Recent Outbreaks of Listeriosis Linked to Fresh, Soft Queso Fresco-Type Cheeses in the U.S.

In 2020,the EU and the U.S. were the world's leading cheese producers, accounting for roughly 70 percent of global production. Overall, global cheese production is projected to continue increasing at least until 2027, as developed nations are expected to increase their milk production by roughly 9 percent.¹

Concurrently, cheese consumption has increased around the world, with the U.S. and the EU being the main cheese-consuming areas.¹ Generally, cheeses can be categorized as soft, semi-soft, soft ripened, or hard, depending on their moisture content and how they are made. These parameters affect their sensory attributes, as well as their ability to support growth of Listeria monocytogenes. Fresh, soft, un-ripened cheeses have not been aged and have high moisture in the range of 40–80 percent, which limits their shelf life.

Queso fresco-type cheeses (QFTCs) include fresh, soft cheeses that have not been extensively aged; these have been classified as "fresh, soft cheeses" in risk assessments conducted jointly by the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA).² These types of soft cheeses are typically white or off-white in color, have high moisture, lower salt content, low acidity, and a shorter shelf life than aged cheeses, and require refrigeration for safety. In the U.S., cheeses must be made from pasteurized milk or cream, with the exception of some cheeses that are aged for 60 days or more. Depending on their country of origin, some of the more popular fresh, soft QFTCs may have different names for the same type of cheese or the same names for different types of cheese, such as Oaxaca, Asadero, Queso Blanco, Queso Fresco, and Panela. In 2009, U.S. producers manufactured approximately 206 million pounds of QFTC, and by 2019, that number climbed to roughly 333 million pounds, with the U.S. per-capita consumption doubling during the same timeframe.³

QFTCs and Listeria

An estimated 1,600 cases of foodborne listeriosis occur annually in the U.S.⁴ The illness is caused by the consumption of foods contaminated with L. monocytogenes and primarily affects those with weakened immune systems, people over the age of 65, and pregnant people. Infection can result in serious outcomes such as septicemia, meningoencephalitis, pregnancy loss, and/or death.

Historically, outbreaks of listeriosis linked to fresh, soft QFTC consumption have been linked to cheeses made from unpasteurized milk. Since 2000, outbreaks have been attributed to fresh, soft Queso Fresco-type cheeses made from pasteurized milk, which have been contaminated by L. monocytogenes after pasteurization.⁵ Proper pasteurization eliminates L. monocytogenes, but cheesemaking involves several steps where the ready-to-eat (RTE) product is exposed to the environment after pasteurization of milk or cream, allowing for potential contamination by L. monocytogenes from the manufacturing environment. Fresh, soft QFTC can support the growth of L. monocytogenes because of their relatively high pH, high moisture content, and low salt levels. Additionally, QFTCs are typically handcrafted by smaller producers where the manual nature of production, which is more common in smaller facilities, can exacerbate this risk. It is crucial that manufacturers identify and control hazards associated with making these products.

Using data from the Foodborne Disease Outbreak Surveillance System, the U.S. Centers for Disease Control and Prevention (CDC) found that 58 outbreaks of listeriosis were reported in the U.S. from 1998 to 2014; of these, 17 (30 percent) of the outbreaks were associated with soft cheeses, resulting in 180 illnesses, 14 fetal losses, and 17 deaths.⁵ Eleven of the 17 cheese-associated outbreaks were due to consumption of QFTC, and the 11 outbreaks accounted for 98 (54 percent) of the cases of listeriosis.⁵ In comparison, the remaining six of the 17 outbreaks were linked to consumption of sheep's milk cheeses, Eastern European-style cheeses, Middle Eastern-style cheese, Italian-style cheese, blue-veined cheese, and soft-ripened cheeses.⁶ Outbreaks of listeriosis due to fresh, soft QFTC often disproportionately affect Hispanic people who are pregnant and their newborns, and can result in pregnancy loss, stillbirth, or death of the newborn.^6,7,8 Additionally, people who are 65 years and older and people with weakened immune systems are at higher risk.⁹ An often underreported aspect of listeriosis outbreaks includes the estimated health care costs. The authors estimate the most recent listeriosis outbreak linked to fresh, soft QFTC in February 2021 to cost approximately $45.96 million in lost consumer health, accounting for underreporting/diagnosis.^10,11

FDA works closely with CDC, state, and local partners to coordinate listeriosis investigations in response to multistate outbreaks. Here, the authors present key findings of outbreak investigations from 2014–2021 that have been linked to the consumption of fresh, soft QFTC where a firm of interest was identified in the U.S. They also outline some of the specific circumstances that small manufacturers of all cheeses may encounter, particularly smaller manufacturers of fresh, soft QFTC. The authors' goal is to provide lessons from these outbreaks and highlight some of the available resources for QFTC manufacturers.

Outbreak Investigations of Listeriosis Linked to QFTCs

Throughout the years, fresh, soft QFTC outbreaks have faced several challenges, including a small number of ill people. Interviews are often completed with a surrogate (someone speaking on behalf of or translating for the ill person), and language barriers, which might require use of a translator, present challenges in obtaining accurate exposure information. L. monocytogenes has a long incubation period (varying from a few days to 90 days) and, as a result, food exposure recollection may be very difficult for ill people when trying to determine the food that made them sick.¹² When exposure information is available, exposures to multiple types of fresh, soft QFTC and/or multiple QFTC brands in the timeframe before illness are often reported.

Fresh, soft QFTC are also sold at deli counters where they may be cut and repackaged, which presents cross-contamination concerns that may skew epidemiologic and traceback data. Additionally, the same fresh, soft QFTC brand may be made by multiple QFTC manufacturers. These limitations significantly impact investigators' ability to identify the source of the outbreak and, as a result, several outbreaks that were suspected to be linked to fresh, soft QFTC remain unsolved.Since 2014, four investigations have been conducted for listeriosis outbreaks linked to the consumption of fresh, soft QFTC made with pasteurized milk (Table 1). Three of the four investigations resulted in the implicated cheeses being recalled from the market to protect public health. All three firms ceased manufacturing cheeses, while the fourth investigation did not confirm a specific brand or firm associated with the outbreak.

TABLE 1. A Summary of Outbreak Investigations of Listeria monocytogenes Infections Linked to QFTCs, 2014–202113,14,15,16

In 2014, two outbreaks of listeriosis were linked to QFTC. The first outbreak investigation, which began early that year, included a total of eight illnesses reported in two states; seven ill people were hospitalized.¹⁴ Five illnesses were related to pregnancy (two mother-newborn pairs, and one in a separate newborn),¹⁴ and three ill people were adults (non-pregnancy related). One person died. All eight ill people reported Hispanic ethnicity. Exposure information was available for five ill people; two were mothers interviewed for the mother-infant pairs.

The five adults reported consuming QFTC from the same grocery chain, and three specifically reported consuming cheese made by the firm of interest. Testing of multiple cheese products collected from the grocery chain was performed by state partners at the Maryland Department of Health and Mental Hygiene and Virginia's Division of Consolidated Laboratory Services (VA DCLS). Pulsed-field gel electrophoresis (PFGE) of the L. monocytogenes isolates from the firm's cheese products (a single strain) were a match to the isolates from ill people in the outbreak, adding further evidence that the firm's cheese products were the source of the outbreak. As a result, the firm voluntarily recalled all potentially contaminated cheeses. Whole genome sequencing (WGS), completed after the recall, confirmed the genetic relationship between the cheese isolates and the clinical isolates from ill people. An investigation at the firm's facility found insanitary conditions and recovered L. monocytogenes isolates from environmental sampling, including the cheese processing room and various pieces of equipment.

Specifically, investigators found the following insanity conditions:

1. The roof was leaking so profusely that water was dripping into the cheese processing room, including onto the cheese processing equipment and storage tanks
2. Standing water was present on the floor throughout the cheese curd processing room in proximity to the cheese vats and in the storage rooms
3. Metal roof/ceiling and metal supports exhibited a rusted appearance with metal flaking, precluding effective cleaning and sanitizing
4. Food residues were found on equipment after cleaning had been performed
5. Openings to milk storage tanks and transfer piping were not capped to prevent contaminants from entering or contaminating food contact surfaces
6. Floors, walls, and equipment were deteriorated and in disrepair, including processing equipment and storage vats with rust holes and floors with rough concrete deterioration.

To immediately ensure that contaminated products would not be distributed, FDA suspended the firm's food facility registration as its QFTC products presented a reasonable probability of serious adverse health consequences or death to humans. Furthermore, a Decree of Permanent Injunction was filed to ensure that longstanding corrective actions would be implemented if the firm resumed operations.¹⁴

The second outbreak investigation took place in late 2014 and included nine ill people from six states, with eight of the nine hospitalized.¹³ Among the nine illnesses, six illnesses were related to pregnancy, with two fetal losses reported and two illnesses in newborns. One death was reported in an infant.^13,17 All nine ill people reported Hispanic ethnicity, and eight of the nine ill people reported consuming QFTC. Food exposures for the pregnant person were collected for infants for the four weeks prior to illness onset. Cheese exposure for one patient was unknown. In July 2014, VA DCLS identified L. monocytogenes in Quesito Casero cheese, which was collected during routine sampling from a retail location. As a result, in August 2014, FDA inspected the firm's processing facility and found failures to comply with Current Good Manufacturing Practice (CGMP) requirements.¹⁸

Additionally, FDA collected environmental samples during the inspection, recovering isolates of L. monocytogenes that were highly related by WGS to isolates from the VA DCLS retail sample of Quesito Casero cheese and all isolates from ill people. This WGS data indicated a high likelihood that these isolates had originated from the same source. The manufacturer voluntarily recalled Quesito Casero cheese and during that month, it expanded the recall on multiple occasions. Another FDA inspection was conducted at the facility in October 2014 in response to a cluster of four listeriosis illnesses (part of the total nine illnesses of the outbreak). The clinical isolates in these four illnesses were closely matched by WGS to the isolates recovered from the Quesito Casero retail cheese sample collected by VA DCLS and the FDA environmental isolates collected in August 2014. At the end of the inspection, the owner informed FDA that the firm would shut down operations and discontinue processing and repackaging of cheese products.¹⁸

The inspection closed on December 16, 2014 and revealed that the firm failed to clean and sanitize equipment in a manner that protects against contamination of food, food contact surfaces, or food packaging materials. Additionally, the firm failed to report a reportable food to the Reportable Food Registry¹⁹ within 24 hours of determining that there is a reasonable probability that the use of, or exposure to, the reportable article of food will cause serious adverse health consequences or death to humans or animals.¹⁸ Based on violations identified during these inspections, FDA issued a Warning Letter to the manufacturer on July 8, 2015.¹⁸ The manufacturer responded to the Warning Letter on July 20, 2015, indicating that dairy products were not being processed at that time and had not been processed since receiving the letter. On November 15, 2016, the U.S. Department of Justice announced that the owner of the firm was sentenced to 15 months in prison for distributing cheese contaminated with L. monocytogenes.²⁰

In late 2019, an outbreak of listeriosis occurred with fresh, soft QFTC identified as a suspect source. Despite the epidemiologic information and inspection issues identified, available epidemiologic and laboratory data did not definitively link the ill people to the fresh, soft QFTC manufacturer. As a result, fresh, soft QFTC was not confirmed as the source of this outbreak.

A total of four ill people from three states were reported. Three ill people were hospitalized. Three illnesses were related to pregnancy, with one resulting in fetal loss. No deaths among the ill people were reported. All four ill people reported Hispanic ethnicity. Two ill people reported consuming the same brand of fresh, soft QFTC, with one specifically reporting Queso Fresco. In January 2020, FDA conducted an inspection at the QFTC manufacturer¹⁵ and subsequently issued a Warning Letter. Environmental and product samples collected during the inspection did not identify L. monocytogenes; however, L. grayi and L. innocua were found in the environment. While typically nonpathogenic to humans, the presence of Listeria species indicates that L. monocytogenes could survive in the same environment.

According to the warning letter,¹⁵ the firm:

1. Did not conduct a hazard analysis for each type of food manufactured, processed, packed, or held at its facility to identify and evaluate known or reasonably foreseeable hazards to determine whether there were any hazards requiring a preventive control
2. Did not establish and implement preventive controls to provide assurances that any hazards requiring a preventive control would be significantly minimized or prevented and the food manufactured, processed, packed, or held by the facility would not be adulterated
3. Did not prepare/have prepared and did not implement a written food safety plan for any of the products manufactured in its facility.

The most recent outbreak of listeriosis linked to the consumption of fresh, soft QFTC occurred in 2021.¹⁶ A total of 13 ill people were reported from four states. Twelve of the 13 ill people were hospitalized. Four illnesses were related to pregnancy, and two resulted in fetal loss. One death was reported among the ill people. Twelve of the 13 ill people reported Hispanic ethnicity. Eight people reported consuming QFTC; among those, seven reported consuming queso fresco. Four people reported the same QFTC brand reported by ill people in the 2019 outbreak discussed above. The Connecticut Department of Public Health collected two samples of the reported brand Queso Fresco cheese and recovered isolates of L. monocytogenes related to the outbreak strain by WGS. In February 2021, FDA initiated an inspection at the fresh, soft QFTC manufacturer. As a result, the QFTC manufacturer ceased production and repackaging at its facility and conducted a voluntary recall. This recall was later expanded to include all cheeses handled at the firm.

General Facility Investigation Observations

These FDA inspections of fresh, soft QFTC firms linked to listeriosis outbreaks showed deviations from regulatory requirements set forth in 21 CFR Part 110 (in the outbreaks in 2017 and before), or 21 CFR Part 117 (in the outbreaks 2019 and after), as noted in the Inspectional Observations FDA Form 483,²¹ a notification to the firm's management of objectionable conditions observed by investigators. All fresh, soft QFTC firms linked to the outbreaks described in this article had fewer than 500 employees—the definition of a small facility, as outlined in 21 CFR Part 117. Most of these facilities were not kept in good repair. Examples included water leaking from the roof, floors in disrepair, and issues with condensate or pooling water. L. monocytogenes or Listeria spp. were detected in the environment during the inspections of some of the firms, and L. monocytogenes was isolated from the product in two of the outbreaks.

Common observations in small fresh, soft QFTC manufacturing facilities associated with listeriosis outbreaks include:

• Inadequate cleaning and sanitizing of facilities resulting in issues such as standing water and cross-contaminated food contact surfaces
• Inadequate pest control
• Poor employee hygiene practices, including improper use of gloves
• Inadequate cleaning, sanitizing, and maintenance of equipment and/or utensils
• Inadequate separation between pasteurized and unpasteurized dairy products
• Failure to manufacture, package, and store products under conditions necessary to minimize the potential for microbial growth and contamination
• Inadequate hazard analysis, lack of food safety plan, and failure to conduct environmental testing for Listeria spp. were also noted for one of the fresh, soft QFTC firms.

Certain Requirements Applicable to Cheese Manufacturers

FDA has established in 21 CFR Part 117 a regulation titled, "Current Good Manufacturing Practice, Hazard Analysis, and Risk Based Preventive Controls for Human Food" (Part 117). Part 117 establishes requirements for CGMPs for human food, for hazard analysis and risk-based preventive controls for human food (PCHF), and related requirements.²² Manufacturers of cheeses, including fresh, soft QFTC, are required to comply with CGMP requirements and, unless specific exemptions apply, PCHF requirements (e.g., very small businesses are "qualified facilities" subject to modified requirements). The CGMP requirements address areas such as employee hygiene, equipment maintenance, training, adequacy of water supply, sanitary design and operation, and process controls, all of which are important in preventing the contamination of food. The PCHF requirements include steps that manufacturers must take to identify and significantly minimize or prevent food safety hazards.

Cheese manufacturers subject to the PCHF requirements must implement a written food safety plan (FSP).^23,24 Within the FSP, these manufacturers must conduct a hazard analysis and determine if any identified hazards require a preventive control. For RTE food, such as fresh, soft QFTC, the hazard analysis must evaluate the hazard of contamination with environmental pathogens (e.g., L. monocytogenes) whenever the RTE food is exposed to the environment prior to packaging and the packaged food does not receive a treatment or otherwise include a control measure (such as a formulation lethal to the pathogen) that would significantly minimize the pathogen.²⁴ When manufacturers determine within the hazard analysis that one or more preventive controls are required, they must establish the preventive control(s) with the required preventive control management components (e.g., monitoring, corrective actions, and verification), as appropriate to the nature of the preventive control and its role in the food safety system.

Examples of preventive controls that could be implemented to control L. monocytogenes in cheese production are outlined in Table 2. The PCHF requirements include environmental monitoring and finished product testing as verification activities as appropriate to the food, the facility, and the preventive control (21 CFR §117.165).²⁴

TABLE 2. Some Provisions of CGMP, Hazard Analysis, and Risk-Based PCHF Regulation Applicable for Control of Listeria monocytogenes in Facilities Subject to 21 CFR Part 117 and Making Soft, Fresh Cheeses

Food Safety and FSMA Resources for QFTC Manufacturers

FDA has developed guidance documents that are available on FDA's website to help cheese manufacturers be better prepared for compliance with the Food Safety Modernization Act (FSMA) and applicable federal rules and regulations.²⁵ Cheese manufacturers will find Draft Guidance for Industry: Control of Listeria monocytogenes in Ready-To-Eat Foods and Draft Guidance for Industry: Hazard Analysis and Risk-Based Preventive Controls for Human Food to be valuable resources.^22,26 These guidance documents, when finalized, will represent FDA's current thinking. They include information about the pathogen, environmental monitoring, and product testing. The FSMA Technical Assistance Network (TAN) is a central source of information for questions related to FSMA rules, programs, and implementation strategies.²⁷ Anyone can submit questions for review and response by subject matter experts.

The Food Safety Preventive Controls Alliance (FSPCA) is a broad-based public-private alliance comprising industry, academic stakeholders, and government representatives, with the goal of developing educational material, training, and outreach programs to support compliance with the prevention-oriented standards of FSMA.²⁸ FSPCA organizes training courses and also develops and shares a plethora of other resources and helpful information. FSPCA Participant Training Courses are held regularly and can provide helpful information to small-scale cheese manufacturers. Additionally, FDA's Food Safety Plan Builder is a tool designed to assist owners and/or operators of food facilities with the development of FSPs that are specific to their facilities and meet the PCHF requirements.²³

Many additional resources are available to fresh, soft QFTC manufacturers. These resources include university food safety extension specialists who can help cheese manufacturers, particularly smaller ones, develop an FSP for their facilities, conduct the hazard analyses, and perform risk assessments of their processes, helping ensure that they are producing the safest product possible. Regulatory partners and food safety experts at state and local government agencies generally inspect milk processing plants and collect samples of milk and milk products to ensure consumer safety. More importantly for small-scale manufacturers, state and local government agencies can share important guidelines, licensing, and compliance information. Cheese manufacturer associations and other trade organizations are groups designed to deliver the latest in food safety news to dairy product manufacturers, processors, and their suppliers. These associations can share guidance materials intended to facilitate the development of food safety risk mitigation steps by manufacturers. Additional resources offered through dairy extension programs, other organizations, and digital resources such as online training, blogs, social media, and podcasts are available.²⁹

Small and very small manufacturers of fresh, soft QFTC may have difficulty accessing, interpreting, and utilizing some of the resources outlined in this section. Additional educational and outreach materials, with a focus on Listeria control for small cheese manufacturers, can be useful, especially when they are tailored to these manufacturers and delivered in the manufacturers' native language. Academic partners, trade organizations, and state departments of health and agriculture can help alleviate this knowledge gap by producing highly focused educational materials to serve small cheese manufacturers.

Conclusion

Listeriosis outbreaks associated with fresh, soft QFTCs continue to present a significant food safety challenge. These outbreaks underscore the importance of compliance with food safety regulations by fresh, soft QFTC manufacturers. The findings of facility inspections, recovery of L. monocytogenes through sample collection and analysis, and recurrence of listeriosis outbreaks due to fresh, soft QFTCs call for attention from industry to focus greater efforts on control of L. monocytogenes in these facilities to prevent contamination of fresh, soft QFTCs and avert future listeriosis outbreaks due to fresh, soft QFTCs.

The industry manufacturing these products must be aware of the risks associated with production and the applicable regulatory requirements. Based on the investigational observations for the listeriosis outbreaks involving fresh, soft QFTC discussed here, QFTC manufacturers should develop strong CGMPs that establish sanitary operations within their facilities and prevent cross-contamination where RTE food is exposed to the environment before packaging. Those subject to PCHF must develop a written FSP that includes a thorough hazard analysis; implement preventive controls where necessary; and implement an environmental monitoring program when required. Even firms not subject to PCHF may consider these actions to minimize risk to public health.

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Evelyn Pereira, M.P.H., is a Biologist at FDA.

Cerisé Hardy, M.P.H., is a Staff Fellow at FDA.

Melinda Hayman, Ph.D., is a Consumer Safety Officer at FDA.

Amanda Conrad, M.P.H., is an Epidemiologist at CDC.

Alexandra Palacios, M.P.H., is an Epidemiologist at CDC.

Stelios Viazis, Ph.D., is a Consumer Safety Officer at FDA.