While it is shocking to see a serious outbreak in such a popular brand, Listeria contamination is a troubling problem for most everyone in the food industry, and the fresh-cut, bagged salad industry is highly vulnerable.
The food industry has controls in place to combat Listeriosis, along with other pathogens; yet outbreaks continue to occur, and we must ask the difficult question… why? A look at the disease transmission factors associated with Listeria monocytogenes point out some unique and difficult problems that can confound our controls.
Listeria has a relatively low infectious dose in the immune compromised; no one knows the amount of Listeria in every case sufficient to cause disease, but it is believed just a 1000 cells can cause disease. It is well adapted to a wide variety of environments, and can readily colonize most surfaces. Listeria can be found in about 7 percent of produce growing areas, and is widely distributed in nature, with a detection rate of about 3 percent of soils sampled in undisturbed areas.
The organism is hardy and survives well. Listeria may colonize a surface, but it can also be transient in an operation and gains entry into a plant with products, in soil, on shoes, and in dust. An effective sanitation program should be taking care of the transient Listeria that finds its way into a plant. From a risk perspective, finding Listeria in a drain or on a floor, is a totally different thing than finding the organism on a food contact surface or in a finished product.
Listeria contaminates cold, wet environments and harbors in drains and on floors and any damp surface if cleaning is not effective. Listeria can create bio-films that resist removal. Cells can multiply in the environment, albeit slowly, perhaps to 10 generations in about seven days at typical refrigeration temperatures. Temperatures around 41° F stop most other human pathogens from multiplication, but not Listeria. Eventually, these cells find their way onto surfaces that contact food, and the food itself.
The pathway for infection is further enhanced by the vulnerability of cut, nutrient rich fruits and vegetables to further support survival and multiplication of the organisms during storage and shipping.
The Listeria phenomenon is a relatively new public health problem in produce and is related to supply chain factors such as centralized processing of fruits and vegetables and extended refrigeration times. Such factors provide ample time and temperature conditions for Listeria to multiply.
Listeria monocytogenes is a very virulent pathogen, having a relatively low infectious dose. Symptoms may not present for at least three weeks after exposure and long incubation periods hamper efforts to identify suspect products, increasing the attack-rate in outbreaks.
Up to 10 percent of humans carry Listeria in their digestive tract, and are asymptomatic. Symptoms in those who become ill can progress rapidly to life-threatening sepsis and eventually to shock. The severity of illness is enhanced in the immune compromised host, and high mortality (~30%) occurs in outbreaks. The elderly, pregnant women, those on immune suppressive drugs, neonates, and those with common disorders of the blood, and internal organs are most at risk for serious infection. The number of at-risk consumers is increasing, along with the demand for fruits and vegetables.
Bagged salads are some of the most popular supermarket items. The convenience of pre-processed vegetables and the perceived health benefits of a vegetable rich diet further increase the exposure.
Due to the ever-present danger that Listeria may be in a processing environment, a food manufacturer must adhere to strict sanitation and testing programs. Together, these controls can effectively reduce the likelihood that Listeria bacteria will reach food-contact surfaces or products.
The first line of defense in a food plant is the design and maintenance of equipment and the environment. All surfaces where food is in contact with processing equipment, even the hard to clean surfaces and hidden areas, must be rigorous cleaned. Effectiveness of cleaning needs to be analyzed and verified. There are several popular testing methods including ATP Bioluminescence, protein and sugar detection swabs, and microbial testing (including rapid DNA detection).
If the surfaces of equipment cannot be properly cleaned and sanitized, and the situation goes unnoticed, the sanitation controls will fail, and a very difficult to remove bio-film will develop.
Produce operations use water processes to move products (in flumes for example), cool product (hydro-coolers), to cool with ice, and to wash products. A large volume of water is typically required in a fresh-cut salad plant. If adequately sized drains are not in place to catch water discharge, or if they are not properly placed, wholesale wetting of the production environment occurs. Given that these processing areas are also refrigerated in fresh-cut operations, adds to the risk of colonization.
Condensation is another enemy; dripping water is especially troublesome in refrigerated areas due to ventilation problems and design issues. Condensation significantly increases the risk of contamination.
Microbial testing is perhaps the most effective prevention strategy, yet it is largely underutilized. Both surfaces and products (raw, in-process, and finished product) should be under a microbial monitoring program. Diligence in executing the sampling program must be maintained; with sampling schedules developed based on risk assessments. Importantly, management needs to plan for an immediate and effective response if unsatisfactory tests results are received.
The keys to prevention of Listeria are vigilance and management oversight, and making the right decisions. Upper management involvement in sanitation problems is critical, along with management support for needed capital improvements in facilities that have issues. Furthermore, sufficient resources must be applied to the frequent and ongoing monitoring of the environment and the verification of sanitation.
No details are available at this time as to conditions in the plant associated with the current bagged salad problem, but history has shown that failures in past Listeria outbreaks occurred in the typical controls; e.g., equipment design and maintenance; environmental sanitation, environmental and product testing; and food safety management decision-making.
While Listeria monocytogenes is a major threat to the food industry, we should not be overly anxious, but have confidence in our food safety program, even if the organism is found somewhere in a plant. A systematic, well-executed, verifiable, and effective sanitation program applied to well-designed and cleanable surfaces will reduce much of the risk. An effective sanitation program along with properly established environmental assays and product-testing programs are the ultimate defenses against “Listeria Hysteria”.
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