Public health efforts have rooted out historical sources of food-borne illness, but others, like nontyphoidal salmonella, remain serious. Vigilant reporting combined with high-tech pathogen “fingerprinting” help doctors stay on top of outbreaks, says expert Marguerite Neill.

Marguerite Neill, associate professor of medicine in the Warren Alpert Medical School of Brown and a physician at Memorial Hospital in Pawtucket, is a national expert on food-borne illness. She has advised the U.S. Food and Drug Administration and the Agriculture Department on matters such as nonpasteurized juices and serves on the National Advisory Committee for Microbiolgical Criteria in Foods, which brings together the FDA, USDA, Centers for Disease Control, and the Department of Defense. She answered David Orenstein’s questions about food safety and how public health officials track outbreaks.

Marguerite Neill: When fresh food was produced locally in small amounts, salmonella outbreaks were also small and local. Foods are now distributed nationwide and remain in commerce for months.
Marguerite Neill When fresh food was produced locally in small amounts, salmonella outbreaks were also small and local. Foods are now distributed nationwide and remain in commerce for months.
Overall, have we made much progress in improving the safety of our food supply?

A hundred years ago common food-borne illnesses would have included tuberculosis, because that can be transported through the milk in the cows, hepatitis A because we didn’t have the type of sanitation that we have now, and typhoid, caused by a type of salmonella. So if one looks at those, the enhancements to food safety are one of the singular public health achievements of the 20th century. Now the interesting piece by comparison is that with nontyphoidal salmonellae today we approach the levels that were seen with typhoid. In the January 2011 issue of the CDC’s Emerging Infectious Disease report there is a major paper that updated the newest estimates on numbers of cases of food-borne illness, which pathogens were causing it, hospitalizations, and deaths. You sit up and take notice because the nontyphoidal salmonellae were the most common cause of an illness that would put you in the hospital, and the most common cause of a death from a food-borne illness.

Why does salmonella remain such a prevalent threat?

A current estimate is that there are 9.4 million episodes of food-borne illness per year in the United States, and nearly 1 million of these are due to salmonella. Amongst those illnesses that result in hospitalization, salmonella is the most commonly identified cause. There are several interrelated reasons why salmonella is so common, but they operate at the interface between pathogen exposure and host susceptibility. Compared to a century ago, enormous improvements in sanitation and refrigeration have yielded cleaner water and safer food. So healthy persons today may paradoxically be more susceptible to salmonella because they lack particular exposures that in the past made people relatively immune. Today there is also a sizeable segment of the population that is immunocompromised (cancer, HIV, transplantation, disease-modifying drugs that are immunosuppressive) and more susceptible to salmonella. So this means that for a given exposure, those with increased susceptibility are more likely to become ill.

On the pathogen side, salmonella differ in virulence, with some serotypes more likely to cause illness than others. Some salmonella are highly host-adapted, meaning they readily colonize one animal species (e.g., chickens) but not others. There is also evidence that salmonella can readily adapt to changes in the food-processing environment, and over time become more resistant to dessication as well as environmental or equipment sanitizers. There have also been enormous changes in food-production practices, some of which have probably fostered exposure to unspoiled-but-nonetheless-contaminated food with salmonella. In the early 1900s, most of Providence’s food came from local farms and was produced in small amounts. If some of it had salmonella, it made only a small number of persons sick, all of whom would be in the immediate area. Today, food processing is done on an industrial scale with a very large geographic distribution. Foods can be distributed over the entire United States, and remain in commerce for months if they have an appreciable shelf life. A food may have a very low contamination rate with salmonella but still cause a low level, geographically dispersed outbreak that affects many persons.

So how do we track these more widespread outbreaks?

The public health system is largely state-based. The state is the unit of measurement for incidence, such as the number of cases of salmonella within a state, within a year. The definition of outbreak is something that is occurring with a frequency that is greater in both time and space than your background rate. So if you said, “Do you normally have 60 cases of nontyphoidal salmonella right around March 20 in Rhode Island?” The answer is no. That’s higher than our background. We also knew that some victims were at a common gathering in a nursing home. We don’t expect several persons in that gathering to develop diarrhea.

Clinical reports come through the infection control practitioners at hospitals. A doctor who saw a family of five who had all eaten at a local restaurant and saw that they were all ill with something, whether it’s hepatitis or diarrhea, can certainly report illness, even if the doctor doesn’t know what it’s due to.

That’s the way it worked until probably about the ’80s into the ’90s.

How has it changed since then?

In the last couple of decades we’ve added the laboratory capability to do “fingerprinting.” These techniques allow the characterization of specific bacteria. Salmonella is one. And after the initial development of this technique, which is called BFGE, it was also then adapted into something that could be digitized. So now it goes beyond just a person in the laboratory who was doing this test comparing the results between two different strains and asking, “Are these the same or do I think they are different?” The digital image can be uploaded into a large database. And this has afforded the opportunity to compare those images using a computer algorithm. Labs upload the images and go about their work, and then receive an e-mail that says the image you just loaded matches another image that was uploaded from Kentucky. This system is called Pulsenet. It has given us the ability to detect a lower frequency and a more geographically dispersed outbreak.

What can people do to reduce their risk?

Make sure you practice adequate refrigeration, thorough cooking, and hand washing, and keep raw food separate from the cooked so you don’t cross-contaminate.