a reasonable standard. Even assuming there was a problem with this company, there is a high likelihood we will never know the cause of the problem. How can we? We will send some people to walk around fields that, maybe, had some salmonella back in January. If you pay people to find things they may speculate on possible causes but even with the FDA, USDA, CDFA and others completely focused in a no-expense-was-too-great search for the cause of the spinach crisis, no cause was ever found. We have a bunch of speculation dressed up in a fancy report.

When Dr. Doyle says that “The most prudent thing to do is stop imports of this product to see if an investigation can determine where the problem originated,” what he means is this: If the only thing that matters in the whole world is food safety, then anytime there is the whisper of a problem, cut off the product until we persuade ourselves we know what caused the problem and have corrected it.

And actually, we would agree with Professor Doyle, except we would also hold that such circumstances — that food safety is the only thing that matters in the whole world — never exist. Reasonable men deal with probabilities, not possibilities; reasonable men balance competing interests such as the benefits of eating fresh fruit and the importance of predictability in trade.

The bottom line here is that the whole thing was announced after the outbreak was over. Such small hangovers as professor Doyle’s wife’s frozen cantaloupe could have been handled in a far more constrained manner. This is an enormous loss to a poor country and for absolutely zero gain in food safety.

Many thanks to Professor Doyle for helping enlighten the industry with regard to food safety.

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