Salmonella are bacteria that can live in the intestinal tracts of humans and other animals. Shown here is a color-enhanced and magnified view of Salmonella typhimurium invading cultured human cells. Photo: Rocky Mountain Laboratories, NIAID, NIH.
How many food-borne illness outbreaks are there in the United States every year?
Tauxe: We get reports from city, county, and state health departments that investigate food-borne outbreaks, and they report about 1,300 outbreaks per year. Each of those outbreaks affects groups of people, who all get sick at the same time from the same infecting organism.
A food-borne illness, commonly referred to as food poisoning, results from eating contaminated food. This type of illness may arise from:
- Food infection, when bacteria or other microbes present in a food infect people who have consumed the food. Escherichia coli, Streptococcus, and Salmonella are a few examples of bacterial pathogens that can cause such illnesses.
- Food intoxication, when people consume food that contains toxins, such as exotoxins. Clostridium botulinum, for example, produces botulism, a potentially deadly toxin found in spoiled canned food, that causes the paralytic disease botulism.
Most illnesses, though, occur from infection rather than from the chemical or natural toxins in foods. Salmonella is the most common bacterial cause of food-borne disease in the United States:
- It causes 1.4 million illnesses and 400 deaths annually.
- The bacteria may come from meat, poultry, produce, or reptile contact.
- There are 2,500 serotypes (closely-related microorganisms) of Salmonella enterica.
Has the pattern of outbreaks changed in recent years?
Tauxe: Using our current system, outbreaks have been constant over the last 10 years. Although, it seems that outbreaks have a wider geographic reach in recent years, which accounts for a larger portion of the total number of outbreaks. Our food supply is more centralized than it used to be; that is, a farm in California, or a factory in Georgia, might be making food that gets sent all over the country; therefore, if something goes wrong, it can cause illness all over the country.
We may be more aware of these multistate outbreaks—where people in a number of different states are infected with contaminated food at the same time—because our surveillance systems have improved. For example, the CDC is now using a DNA fingerprinting technology in all the state health departments that helps us identify these illnesses as they spread across the country. In addition to better tracking, we are also changing the way we monitor food production and where food comes from.
How does the CDC track a disease that all of the sudden shows up in many states?
Tauxe: We actually track infections in several ways. We call this disease surveillance, and all U.S. states participate in the program.1 There is an agreement between CDC and the states about which diseases to track. For instance, we track a number of infections that are often transmitted through foods—like Salmonella, Escherichia coli (E-coli) and Listeria. These diseases are notifiable, which means when a doctor diagnoses them—that is, when a patient or a laboratory finds this germ in a sample from a patient—they are obligated to report that to their local health department. These reports from doctors and laboratories are the primary way we track many illnesses that come potentially from contaminated food.
In addition, for some of these diseases, we now have another layer of surveillance that is based on the DNA fingerprint.2 Bacteria have DNA and viruses have DNA or RNA. By looking at elements on the microbe’s DNA, we can acquire a fingerprint, which allows us to identify the source of microbe. The network for DNA fingerprinting is new; we started it in 1996. We got full national participation for the Salmonella and E-coli pathogens in 2001—for all 50 U.S. states. Now CDC is adding other pathogens to the list for DNA fingerprinting and setting up multistate networking to monitor them.
With newer technologies at your disposal, have you gained new insights into food-borne illnesses?
Tauxe: Yes, we certainly have found new issues with these illnesses. When we set this system up, we were not sure what we were going to find. It was a little bit like sending up NASA’s Hubble Space Telescope and opening a new window in space. When the system was up and running, however, we started to find that there were outbreaks of infection spread across the country that we had not seen before. When we began to look at the foods involved, we often found that the infections were being caused by foods we did not know were at risk for contamination.
For example, in 2007, there was a big outbreak related to peanut butter, and we had another similar outbreak in early 2009. Peanut butter had never been recognized as a problem before. Everyone assumed peanut butter was safe because the peanuts are roasted and ground up to make the peanut butter. It seemed like a safe manufacturing process.
These two outbreaks, however, proved there are ways that peanut butter can become contaminated during manufacturing, which means there is a whole new layer of attention required. Actually, more research needs to be done as well. It turns out, that Salmonella in peanut butter is harder to kill because it can survive higher temperatures than Salmonella in milk, water, or meat. This discovery means that there is something new about Salmonella that we did not know—even though Salmonella has been studied for over a hundred years.
Why are foods that are not usually connected to illness, like leafy greens, now causing outbreaks, too?
Tauxe: We ask ourselves the same question. We are used to thinking that foods derived straight from animals, such as raw meat, raw eggs, and raw milk, are more likely to be contaminated with animal bacteria. This makes sense, and almost everybody seems to understand this possibility, so we cook our meat; we pasteurize our milk; and we pasteurize and/or cook our eggs.
Fresh produce, however, was not traditionally part of the larger concern. We are now seeing outbreaks related to produce, such as tomatoes, melons, jalapeño peppers, lettuce, and spinach. It is not entirely clear what is occurring here; however, one noticeable difference between these foods and food from animals is that production methods on produce farms are changing a lot. The lettuce or spinach field is not the same as it was 25 years ago. The product for consumption used to be a mature plant; somebody would come along with a knife and harvest it one-by-one, by hand. Then it would be taken to another facility for washing. The harvesting process did not involve a lot of manipulation—it was a simple method.
Today, in some of these instances, the product is a three-inch tall baby plant that is harvested by a giant mowing machine. The person driving is at the back end and she cannot really see where she is going. This procedure is very efficient and very inexpensive. However, the machine picks up not only the plant but also anything else that is in the soil—sticks, stones, and anything else. As a consequence, all these ancillary chunks are added into the mix. Nobody actually reviews what is being scooped up in this process until much later, when contamination has alreadty occurred. Mechanized harvest, therefore, may add to the chances of contamination for some products.
Another factor is that harvest workers are out in the field doing much more with the produce than they used to. Take lettuce farming as an example. Previously, harvesting consisted of picking the lettuce from the field, putting the lettuce in a bin, and sending it off to be processed somewhere else. Now, however, the person in the field can take a specialized knife and actually core out the heart of the lettuce, peel off some of the outer leaves, and actually begin to process the lettuce in the field—where there is nowhere to wash or otherwise sanitize your hands. This is a newer process, and it helps keep the costs down; nevertheless, it may not be such a good idea for public health reasons.
Do we have systems in place in the United States to prevent outbreaks?
Layers of partners in the fight against food-borne illness. Chart: Robert Tauxe, CDC slide presentation.
Tauxe: There are a lot of partners in food safety—including the farmers and the workers in the fields. They may not know that they are partners, but they are. I think that there are a number of good and common sense practices that they can follow, called Good Agricultural Practices (GAPs). Because fresh produce has not been thought of as a large source of infection, there has not been as much thought given to what those GAPs are for fresh produce, but GAPs are constantly being revised, reviewed, and updated. I think there are ways of producing food in ways to minimize infection; but, we must remember that food sources are living organisms. They are plants growing in the sun, visited by birds and insects, and touched by human hands. They are never going to be completely germ free.
Using common sense, however, can make contamination less likely. For example, farmers should not allow wild pigs to gallop through the produce fields. They need to keep fresh manure away from the plants or produce. I think the more we learn about the ecology of an agricultural area, there will be some relatively simple and straightforward things that farmers can implement that will make this less likely to occur.
Can your surveillance system also help identify whether our food supplies have been contaminated by pathogens accidentally or by someone with ill intentions?
Tauxe: Could we tell if someone wanted to make a group of people sick, by putting something into our food? I am not certain we can. What we would see, perhaps, is that a bunch of people might become ill, or a food might be contaminated with a particular strain, but I do not think we would know why. Epidemiology can tell us something about when and where the contamination occurred, but not about motive. That takes a forensic approach If the particular infecting strain is bizarre or is not typically found in that kind of food, it might raise question about whther it was intentional. This scenario would raise suspicion, and we might wonder: could that have happened naturally, or does it mean somebody must have put it there? Whatever the circumstances of the outbreak, we use good old-fashioned disease detective work. We call it “Shoe Leather Epidemiology”—we go out and investigate the outbreaks to find the underlying cause of the problem.
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