Elementary? Not Quite! Cracking the Case of a Salmonella Outbreak

Published - Written by John Staples

“You know my methods, Watson. There was not one of them which I did not apply to the inquiry. And it ended by my discovering traces, but very different ones from those which I had expected.”

– Sherlock Holmes in Sir Arthur Conan Doyle’s “The Crooked Man” (1893)

Like Sherlock Holmes and his arch-rival Moriarty, public health sleuths seem forever locked in a deadly contest of wits with a formidable foe: Salmonella. George Soper’s early tracking of Typhoid Mary is the stuff of public health legend. More contemporary non-typhoidal contamination of things like ice cream, marijuana, and hundreds of millions of eggs has caused spikes of illness and even death, prompting massive recalls, public warnings, and Congressional hearings.  Although modern investigations would be more readily recognizable to CSI: Miami’s Lieutenant Horatio Caine than to Conan Doyle’s hero of 221B Baker Street, both would agree that the game is still very much afoot.

Another installment of this gripping serial is provided in this week’s NEJM: Dr. Casey Barton Behravesh and colleagues from the Centers for Disease Control and Prevention (CDC) describe their investigation into an outbreak of diarrheal illness caused by Salmonella enterica, serotype Saintpaul. The case began back in May 2008, when officials in New Mexico alerted the CDC that they’d been informed about approximately 19 cases of Salmonella enterica diarrhea. It was an outbreak, and the chase was on.

Officials in Texas and New Mexico began by generating hypotheses about potential contaminated food sources. Affected individuals were interviewed about the foods they’d eaten in the week prior to illness onset. Suspect foods were those reported to be consumed by more than 50% of case subjects: Raw tomatoes, eggs, ice cream, potatoes, milk, tortillas, cold breakfast cereal, raw onions, salsa, ground beef, chicken, and lettuce (with avocado and guacamole added to the list as known previous offenders). These suspect foods were then evaluated in an expanding number of states by subject- and household-level matched case-control studies. Restaurant and event clusters were investigated by similar means, using unaffected meal companions or unrelated restaurant patrons identified by credit-card receipts as control subjects.

Iterations of this shoe-leather epidemiology identified a number of significant risk factors for infection: Eating at a Mexican-style restaurant, having jalapeño and serrano peppers in the home, and consuming raw tomatoes, pico de gallo, tortillas, guacamole, garnish, or salsa. The Food and Drug Administration (FDA) then did environmental restaurant assessments and distribution network traceback investigations for tomatoes and hot peppers. The tracebacks converged on two farms in Mexico.  On “Farm B,” the outbreak strain was isolated from agricultural water, two environmental samples, and from some of the foodstuffs ultimately surmised to be guilty: Serrano and jalapeño peppers. Tracebacks and environmental swabs suggested that the early lead on tomatoes was probably a red herring, their only guilt being the company they often keep on dinner tables across the Southwest.

“Foodborne illness is a remarkably important topic, both scientifically and from the perspective of public policy,” says NEJM deputy editor and infectious disease specialist Dr. Lindsey Baden, “When the safety of our food breaks down, any one of us might be at risk.” Indeed, by the time the outbreak had subsided, it had grown to include 1500 identified case subjects in 43 states – a number that almost certainly vastly underestimates the true number affected individuals.

As published recently in NEJM, ongoing developments in food safety regulation (through the as-yet unfunded Food Safety Modernization Act), potential changes to the food production process, and innovations in the molecular tracking of isolates during an outbreak all offer some hope that food-borne disease epidemics might be prevented, tracked, and contained more effectively in future. Until salmonella outbreaks are eliminated, though, public health officials are likely to agree with the ever-astute Mr. Holmes: “There is no branch of detective science which is so important … as the art of tracing footsteps.¹”

¹Final quote from Sir Arthur Conan Doyle’s “A Study in Scarlet (1887).”