At least three outbreaks of listeriosis associated with seafood have been reported. Listeria monocytogenes is widely distributed in the general environment including fresh water, coastal water and live fish from these areas. Contamination or recontamination of seafood may also take place during processing and low levels (
The present situation regarding Listeria monocytogenes and ready-to-eat (RTE) seafood is discussed. An updated regulatory policy on L. monocytogenes directs inspection and compliance action to those RTE foods capable of supporting growth of the organism and is based on a combination of inspection, environmental sampling and product testing. The incidence of L. monocytogenes in imported seafood products in 1996-1997 and 1997-1998 was 0.88 and 0.3%, respectively. With respect to domestic products, an analysis of 347 RTE foods in 1997-1998 and 1998-1999, at one of the large fish inspection labs in the Maritimes, revealed an absence of L. monocytogenes. The only seafood product linked to suspect cases of listeriosis in Canada was imported.
The first lesson learned from this outbreak was that vacuum-packed rainbow trout is not only an excellent medium for the growth of Listeria monocytogenes, but may also cause human listeriosis. Another lesson is that one single fish processing plant may spread multiple clonal types of L. monocytogenes by selling contaminated products to consumers. Thus, when investigating fish-borne outbreaks of listeriosis one should identify and type several isolates of L. monocytogenes from each food and environmental sample, since multiple clonal types might be present. The outbreak described in this paper involved at least eight human cases, three clonal types of L. monocytogenes, and lasted for 11 months. During the outbreak investigation, L. monocytogenes was also isolated from another brand of rainbow trout found in the refrigerator of one of the patients. These latter isolates belonged to a clonal type not associated with the outbreak. However, this clonal type is of considerable interest since it has been associated with foodborne outbreaks of listeriosis in several countries, and is also the second most common clonal type among human clinical isolates of L. monocytogenes in Sweden. Besides the described outbreak, it is likely that vacuum-packed, cold-smoked and gravad rainbow trout have been involved in additional cases of foodborne listeriosis in Sweden.
Human listeriosis outbreaks in Canada have been predominantly caused by serotype 1/2a isolates with highly similar pulsed-field gel electrophoresis (PFGE) patterns. Multilocus sequence typing (MLST) and multi-virulence-locus sequence typing (MVLST) each identified a diverse population of Listeria monocytogenes isolates, and within that, both methods had congruent subtypes that substantiated a predominant clone (clonal complex 8; virulence type 59; proposed epidemic clone 5 [ECV]) that has been causing human illness across Canada for more than 2 decades.
The investigation of the bacterial populations' heterogeneity contributes to the control of natural foci, causative agents of nosocomial infections, to the analysis of the microbial evolution. Multilocus sequence typing (MLST) was employed for the analysis of the diversity and features of the distribution of polyhostal ubiquitous microorganisms of the genera Burkholderia, Leptospira, and Listeria, which belong to three bacterial phyla: Proteobacteria, Spirochaetes, and Firmicutes. According to the bacterial samples analysis microbial genotypes prevalent and unique to Russia were identified; their occurrence in different Federal Regions was investigated; their similarity with global spread genotypes was appreciated. Obtained results allowed identifying common regularities of the selection of the microorganisms capable to cause the diseases of human and animals. The formation of genotypes that are most pathogenic for the host was demonstrated for all groups of bacteria. Leptospira spp. and Listeria monocytogenes strains with these genotypes have been circulating for a long time, being supported by natural foci. The formation of a wide variety of genotypes with different pathogenicity was demonstrated in the local geographic areas. In Russia, the zonal difference in all three groups of bacteria is most clearly traced to the Far Eastern Federal Region. The results are thought to contribute to analyzing the factors of selection and the phylogeny of the taxa under study.
In this study, we investigated the fate of Listeria monocytogenes , pathogenic Yersinia enterocolitica , and Escherichia coli O157:H7 gfp(+) inoculated in low numbers into ready-to-eat baby spinach and mixed-ingredient salad (baby spinach with chicken meat). Samples were stored at recommended maximum refrigerator temperature (8°C in Sweden) or at an abuse temperature (15°C) for up to 7 days. Mixed-ingredient salad supported considerable growth when stored at 15°C during shelf life (3 days), with populations of L. monocytogenes , pathogenic Y. enterocolitica , and E. coli O157:H7 gfp(+) increasing from less than 2.0 log CFU/g on day 0 to 7.0, 4.0, and 5.6 log CFU/g, respectively. However, when mixed-ingredient salad was stored at 8°C during shelf life, only L. monocytogenes increased significantly, reaching 3.0 log CFU/g within 3 days. In plain baby spinach, only pathogenic Y. enterocolitica populations increased significantly during storage for 7 days, and this was exclusively at an abuse temperature (15°C). Thus, mixing ready-to-eat leafy vegetables with chicken meat strongly influenced levels of inoculated strains during storage. To explore the food safety implications of these findings, bacterial numbers were translated into risks of infection by modeling. The risk of listeriosis (measured as probability of infection) was 16 times higher when consuming a mixed-ingredient salad stored at 8°C at the end of shelf life, or 200,000 times higher when stored at 15°C, compared with when consuming it on the day of inoculation. This indicates that efforts should focus on preventing temperature abuse during storage to mitigate the risk of listeriosis. The storage conditions recommended for mixed-ingredient salads in Sweden (maximum 8°C for 3 days) did not prevent growth of L. monocytogenes in baby spinach mixed with chicken meat. Manufacturers preparing these salads should be aware of this, and recommended storage temperature should be revised downwards to reduce the risk of foodborne disease.
Prepacked ready-to-eat mixed ingredient salads (RTE salads) are readily available whole meals that include a variety of ingredients such as raw vegetables, cooked meat, and pasta. As part of a trend toward healthy convenience foods, RTE salads have become an increasingly popular product among consumers. However, data on the incidence of foodborne pathogens in RTE salads are scarce. In this study, the microbiological safety of 141 RTE salads containing chicken, ham, or smoked salmon was investigated. Salad samples were collected at retail and analyzed using standard methods for Listeria monocytogenes, Shiga toxin-producing Escherichia coli (STEC), pathogenic Yersinia enterocolitica, Salmonella, and Campylobacter spp.L. monocytogenes was isolated from two (1.4%) of the RTE salad samples. Seven (5.0%) of the samples were positive for the ail gene (present in all human pathogenic Y. enterocolitica isolates) and three (2.1%) of the samples were positive for the Shiga toxin genes stx1 and/or stx2. However, no strains of pathogenic Y.enterocolitica or STEC were isolated. Thus, pathogens were found or suspected in almost 1 of 10 RTE salads investigated, and pathogenic bacteria probably are present in various RTE salads from retail premises in Sweden. Because RTE salads are intended to be consumed without heat treatment, control of the ingredients and production hygiene is essential to maintain consumer safety. The recommended maximum storage temperature for RTE salads varies among countries but can be up to 8°C (e.g., in Sweden). Even during a short shelf life (3 to 5 days), storage at 8°C can enable growth of psychrotrophs such as L. monocytogenes and Y. enterocolitica. The maximum storage temperature should therefore be reduced.
Denmark has a high incidence of invasive listeriosis (0.9 cases/100,000 population in 2012). We analyzed patient data, clinical outcome, and trends in pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) of Listeria monocytogenes strains isolated in Denmark during 2002-2012. We performed 2-enzyme PFGE and serotyping on 559 isolates and MLST on 92 isolates and identified some correlation between molecular type and clinical outcome and patient characteristics. We found 178 different PFGE types, but isolates from 122 cases belonged to just 2 closely related PFGE types, clonal complex 8 and sequence type 8. These 2 types were the main cause of a peak in incidence of invasive listeriosis during 2005-2009, possibly representing an outbreak or the presence of a highly prevalent clone. However, current typing methods could not fully confirm these possibilities, highlighting the need for more refined discriminatory typing methods to identify outbreaks within frequently occurring L. monocytogenes PFGE types.
Cites: Int J Food Microbiol. 2001 Apr 11;65(1-2):55-6211322701
The objective of the present work was to develop a quantitative risk assessment model in which the exposure and risk of acquiring listeriosis from consumption of packaged smoked or gravad salmon and rainbow trout were estimated. An Excel spreadsheet model was constructed in which variables were represented by distributions based on surveys of L. monocytogenes in these food products, and on demographic and consumption data. Growth or inactivation was not included in the model. The model was run through Monte Carlo simulations using the @Risk software (Palisade Corporation). The probability of illness per serving was calculated using two dose-response models from the literature. The first was an exponential model in which the species specific constant R, that helps define the dose-response curve, previously has been estimated to be 1.18 x 10(-10) based on German data (GR). In this study, R was estimated to 5.6 x 10(-10) based on Swedish data. The second model was a flexible Weibull-Gamma model (WG), with different coefficients for high- and low-risk groups. The exponential model (GR), although conservative and generally overestimating the risk, still predicted a lower probability of illness than the WG-model. The estimated mean risk per serving was 2.8 x 10(-5) (GR, high-risk group), 2.0 x 10(-3) (WG, low-risk group) and 0.016 (WG, high-risk group), respectively. The average number of reported listeriosis cases in Sweden is 37 per year. In comparison, the mean number of annual cases predicted by the risk assessment model was 168 (range 47 to 2800, GR, high-risk group), and 95 000 (range 34 000 to 1.6 x 10(6), WG high-risk group), respectively. If 1 to 10% (uniform distribution) of strains, instead of all, were considered virulent, the mean number of predicted cases would decrease to nine (GR) and 5200 (WG), respectively. The mean annual cumulative individual risk in the high-risk group based on a monthly exposure was estimated to be 4.0 x 10(-4) (range 8.0 x 10(-8) to 5.4 x 10(-3), GR). This risk increased to 1.5 x 10(-3) (range 1.7 x 10(-5) to 9.2 x 10(-3), GR) based on a weekly exposure. The risk assessment model was most sensitive to the input distribution describing the level of contamination and to a lesser degree on the prevalence of L. monocytogenes, the proportion of virulent strains, and serving sizes. A lack of data on the prevalence and concentration of L. monocytogenes in these products, dose-response data and quantitative information on the proportion of virulent strains were identified.
An outbreak of listeriosis in Denmark occurred in May 2009. Multilocus variable number of tandem repeats analysis typing, later confirmed by pulsed-field gel electrophoresis typing, showed that isolates from eight patients had identical patterns and were distinguishable from Listeria monocytogenes isolates from other Danish patients. Seven out of eight patients had received a meal with beef from the same meals-on-wheels delivery catering company 3 weeks prior to onset of disease. Two patients died of their infection. Large-scale delivery of precooked meals to a vulnerable population represents a threat if proper measures against listeriosis are not taken.