Foodborne Illness and Staying Safe (part 3) - How to Prevent Foodborne Illness Caused by Parasites

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2. How to Prevent Foodborne Illness Caused by Parasites

Not long ago, I overheard the fishmonger at one of my local grocery stores (which shall remain nameless to protect the guilty) tell a customer that it was okay to use the salmon he was selling for making sushi. Given that the fish wasn’t labeled as “previously frozen” and that it was in direct contact with other fish in the case, there wasn’t any real guarantee that it was free from harmful parasites or bacteria, two of the biggest concerns that consumers need to manage for food safety. What’s a shopper to do in response to the disappearance of the true fishmonger?

For one, start by understanding where the risks actually are. Not all fish and meats share the same set of risks for foodborne pathogens. Salmonella, for example, tends to show up in land animals and improperly handled vegetables, while bacteria such as Vibrio vulnificus show up in fish that are exposed to the brackish waters of tidal estuaries, such as salmon. Deep-water fish, such as tuna, are of less concern. Because of these differences, you should consider the source of your ingredients when thinking about food safety, focusing on the issues that are present in the particular food at hand.

With uncooked and undercooked fish, one concern is parasites. Parasites are to fish as bugs are to veggies: really common (if you’ve eaten fish, you’ve eaten worms). But on the plus side, most parasites in seafood don’t infect humans. However, there are those that do, Anisakis simplex and tapeworms (cestodes) being the two parasites of general concern. A. simplex will give you abdominal pains, will possibly cause you to vomit and generally feel like crap, and will possibly take your doc a while to figure out. It’s not appendicitis, Crohn’s disease, nor a gastric ulcer, and with only around 10 cases diagnosed per year in the United States, chances are your doc won’t have encountered it before. On the plus side, humans are a dead-end host for A. simplex. The bacteria will die after about 10 days, at which point you’ll go back to feeling normal. (Unless you have an extreme infection, in which case, it’s off to surgery to remove ’em.) That leaves tapeworms as the major parasitic concern in fish.

For cooked dishes—internal temperature of 140°F / 60°C—there is little risk from these parasites directly. Cooking the fish also cooks the parasite, and while the thought of eating a worm might be unappetizing, if it’s dead there’s little to worry about other than the mental factor. (Just think of it as extra protein.)

Of course, raw and undercooked seafood is another matter entirely. Cod, halibut, salmon? Fish cooked rare or medium rare? Ceviche, sashimi, cold-smoked fish? All potential hosts for roundworm, tapeworms, and flukes. Fortunately, like most animals, few parasites can survive freezing.


Some parasites do survive freezing. Trichomonas—parasitic microorganisms that infect vertebrates—can survive temperatures as cold as liquid nitrogen. Yikes!

For the FDA to consider raw or undercooked fish safe to eat, it must be frozen for a period of time to kill any parasites that might be present:

FDA 2005 Food Code, Section 3-402.11: “[B]efore service or sale in ready-to-eat form, raw, raw-marinated, partially cooked, or marinated-partially cooked fish shall be: (1) Frozen and stored at a temperature of –20°C (–4°F) or below for a minimum of 168 hours (7 days) in a freezer; [or] (2) Frozen at –35°C (–31°F) or below until solid and stored at –35°C (–31°F) or below for a minimum of 15 hours...”

The second concern with undercooked fish is bacteria. While freezing kills parasites, it does not kill bacteria; it just puts them “on ice.” Researchers store bacterial samples at –94°F / –70°C to preserve them for future study, so even super-chilling food does not destroy bacteria. Luckily, most bacteria in fish can be traced to surface contamination due to improper handling—that is, cross-contamination from surfaces previously exposed to contaminated items.


Don’t put cooked fish or meat on the same plate as the raw food! In addition to being potentially dangerous, that’s just gross.

If your grocery store sells both raw and “sashimi-grade” fish, the difference between the two will be in the handling and care related to the chances of surface contamination, and in most cases the sashimi-grade fish should have been previously frozen. The FDA doesn’t actually define what “sashimi grade” or “sushi grade” means, but it does explicitly state that fish not intended to be completely cooked before serving must be frozen before being served.

If you don’t have access to a good fish market or find the frozen fish available at your local grocery store unappealing, and you plan on serving undercooked fish, you can kill any parasites present in the fish by freezing: check that your freezer is at least as cold as –4°F / –20°C, and follow the FDA rule of keeping the fish frozen for a week. If you happen to have a supply of liquid nitrogen around—you know, just by chance—you can also flash-freeze the fish, which should result in better texture and cut the hold time down to less than a day.

Luckily for oyster lovers, the FDA excludes molluscan shellfish, as well as some types of tuna and some farm-raised fish (those that are fed only food pellets that wouldn’t contain live parasites) from the freezing requirement.

Doug Powell on Food Safety

Is there a tension between safety and quality in cooking, and if so, are there methods to achieve both?

Safety and quality are two very different things. Quality is something that people love talking about, whether it’s wine, or organic food, or how it was grown, and people can talk themselves to death about all that. My job is to make sure they don’t barf.

For somebody cooking at home, it’s easy for them to see a difference in quality. It’s very hard for them to see a difference in safety until they get sick, I imagine?

There are tremendous nutritional benefits to having a year-round supply of fresh fruits and vegetables. At the same time, the diet rich in fruits and vegetables is the leading cause of food-borne illness in North America because they’re fresh, and anything that touches them has the potential to contaminate. So how do you balance the potential for risk against the potential benefits? Be aware of the risks and put in place safety programs, beginning on the farm.

If you look at cancer trends in the 1920s, the most predominant cancers were stomach cancers. All everyone ate during the winter were pickles and vinegar and salt. Now that’s almost completely eradicated because of fresh food. But now you have to prevent contamination from the farm to the kitchen, because more food is eaten fresh. There are tradeoffs in all of these things. In preparing hamburger and chicken, there is an issue with cooking it thoroughly and validating that with a thermometer, but most of the risk is actually associated with cross-contamination. Potatoes are grown in dirt, and birds crap all over them, and bird crap is loaded with salmonella and campylobacter. When you bring a potato into a kitchen or a food service operation, it’s just loaded with bacteria that get all over the place.

What’s the normal time between ingestion and symptoms?

It’s around one to two days for salmonella and E. coli. For things like listeria, it can be up to two months. Hepatitis A is a month. You probably can’t remember what you had yesterday or the day before, so how can you remember what you ate a month ago? The fact that any outbreak actually gets tracked to the source I find miraculous. In the past, if a hundred people went to a wedding or a funeral, they all had the same meal. They all showed up at emergency two days later, and they would have a common menu that investigators would look at to piece it together. Nowadays, through DNA fingerprinting, it’s easier. If a person in Tennessee and a person in Michigan and a person in New York have gotten sick from something, they take samples and check against DNA fingerprints. There are computers working 24/7 along with humans looking to make these matches. And they can say these people from all across the country, they actually have the same bug, so they ate the same food.

Think of spinach contamination in 2006. There were 200 people sick, but it was all across the country. How did they put those together? Because they had the same DNA fingerprint and they were able to find the same DNA fingerprint in E. coli in a bag of spinach from someone’s kitchen. Then they were able to find the same DNA fingerprint from a cow next to the spinach farm. It was one of the best cases with the most conclusive evidence. Normally, you don’t have that much evidence.

What to do about it isn’t very clear-cut, but when you look at most outbreaks, they’re usually not acts of God. They’re usually such gross violations of sanitation that you wonder why people didn’t get sicker earlier. With a lot of fresh produce outbreaks, the irrigation water has either human or animal waste in it, and they’re using that water to grow crops. These bugs exist naturally. We can take some regulatory precautions, but what are we going to do, kill all the birds? But we can minimize the impact.

When farmers harvest crops, they can wash them in a chlorinated water system that will reduce the bacterial loads. We know that cows and pigs and other animals carry these bacteria and they’re going to get contaminated during slaughter. So we take other steps to reduce the risk as much as possible, because by the time you get it home and go to make those hamburgers, we know you’re going to make mistakes. I’ve got a PhD, and I’m going to make mistakes. I want the number of bacteria as low as possible so that I don’t make my one-year-old sick.

Is there a particular count of bacteria that is required to overwhelm the system?

It depends on the microorganism. With something like salmonella or campylobacter, we don’t know the proper dose response curves. We work backward when there is an outbreak. If it’s something like a frozen food, where they might have a good sample because it’s in someone’s freezer, we can find out more. With something like salmonella or campylobacter, it looks like you need a million cells to trigger an infection. With something like E. coli O157, you need about five.

You have to take into account the lethality of the bug. For 10% of the victims, E. coli O157 is going to blow out their kidneys and some are going to die. With listeria, 30% are going to die. Salmonella and campylobacter tend not to kill, but it’s not fun. So all of these things factor into it. A pregnant woman is 20 times more susceptible to listeria. That’s why they are warned to not eat deli meat, smoked salmon, and refrigerated, ready-to-eat foods. Listeria grows in the refrigerator and they’re 20 times more susceptible and it can kill their babies. Most people don’t know that either.

Are there any particular major messages that you would want to get to consumers about food safety?

It’s no different than anything else, like drunk driving or whatever other campaign: be careful. The main message about food in our culture today is dominated by food pornography. Turn on the TV and there are endless cooking shows, and all these people going on about all these foods. None of that has anything to do with safety. You go to the supermarket today, you can buy 40 different kinds of milk and 100 different kinds of vegetables grown in different ways, but none of it says it’s E. coli–free. Retailers are very reluctant to market on food safety, because then people will think, “Oh my god, all food is dangerous!” All they have to do is read a newspaper, and they’ll know that food is dangerous.

A lot of the guidelines I see talk about the danger zone of 40–140°F / 4–60°C.

A lot of those guidelines are just complete nonsense. The danger zone is nice and it’s important not to leave food in the danger zone, but at the same time it doesn’t really get into any details. People learn by telling stories. Just telling people “Don’t do this with your food” doesn’t work; they say, “Yeah, okay, why?” I can tell you lots of stories of why or why not. The guidelines aren’t changing what people do and that’s why we do research on human behavior, how to actually get people to do what they’re supposed to do. As Jon Stewart said in 2002, if you think those signs in the bathrooms (“Employees must wash hands”) are keeping the piss out of your food, you’re wrong! What we want to do is come up with signs that work.

I’m wondering what your signs look like?

We have some good ones! Our favorite picture is the skull in the bed of lettuce! The dead person from carrot juice is pretty good, too.

A Final Note on Food Safety

The safest way of preventing bacterial and parasitic infections from seafood and meats is with proper cooking. The USDA recommends cooking fish to a minimum internal temperature of 145°F / 63°C, ground beef to a minimum internal temperature of 160°F / 71°C, and poultry to 165°F / 74°C.

If you enjoy your fish cooked only to a rare point or even raw in the middle and you’re concerned about parasites, give frozen fish a chance. I’ve found distinct differences in the quality of frozen fish. Some stores sell frozen product that’s downright bad—mushy, bland, uninspiring—but this isn’t because the fish was frozen. Some of the best sushi chefs in Japan are finding that quick-frozen tuna is exceptionally good. Frozen at sea right after it’s caught (in a slurry of liquid nitrogen and dry ice), the tuna doesn’t have much time to break down and so maintains its quality during transportation.

One last comment on keeping yourself safe in the kitchen: the biggest issue isn’t contaminated food from the store, but cross-contamination while preparing it at home. Avoid cross-contamination by washing your hands often, especially both before and after working with raw meat. Use hot water and soap, and wash for a good 20 seconds.

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