Listeria - Why is it Not Going Away? Part 1
Craig Kahlke


Before late December of 2014, I remember talking food safety on many occasions with packers, marketers and fieldmen of the commercial apple industry in Western NY. One "fact" that we would commonly share was that, to our knowledge, no one had ever "gotten sick" (contracted a foodborne illness) as a result of eating a fresh apple. Each subsequent time I uttered this phrase aloud, I felt slightly more uneasy, as if I was tempting fate. When the initial reports came of a Listeria outbreak linked to apples packed from a small facility in California, I was not alarmed, as they were first linked to caramel apples. Therefore, most of us thought the contamination occurred during the processing step. As the news trickled out from the FDA inspection, it was later found that the surfaces of fresh Gala and Granny Smith apples also tested positive for Listeria. Now they had my attention, as well as the attention of the entire apple industry in the U.S. News of the outbreak spread like wildfire, and before the outbreak was over, 35 people from the U.S. and Canada were sickened, 7 died, with 3 deaths were directly caused by listeriosis. In addition, news of the outbreak reached overseas and delayed exports for weeks. It did not matter that this small packer did not export. Suddenly we all wanted to know how to test for L. monocytogenes, and what to do if a positive sample was found.
To answer these and other questions, we have to look back. Like E. coli, Listeria can be ubiquitous in the environment. Historically, Listeria contamination has typically been associated with processed meats and cheeses made from unpasteurized milk.

Immuno-compromised individuals such as children under 5, pregnant women, and those over the age of 65, are particularly susceptible to developing Listeriosis. For most people, Listeria infection causes influenza-like symptoms including a fever, however it also can cause more serious health conditions such as meningitis, septicemia (blood infection), and fetal death in pregnant women. Listeria outbreaks are also very difficult to track since the time from ingestion of the contaminated food to onset of the illness can be anywhere from 3-70 days!

Sadly, the deadliest foodborne outbreak in the U.S. in nearly 100 years occurred in the summer and fall of 2011 when cantaloupe were contaminated with Listeria monocytogenes and were traced back to a farm in Colorado. Although the outbreak began in July, illnesses were still being reported in December, due to the long incubation period of this pathogen in some individuals. When the outbreak was over, 33 people had died and 147 were sickened in 28 states. The FDA found positive L. monocytogenes samples at a broker of the cantaloupe that linked them to one farm (Jensen Farms) in a cooperative of Eastern Colorado cantaloupe growers. Subsequently, the FDA found positive Listeria samples at Jensen Farm's packing facility, including on the grading belts, conveyor, felt rollers in the facility and on equipment that was originally designed for potatoes. Lack of proper sanitation of this equipment allowed L. monocytogenes to become established within the equipment and subsequently contaminate cantaloupes that moved through the washer and pack line. It is not certain how Listeria was first introduced into the facility, but the investigation report proposes that low levels of sporadic L. monocytogenes could have been present in the field where the cantaloupes were grown. The report also mentions that a truck used to haul culled cantaloupes to a cattle operation was parked next to the packing facility and could have introduced contamination via soil and manure from the tires.

Contributing to the pathogen's growth, there was no pre-cooling step to remove field heat from the cantaloupes before cold storage, which in turn, may have caused condensation on the cantaloupe in the cooler.

One of the major differences between L. monocytogenes and other pathogens such as E. coli and Salmonella sp. is that L. monocytogenes continues to multiply at refrigeration temperatures. Thus, the potential for the organism to reach levels that can make a healthy person ill is greatly increased if the environmental conditions are right. It is important to note that Jensen farms received a 96/100 score in an early season Primus food safety audit, so the growers likely thought their food safety program was well implemented and effective at reducing risks. In hindsight, this outbreak highlights many lessons including the importance of equipment sanitation and the risks that L. monocytogenes presents in fresh produce operations. In addition, it provides many points for discussing the value of third party audits as well as the importance of the competence and training of auditors, and the misconception that an auditor could be in expert in all areas of produce food safety.

Traditionally, Salmonella had been the main microbial risk to cantaloupe, and apples did not have any reported foodborne illness outbreaks associated with either of these pathogens. So what has changed? There is no clear answer. One contributing possibility is that Listeria detection and serotyping methods have greatly improved in recent years. Molecular subtyping can link illnesses with a single outbreak strain. In addition, state health departments are being encouraged to report suspected outbreaks quicker and are able to access a national outbreak surveillance network, PulseNet, which is overseen by the CDC. Information can be quickly shared and compared, such as genetic fingerprints of strains that can be isolated in different parts of the country, which allows public health officials to get a jump on potential sources of the foodborne illness. There also is the regulatory standpoint. Regulators are looking for it, food processing and packing plants are looking for it, and once the number activities designed to detect a pathogen are increased, it is more likely to be found- it's inevitable that the number of recalls will increase. Despite the increased testing, outbreaks, and illnesses, the death rate due to L. monocytogenes has remained fairly consistent over the past few decades.
A major concern is that this organism causes a higher rate of hospitalization than any other foodborne pathogen; the hospitalization rate of individuals that acquire this pathogen is 95%.

Equally scary is that this disease is the leading cause of death from a foodborne pathogen. Even though there are only 1,600 cases of listeriosis per year in the U.S., those cases result in about 260 deaths.

Compare that to Salmonella, which is responsible for about 1.2 million cases of illness. Those same 1.2 million cases of salmonellosis result in about 450 deaths. This comparison really illustrates the fact that Listeria is a very deadly pathogen for the individuals who contract it.

PREVENTION: Since Listeria is often associated with wet, cool environments, it is critical that standing water be eliminated from packing areas and that all packing equipment be cleaned and sanitized and allowed to dry at the end of each day. Many newer packing lines and packing equipment available today utilize principles of sanitary design, in which the equipment is constructed with materials that are more resistant to microbial growth and designed in a manner which facilitates easy cleaning and sanitizing. Wood surfaces are not recommended in a packing house, as wood is porous and cannot be sanitized. Only food-grade lubricants should be used on equipment which is likely to come in contact with food contact surfaces.

Many more questions need to be answered. What do you do if you are testing for L. monocytogenes in your packinghouse and find a positive sample? Do you report it if it is only on a non-contact food surface such as in a drain from under a dump tank? With legal ramifications, questions like these have no easy answer. However, there is some hope on the horizon. There is a huge amount of research with L. monocytogenes going on in the U.S. and abroad to help answer these questions. In fact, at an upcoming food safety conference, there is going to be a large panel convening discussion about what we've learned from the 2014 L. monocytogenes outbreak associated with caramel apples. The Center for Produce Safety (CPS) is a well-funded organization that provides and shares ready-to-use, science-based solutions to prevent or minimize produce safety vulnerabilities based on applied research grants. Their annual conference later this month in Seattle will shed light on what can be done form a practical standpoint to reduce risk. Although I'll not be attending the conference, other food safety experts from Cornell will, and will share what they've learned to help us. There will be more to follow after the conference, including what can be done at the farm level. Regardless of the type of facility you have, all packinghouses can reduce risks by doing some basic cleaning and sanitizing activities.