1 Institut de Veille Sanitaire, St. Maurice, France.
2 Centre National de Référence des Listeria, WHO Collaborating Center for Foodborne Listeriosis, Institut Pasteur, Paris, France.
3 Direction Générale de l'Alimentation, Ministère de l'Agriculture, Paris, France.
4 Direction Générale de la Concurrence, de la Consommation, et de la Répression des Fraudes, Ministère de l'Economie, Paris, France.
5 Direction Générale de la Santé, Ministère de la Santé, Paris, France.
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
disease outbreaks; food contamination; food poisoning; Listeria infections; Listeria monocytogenes
Abbreviations: CI, confidence interval; InVS, Institut de Veille Sanitaire; NRC, National Reference Center for Listeria; OR, odds ratio
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
From October 1999 to February 2000, a small but nationwide outbreak of listeriosis occurred, and it received extensive media attention when the incriminated product, "rillettes," was withdrawn from the market. In the aftermath of the recall, there was a second, more extensive nationwide outbreak of listeriosis caused by a strain of L. monocytogenes different from the strain that caused the first outbreak. In this report, we describe the investigation of these consecutive outbreaks.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Case definitions
In this study, a case of listeriosis was defined as illness in a resident of France for whom L. monocytogenes was isolated from a normally sterile site, or from any site within the first 30 days of life in the case of a newborn infant. A case was related to outbreak 1 if the strain, isolated from the patient between October 18, 1999, and February 10, 2000, had the same serovar, phagovar, and DNA macrorestriction pattern as the first outbreak strain. A case was related to outbreak 2 if the strain, isolated between November 12, 1999, and February 28, 2000, had the same serovar and DNA macrorestriction pattern as the second outbreak strain. Sporadic cases were defined as those occurring in patients infected by a strain of L. monocytogenes other than the two outbreak strains.
Case patients were categorized as either being pregnant, being previously healthy, or having an underlying medical condition. Pregnancy-associated cases were defined by the isolation of L. monocytogenes from a pregnant woman or her fetus or newborn infant within the first 30 days of life. A mother-infant pair (even in the case of twins) was counted as a single case. A patient was defined as having an underlying medical condition if he or she suffered from a severe and potentially life-threatening illness or suffered from a disease (malignancy, hepatic or renal failure, diabetes, human immunodeficiency virus/acquired immunodeficiency syndrome) or had received a treatment (chronic hemodialysis, corticosteroid therapy, chemotherapy, organ transplantation) for which an increased risk of listeriosis had been shown in previous studies (3, 4
). All patients without an underlying medical condition whose cases were not associated with pregnancy were considered previously healthy.
Case-finding
Case patients were identified through the French surveillance system. During outbreak 2, information on the outbreak was sent to the health authorities of European member states through the Health Surveillance System for Communicable Diseases (5) and to all participants in the international network for surveillance of human gastrointestinal infections (Enter-net (6
)), with a request to notify the InVS of all cases of listeriosis in persons who might have consumed French meat products and to send the strains isolated from these patients to the NRC.
Case-control study
During outbreak 2, we carried out a case-control study to identify risk factors for infection with the outbreak strain. Control subjects were people with sporadic cases of listeriosis diagnosed in France between November 28, 1999, and March 1, 2000. Information on underlying medical conditions and food consumption was obtained from the questionnaires that had been completed at the time of illness as part of the routine enhanced surveillance of listeriosis. Only patients interviewed before their case/control status was known were included. Some patients underwent a second interview for completion of information on places of purchase of foods, but only information obtained during the first interview was used for the analysis of the case-control study. To examine the risk associated with each exposure, we calculated univariate odds ratios and their exact 95 percent confidence intervals (7). Differences in categorical variables were compared using the
2 test with Yates' correction or Fisher's exact test, as appropriate. Associations at the 5 percent level were considered statistically significant. Exposures associated with infection with the outbreak strain by univariate analysis (p < 0.1 or odds ratio > 3) were included in a logistic regression model (EGRET statistical software; Statistics and Epidemiology Research Corporation, Seattle, Washington), allowing for the date of interview (before or after media reports on the incriminated product), pregnancy status, and underlying medical conditions. The final model was obtained through stepwise deletion of variables on the basis of statistical and epidemiologic criteria (8
).
Microbiologic and environmental investigations
All isolates were serotyped (9) and characterized by DNA macrorestriction pattern analysis using Asc1 and Apa1 as endonucleases (10
), and until February 17, 2000, all isolates were phage-typed (11
). Since difficulties were encountered in the interpretation and reproducibility of the phagovar of the second outbreak strain, it was decided to characterize the strains by serovar and DNA macrorestriction pattern only.
The NRC compared with the outbreak strains L. monocytogenes isolates from food and environmental sources submitted to them by state and private food hygiene laboratories in the year prior to the outbreaks. For the investigation of outbreak 2, L. monocytogenes strains isolated from food and environmental sources recovered since May 1999 by the Veterinary Services (Ministry of Agriculture) and Consumers' Affairs inspectors were also submitted to the NRC for comparison with the outbreak strain.
District officers of the Veterinary Services and Con-sumers' Affairs inspectors analyzed the supply channels of the retail stores where case patients had purchased their meat products, in order to identify a common supplier or manufacturer. Samples of meat products offered for sale at the delicatessen counters in these retail stores were collected and examined for contamination with L. monocytogenes. In outbreak 1, an investigation of the incriminated production plant was carried out. In outbreak 2, no particular manufacturer could be incriminated; therefore, investigations were carried out in the major production plants that distributed the incriminated product nationally and in all production plants and suppliers that repackaged foods whose meat products were on sale in the places of purchase of at least 10 case patients. These investigations included review of production and product distribution records, noncompliance reports, and results of microbiologic monitoring by the plants and the Veterinary Services. Samples from food products and environmental sources were collected and examined for L. monocytogenes contamination.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
From its collection of isolates obtained from food and environmental sources, the NRC identified 21 L. monocy-togenes strains with the same serovar and phagovar as the outbreak strain. Seven of these strains had been recovered from brand X rillettes, produced by the same manufacturer that supplied supermarket chain Y. During the inspection of production plant X, review of the manufacturer's microbiologic monitoring of shelved stock revealed that on several occasions from September through December 1999, L. monocytogenes had been isolated, in concentrations of <10 colony-forming units per gram, from finished product sampled towards the end of its shelf life. The investigation did not identify a precise source of contamination. The strains isolated from the incriminated rillettes were later shown to have the same DNA macrorestriction pattern as the outbreak strain, whereas the isolates recovered from the other foods all had a different pattern.
On January 6, 2000, the manufacturer recalled all of its products (rillettes and jellied pork tongue), and consumers were advised to discard the affected products or return them to the place of purchase. The recall received extensive media attention. Persons who had eaten the products were advised to consult their physician in case of fever, isolated or accompanied by headache, within 2 months after consumption. Consumers were informed that, because of the long incubation period, it was possible that cases would continue to occur in the weeks following the recall.
Outbreak 2
Between November 12, 1999, and February 28, 2000, 32 cases of illness caused by a serovar 4b L. monocytogenes strain with identical DNA macrorestriction patterns different from the pattern of the first outbreak strain were identified (figure 1). Nine pregnancy-associated cases and 23 adult cases occurred in 26 departments throughout France. Adult case patients were a median of 55 years old (range, 2588 years), and 18 (78 percent) were male. Of the adult cases, 11 (48 percent) had an underlying medical condition and 12 (52 percent), all males, were previously healthy adults with a median age of 52.5 years (range, 2581 years). All previously healthy adults had central nervous system listeriosis, whereas among the case patients with an underlying medical condition, only four (36 percent) (p = 0.001) had central nervous system listeriosis and seven (64 percent) had bacteremia. Five adults, all with an underlying medical condition, died. The infection resulted in one spontaneous abortion and seven premature births. Four of the premature neonates died.
Case-control study. Twenty-nine case patients and 32 control subjects were enrolled in the case-control study. Two case patients were excluded because no food history had been obtained due to death of the patient and nonavailability of a proxy. One case patient was excluded because his case status was known at the time of interview and media reports on the incriminated product had been released.
Case patients and control subjects did not differ significantly by age, sex, pregnancy status, or underlying medical condition. Like the case patients, control subjects resided throughout France (data not shown).
Case patients were significantly more likely than control subjects to have eaten jellied pork tongue (odds ratio (OR) = 28.0, 95 percent confidence interval (CI): 3.2, 1,222.1), cervelas sausage (OR = 9.6, 95 percent CI: 1.1, 444.2), cooked ham (OR = 5.0, 95 percent CI: 1.1, 31.0), or pâté de campagne (OR = 4.6, 95 percent CI: 1.5, 14.6) (table 1). Having eaten at least one meat product purchased from a delicatessen counter was significantly associated with infection with the outbreak strain (OR undetermined, 95 percent CI: 3.1, undetermined) (table 1), and this association was independent of the consumption of jellied pork tongue (after adjustment for consumption of jellied pork tongue, the Mantel-Haenszel OR was undetermined (95 percent CI: 1.4, undetermined)). In multivariate analysis, after controlling for consumption of other meat products, only consumption of jellied pork tongue (OR = 75.5, 95 percent CI: 4.7, 1,216.0) and consumption of pâté de campagne (OR = 8.9, 95 percent CI: 1.7, 46.1) were significantly associated with infection with the outbreak strain (table 2). Fourteen (48 percent) of the 29 case patients included in the case-control study reported having eaten jellied pork tongue. The proportion of case patients reporting consumption of jellied pork tongue was constant throughout the outbreak. All 15 case patients who did not report having eaten jellied pork tongue had eaten either a type of pâté (one case patient) or ham (three case patients) or both (11 case patients).
|
|
L. monocytogenes isolates recovered from 31 patients identified in Germany, Austria, Belgium, Denmark, and Spain were received by the NRC. One strain from a patient in Austria had the same serovar and DNA macrorestriction pattern as the outbreak strain. An investigation carried out by Austrian health officials concluded that it was highly unlikely that the patient had consumed French meat products.
Environmental and laboratory investigations. In total, 69 places of purchase of meat products reported by case patients were investigated. Among these were 22 possible places of purchase of jellied pork tongue. Analysis of distribution channels allowed us to eliminate certain manufacturers on the basis that their product had not been offered for sale in any of these shops, and it identified two major producers that had supplied the majority of the possible places of purchase with jellied pork tongue. However, the findings did not permit any given supplier or processing plant to be incriminated.
Characterization of 3,206 L. monocytogenes isolates recovered from different foods and environmental sources yielded 14 isolates with the same serovar and DNA macrorestriction pattern as the outbreak strain: nine isolates recovered from raw meat and dairy products between March and November 1999 and five isolates recovered from rillettes, raw sausage, and cheese in 2000. None of these foods were associated with infection with the outbreak strain in the case-control study, and very few case patients reported their consumption. With the exception of the rillettes, the contaminated foods either had not been commercialized or had been commercialized in a very restricted geographic area. The two strains isolated from rillettes had been recovered from two opened packages from different manufacturers at the delicatessen counters of two different shops. Inspection of both manufacturing plants and extensive sampling of their environment and products failed to yield the outbreak strain.
A posteriori, based on the distribution of cases over time, we determined the period in which consumers had most likely been exposed to the contaminated product. We considered that the outbreak had started in early December 1999 when cases were clustered in time. Based on a median incubation period of 21 days, exposure was estimated to have been maximal around December 30. The period of exposure started, at the latest, on December 13, 3 days before the occurrence of the first case, corresponding to the minimum incubation period described in the literature (figure 1) (12). The period of exposure ended, at the earliest, on February 9, 2000, 19 days before the occurrence of the last case (in a previously healthy adult), corresponding to the maximum incubation period described in the literature for a case without an underlying medical condition (13
).
Control measures. On February 17, 2000, meat processing and distribution professionals were informed of the outbreak and reminded of food hygiene precautions. On February 19, a press conference was organized informing the public of the occurrence of the outbreak and reminding pregnant women and immunocompromised persons of the general dietary recommendations for prevention of listeriosis. On February 22, based on preliminary results of the case-control study, health authorities advised the public not to consume any jellied pork tongue. Since the manufacturer of the incriminated product had not been identified, there was no official recall of products. However, the majority of retail shops removed all brands of jellied pork tongue from their shelves.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In spite of extensive sampling in retail stores and production plants, the outbreak strain was not recovered from the incriminated products. However, this failure to isolate L. monocytogenes from the incriminated products does not disprove the epidemiologic evidence regarding their role as the vehicles of transmission. The period of exposure to the con-taminated products was estimated to have ended around February 9, 2000, which suggests that sampling took place when the contaminated product was no longer in circulation. In addition, enhanced sanitation efforts made by meat producers and retailers after the media reported the outbreak could have reduced the likelihood of detection of the organism.
The failure to identify a common manufacturer or supplier does not indicate the absence of a common source. Rather, it can be explained by the limitations of information obtained through food questionnaires that collect data over a 2-month period and the difficulty of tracing the origin of products served at delicatessen counters on the basis of this information. Firstly, case patients did not identify any particular brand of products purchased at the delicatessen counter. Secondly, the majority of case patients mentioned several points of purchase of meat products; they were often unable to specify in which of these shops they had actually purchased their jellied pork tongue, or they had purchased jellied pork tongue at several different shops. Most retail shops surveyed had different brands of jellied pork tongue for sale, either concurrently or consecutively. Therefore, without an exact date of purchase of the contaminated product, it was very difficult to identify the exact supplier and manufacturer of the jellied pork tongue that had contaminated a particular case patient.
Case-case comparison (16), a case-control method made possible by laboratory typing techniques, was successfully applied in a recent outbreak of listeriosis in the United States (17
). In our investigation, we decided to apply this methodology using patients with sporadic cases of listeriosis as controls, for several reasons. Firstly, patients with sporadic cases are likely to have come from the same source population that gave rise to the outbreak cases, since they have been shown to be susceptible to development of the disease and would have been included as cases had they also developed the epidemic form of the diseasethus reducing the potential for selection bias. Secondly, because they experienced a disease similar to that of the cases, sporadic case patients used as controls may be more likely than healthy individuals to recall exposures with the same level of accuracy as epidemic case patientsreducing the potential for recall bias. In the context of the extensive media coverage of the recall of brand X rillettes and jellied pork tongue in early January 2000, the ability to obtain exposure information from questionnaires completed before the subject and the investigator were aware of the subject's case/control status was especially valuable, since it is unlikely that accuracy and completeness of data would be dependent on whether the subject later became a case. The disadvantage of using sporadic case patients as controls is that they differ from the non-ill susceptible population with regard to exposures that led to their infection. However, several studies have shown that sporadic listeriosis is associated with a large variety of different foods, including soft cheese, prepared salads, precooked and undercooked chicken, and cold meat products (3
, 18
20
). Since patients with a broad range of L. monocytogenes subtypes were included in the control group, the wide variety of different exposures involved should have minimized any bias. Although we cannot exclude the possibility that subjects with sporadic cases were less exposed to meat products, it is unlikely that this selection bias was sufficiently strong to account for the increased risk observed.
The second outbreak was the first occasion on which consumption of a product (jellied pork tongue) was discouraged on the basis of epidemiologic evidence alone, without incriminating a specific brand, dramatically reducing its consumption in the subsequent weeks. These outbreaks have also fueled a debate on regulation of the kinds of products known to be at risk for L. monocytogenes contaminationspecifically about the relevance and feasibility of implementing zero tolerance at the level of production, maintenance of the cold chain from producer to consumer, and the need to restrict shelf life for these types of products. At the time of the outbreak, contamination with L. monocytogenes could be tolerated in ready-to-eat meat products that were manipulated between submission to a listericidal treatment and final packaging, if studies were available showing that the contamination would not exceed 100 colony-forming units per gram at the end of the product's shelf life (21). In response to the outbreak, the French food safety agency recommended a zero-tolerance policy at production level for these ready-to-eat meat products (22
). In France, manufacturers fix the shelf life of their products, which can be up to 42 days for rillettes and jellied pork tongue. In response to outbreak 1, members of the meat processing association reduced their shelf life for rillettes to 28 days.
These two outbreaks show that the epidemic potential of listeriosis remains a reality, even in a context of decreasing listeriosis incidence resulting from Listeria control programs at the level of food production and distribution. A similar observation was made in the United States, where an outbreak of at least 100 cases occurred in 19981999, in spite of an important reduction in incidence (17). The consecutive occurrence of these outbreaks underlines the importance of timely ongoing case-reporting and systematic characterization of L. monocytogenes isolates, allowing detection of even small outbreaks at an early stage and discrete investigation of different clusters.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
NOTES |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|