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A new Centers for Disease Control and Prevention (CDC) report analyzing outbreaks of foodborne illnesses spanning over several years revealed some surprises.

Ranking at the top of the list for number of illnesses between 2009 and 2015 were chicken, pork — and seeded vegetables with 2,572 illnesses.

Eggs, fruits, and leafy and stem vegetables followed, notably above beef, turkey, dairy, fish, and even mollusks. Much lower on the list, with only 86 and 74 reported illnesses, respectively, were game and crustaceans.

Why are vegetables ranked so high?

While chicken and pork are probably no surprise to most, the high ranking of vegetables — particularly as compared to mollusks and crustaceans — could be more of an unexpected twist for the general public.

Especially considering the increase of plant-based diets.

“Vegetables are grown very close to the ground, some seeded vegetables — like squash — actually vine along the ground,” said Dr. Amy Edwards, assistant professor in the Division of Pediatric Infectious Diseases at University Hospitals Rainbow Babies and Children’s Hospital in Ohio.

“If not cleaned or cooked properly, they can be covered in bacteria,” she said.

Dr. William Schaffner, professor of preventive medicine in the Division of Infectious Diseases at Vanderbilt University in Tennessee, said there are many reasons for contamination by pathogens of our food supply.

First and foremost, he said, we must realize that the origins of our food are not sterile.

“Lettuce grows on the earth, tomatoes come out of the earth,” he said. “Chickens, well look at chickens… they don’t wash their hands after they eat.”

And while chicken can be decontaminated by cooking, vegetables like lettuce present more of a challenge if they carry a large amount of bacteria.

“Lettuce is washed off but it isn’t cooked,” he said. “The bacteria often remain on the lettuce even though you rinse it off.”

He said leafy vegetables like spinach and lettuce can arrive already contaminated from the field or when they’re rinsed with contaminated water.

Add to that the fact that many of these foods come in from outside the country from areas with no real Food and Drug Administration (FDA) oversight — and there’s a much larger potential for food pathogens.

Besides lettuce, cucumbers may also be a source of contaminated vegetables.

“For seeded vegetables, you can think of something like cucumbers,” said Edwards. “Cucumbers have very thin skin and typically grow in a vine along the ground. So, it has the potential of carrying salmonella.”

Schaffner said fruits that have been involved in outbreaks include mangos and cantaloupes.

“Depends on the circumstance,” he said of the cause of such fruit and vegetable outbreaks. “Sometimes wild animals have entered the growing fields and have contaminated the vegetable with their droppings. At times contaminated water has been used for irrigation or for washing the product.”

What’s the deal with shellfish?

While vegetables now suddenly seem less clean than previously thought, shellfish (haven’t we all had a bad clam at some point?) are on the opposite end, according to the CDC ranking.

Schaffner attributes some of this to simple math.

“There’s a lot more spinach consumers than there is clam [consumers],” he said.

But another factor has to do with increased regulation that prohibits collecting shellfish like clams and oysters from polluted waters in the United States.

“Our having done that has really resulted in many fewer outbreaks,” he said.

Other surprising food facts

Another food type that unexpectedly ranks low is game meat — especially as compared to the high-ranking chicken and pork.

Why isn’t wild game a greater potential threat?

“There are a few ways to answer this question,” said Edwards. “The first is that these are in absolute numbers, fewer people eat wild game than eat store-bought chicken and pork, and so of course, the number of people who get sick will be lower.”

But there’s more to it. Wild game wouldn’t necessarily harbor more pathogens than farm-grown meat, Edwards explained.

“Wild game is killed and cleaned by an individual person, one at a time, whereas store-bought meat is done in more of an assembly-line manner,” Edwards said. “If the person cleaning the wild game knows what they are doing, there is less chance for contamination because they only have to focus on that one animal.”

She pointed out that with an assembly line, “if one animal gets through with pathogenic bacteria on it, you have the opportunity to infect other surrounding meat and impact a larger number of people.”

Tips to keeping your food safe to eat

Schaffner offered some advice for preventing foodborne illness:

Pay attention to any alerts about food contamination and follow all suggestions.

And also, with summer picnics in full swing, there’s an increased risk of foodborne illness. Don’t leave food out on a picnic table for hours in the summer heat.

“That’s exactly what the bacteria like and they will multiply in the food,” he said. “Hot or cold, not in-between.”