By: Ryan Robinson PhD

Recent news that a popular cashew butter brand was contaminated with the dangerous pathogenic bacteria Listeria monocytogenes has caused understandable concern among consumers in the United States. Questions continue to arise from worried individuals who want answers regarding how frequently food is contaminated and how often this contamination leads to illness.

Listeria undoubtedly has a long and storied history of causing illness in the US and abroad, but it’s study only dates back about a century. First identified in the 1920s, the tiny organism rapidly became a focus for medical microbiologists due to its potential to cause life-threatening illness. Over time, studying and tracking Listeria cases became a critical concern for government health agencies. Today listeriosis (the disease caused by Listera monocytogenes) is as a significant enough public health crisis so as to be classified as a “notifiable disease”. This means that information regarding any listeriosis infection observed by local healthcare providers should be forwarded to CDC so that incidents can be meticulously tracked.

This intricate and detailed tracking can help us get a better handle on the risk that various infectious agents play. Data published by the CDC on overall trends in foodborne illnesses can give some insight into the incidence of illness over time, and analyzing the incidence of Listeria infection can help us better understand its prevalence.

Upon review of the data, one will find some good news: Listeria infections occur infrequently in the United States (1.23 cases per 1000 people), and the rate of infection appears to be steady. The bad news is that we do not appear to be making significant inroads into fighting Listeria contamination with any significant haste. The incidence of Listeria infection did not change in a statistically significant manner between 2010 and 2012.

In addition to simply tracking individual infectious incidents, outbreaks are also tracked. Each observed listeriosis outbreak is monitored, and the data is eventually compiled into a report and published on the CDC’s website. Confirmed outbreaks are infrequent for a variety of reasons, but still happen more often than anyone would like. The CDC has confirmed at least one outbreak for every year on record this decade.

For interested parties, we have compiled a brief highlight of some of the most notable recent outbreaks cited by the CDC below.

  • Vulto Creamery Outbreak – Raw Milk Cheese – 8 cases, 2 deaths
    First Infection Observed: September 2016
    Thus far, only a single listeriosis outbreak has been noted in 2017. This outbreak was linked to cheese distributed by Vulto Creamery, and caused a nationwide recall extending to all raw-milk cheese products produced by this creamery. The Vulto creamery outbreak was directly responsible for eight illnesses and two deaths, and while contaminated products were distributed nationwide, the small number of reported illnesses were identified in four states (New York, Vermont, Connecticut, and Florida).
  • CRF Frozen Foods – Frozen Vegetables – 9 cases, 3 deaths
    First Infection Observed: September 2013
    In 2016 nine individuals across four states contracted listeriois from contaminated frozen vegetables. 11 different frozen vegetable products produced by CRF Frozen Foods were initially identified, but the recall eventually extended to more than 450 different food products, either produce by CRF directly, or containing contaminated food sold by CRF.

It is important to note that contamination of frozen foods is particularly troubling. The CDC regards this outbreak investigation as closed, but they note that as frozen foods typically have an extended shelf life, the contaminated food products may remain undetected in an freezers across the United States for months or years. As consumers use these products they may become ill, and the case count of an ongoing outbreak can continue to rise.

  • Miller’s Organic Farm – Raw Milk – 2 cases, 1 death
    First Infection Observed: 2014
    Proving again the danger associated with consuming unpasteurized dairy products, two individuals (one from California, and a second from Florida) likely became ill after consuming unpasteurized chocolate milk from Miller’s Organic Farm in Bird-in-Hand, Pennsylvania.

Following the death of the patient in the Florida case, it took nearly two years for CDC investigators to trace the root cause. Eventually whole-genome sequencing of cultures taken from both patients, and from the contaminated dairy product were used to link these geographically distant cases to a single cause.

While infecting relatively few individuals, this case is particularly notable it highlights both the danger associated with consumption of unpasteurized dairy, and the exceptional work performed by the diligent public-health investigators at the CDC.

  • Dole Foods – Packaged Salad Mixes – 19 cases, 1 death
    First Infection Observed: July 2015
    One of the larger identified outbreaks in recent history came from packaged salad mix originating from a Dole processing facility in Springfield, OH. This exceptionally large outbreak spanned 9 states. Further genomic analysis also identified incidents in Canada that likely originated from the same source.
  • Karoun Dairies – Soft Cheeses – 30 cases, 3 deaths
    First Infection Observed: June 2010
    This massive, multi-year outbreak was prompted by contaminated cheese products distributed by Karoun Dairy, and marketed under a variety of different brand names. Karoun, Arz, Gopi, Queso Del Valle, Central Valley Creamery, and Yanni. The first infectious incidents were classified in 2010, but the CDC investigation was not closed until 2015, with the final recorded case was reported in August of that year.

    Particularly notable in this case is the fact that contaminated products remained on store shelves for nearly five years. The first infection was observed in 2010, and products were not recalled until September of 2015. Ostensibly, this case speaks volumes to those who are calling for more stringent food-safety monitoring and regulation.

A note on tracking recorded outbreaks and the greater damage done by foodborne illness

It is worth stating that while studying recorded outbreaks and infections can help us better grasp the damage done by infections like listeriosis, they fall short of properly describing the total damage done.

A reported outbreak must be based on reported infections, and infections may not be reported for a variety of reasons. Some infected individuals may not be aware they are infected, the early symptoms of Listeria mimic other common illnesses (nausea, vomiting, fever, and body ache). Some high-risk cases may pass away before a confirmative diagnosis is ever made. Even if a notifiable disease is recognized and diagnosed by a health-care provider it may not be reported to health authorities at the local, state, or federal level.

Not all illnesses will become an outbreak, but all illnesses have a real human cost. We each have a role to play in preventing foodborne illness, and while we expect our government health authorities to be vigilant in monitoring, tracking, and preventing the spread of outbreaks, we must also carefully monitor our own symptoms and our own consumption habits.