A new report on Listeria monocytogenes infections underscores the groups at greatest risk—older people and pregnant women—and sheds light on why progress has stalled in the battle against one of the deadliest foodborne diseases, federal officials said yesterday.
The new information is from an analysis of infections from 2009 through 2011 that appeared yesterday in a Vital Signs report from the US Centers for Disease Control and Prevention (CDC) published in Morbidity and Mortality Weekly Report (MMWR). It covered 1,651 listeriosis cases that occurred during that time, including 224 that were part of 12 outbreaks affecting 38 states.
Better understanding of the disease and efforts to reduce contamination in foods such as hot dogs and retail deli meat helped decrease the incidence of Listeria infections by 24% from 1996 through 2001, but investigation of the latest outbreaks found other food sources, such as soft cheeses and fresh produce, and food safety gaps, according to the CDC's report.
At a media briefing yesterday, CDC Director Tom Frieden, MD, MPH, said the report's key findings are that people age 65 and older are four times more likely than the general population to get sick from Listeria and that pregnant women are 10 times more likely to be infected. The risk for pregnant Hispanic women is even more pronounced—24 times more likely than the general public.
"The bottom line is that progress reducing rates of Listeria infection has stalled. Rates have not budged in more than a decade," he said.
Federal officials at the briefing described why Listeria is so difficult to battle and why it is showing up in foods that haven't typically been linked with past outbreaks. Michael Taylor, JD, deputy commissioner for foods and veterinary medicine at the Food and Drug Administration (FDA), said L monocytogenes is an environmental contaminant that can be present anywhere food is being prepared.
"That can happen at the growing stage for produce, it can happen in processing facilities for cheese and other products, and it can happen at retail," he said.
Elizabeth Hagen, MD, undersecretary for food safety at the US Department of Agriculture (USDA), said the broad threat requires a comprehensive effort to prevent contamination and reduce the illness risk, such as a recent intra-agency assessment of L monocytogenes at retail delis.
"We can prevent illnesses and save lives by taking some simple steps. Controlling temperatures in the deli case and putting in measures to prevent cross-contamination can have a really powerful impact to public health," she said. "We've had a lot of success reducing Listeria in the past, and we're going to have more success in the future thanks to the collaboration we have going on today."
Yesterday's report is mainly based on data from a nationwide CDC-led Listeria surveillance system and FoodNet, a population-based surveillance system that involves the CDC, USDA, and 10 state health departments.
The CDC said 21% of infections in the study period were deadly—a figure that includes fetal losses. The fatal cases were mostly in older people and as miscarriages or stillbirths. It noted that pregnant women who have listeriosis often have only mild symptoms or fever, but the illness can lead to miscarriage or premature labor, as well as illness or death in newborns.
In younger, nonpregnant people, the Listeria threat was greatest in people with weakened immune systems, such as those undergoing cancer treatment or people with underlying conditions such as kidney disease or diabetes, according to the report.
Though US and Canadian health officials have pointed out the high risk of Listeria contamination in soft cheeses made from unpasteurized milk, the CDC report said those made from pasteurized milk also pose a threat. Cheese was implicated in six outbreaks covered in the report, and five were made from pasteurized milk: four were Mexican-style cheese, such as queso fresco, and one involved chive and ackawi cheese (a white brined cheese.)
The report said that although pasteurization eliminates Listeria, contamination can occur after the process, and the bacteria can grow in moist, refrigeration environments and can thrive when the product is contaminated.
Raw produce items linked to recent Listeria outbreaks included cantaloupe and pre-cut celery.
New prevention efforts are needed to regain progress toward reducing the Listeria illness burden, including quicker illness detection, speedier outbreak investigations, and heightened awareness for food producers and consumers, the report said.
In a CDC press release, Frieden said new cutting-edge molecular technologies are needed to help link illnesses and outbreaks as a way to drop the number of illnesses and deaths. He said President Obama's fiscal year 2014 budget includes $40 million for the CDC's Advanced Molecular Detection Initiative.
The CDC said it also has plans to test an advanced DNA fingerprinting method (whole-genome sequencing) on Listeria, which could identify and control outbreaks faster.
Hagen said in the release that changes in policy and industry practices have played an important role in reducing the Listeria burden in the past. "However, important work remains if we hope to continue this momentum. Additional research and continual monitoring of evolving risks will allow us to develop policies that further reduce these illness rates," she said.
Jun 4 CDC MMWR Vital Signs report
Jun 4 CDC telebriefing transcript
Jun 4 CDC press release
May 13 CIDRAP News story "New tool for retail delis targets Listeria contamination"