By Craig W. Hedberg, PhD
University of Minnesota
See related news article, "CDC suspects turkey meat in listeriosis outbreak, now at 43 cases," and follow-up commentary, "Listeria's unique challenge calls for unique response," published Nov 27, 2002
Oct 9, 2002 – On August 30, 2002 (day 0), the Pennsylvania Department of Health announced that it was investigating an apparent outbreak of listeriosis involving 20 cases. The department reported that epidemiologic and laboratory investigations were ongoing, and that specimens were being collected and forwarded to the Centers for Disease Control and Prevention (CDC) for molecular subtyping. This was a good response. CDC's development of a national molecular subtype surveillance system, called PulseNet, has been a flagship investment in our public health infrastructure during the past decade.
This has been followed by a series of press releases from federal agencies updating the progress of the investigation:
- September 13 (day 14): Recent increase in cases in Pennsylvania and monitoring in adjacent states. FDA issued an advisory and noted that the specific food had not been identified. Consumers were given general warnings about Listeria.
- September 25 (day 26): 31 cases with 5 deaths in 5 states. CDC noted that federal and state public health officials were working together to determine the food that caused the illnesses.
- October 2 (day 33): 36 cases with 6 deaths in 8 states. USDA announced that in cooperation with CDC and state health officials, it was continuing its aggressive investigation into the origins of the outbreak.
- October 8 (day 39): 43 cases with 7 deaths in 7 states. CDC blames sliced deli-style turkey meat, but cannot identify the specific source.
How can this be? Five separate press releases assured us that the outbreak was being aggressively pursued with a high degree of coordination among federal and state agencies. After a month of high-profile investigation, how can we be satisfied with "sliced turkey deli meat" as a vehicle? What is wrong here?
Several things are wrong. First, the public health system is tremendously fragmented. CDC cannot control what state health departments do, and most state health departments cannot control what local public health agencies do. The result is that multistate outbreaks are usually investigated independently by local or state health departments, often (as in this case) with coordination by CDC. However, central coordination does not address the frequent lack of resources or expertise at the state or local level to conduct the investigation.
Second, molecular subtyping has increased our ability to detect outbreaks (by linking apparently unrelated cases) and our ability to investigate outbreaks (by excluding cases that are not part of the outbreak). However, in addition to having a specific case definition, it is necessary to collect detailed exposure information to identify the source of the outbreak. In a foodborne outbreak this means that brand name and location-of-purchase information needs to be collected for a wide range of food items from all subjects, as quickly as possible.
In many investigations, detailed source information is collected only for items that appear to be associated with illness. This is a mistake: It delays the investigation and allows investigators to miss important exposures caused by common food items. When Schwan's ice cream was implicated as the cause of a large, multistate outbreak of salmonellosis in 1994, it wasn't because of the skill or insight of the investigators, it was because detailed brand and source information was collected on many food items, including ice cream. With this information the vehicle was identified in a couple of days. Without it the vehicle would never have been identified.
In 1998, it took over a month to identify the source of a multistate outbreak of listeriosis that was ultimately traced to a Michigan meat-processing plant operated by Sara Lee. Perhaps a month is just how long it takes to find the source of Listeria. I would like to believe we can do better than that.
As we think about our public health system and its preparedness to respond to intentional acts of food contamination, I am certain we need to do better.
Dr Hedberg is an associate professor in the Division of Environmental and Occupational Health, School of Public Health, University of Minnesota, Minneapolis.