Listeriosis is a foodborne disease with a high fatality rate, and infection is mostly transmitted through ready-to-eat (RTE) foods contaminated with Listeria monocytogenes, such as gravad/smoked fish, soft cheeses, and sliced processed delicatessen (deli) meat. Food products/dishes stored in vacuum or in modified atmospheres and with extended refrigerator shelf lives provide an opportunity for L. monocytogenes to multiply to large numbers toward the end of the shelf life. Elderly, pregnant women, neonates, and immunocompromised individuals are particularly susceptible to L. monocytogenes. Listeriosis in humans manifests primarily as septicemia, meningitis, encephalitis, gastrointestinal infection, and abortion. In the mid 1990s and early 2000s a shift from L. monocytogenes serovar 4b to serovar 1/2a causing human listeriosis occurred, and serovar 1/2a is becoming more frequently linked to outbreaks of listeriosis, particularly in Europe and Northern America. Consumer lifestyle has changed, and less time is available for food preparation. Modern lifestyle has markedly changed eating habits worldwide, with a consequent increased demand for RTE foods; therefore, more RTE and take away foods are consumed. There is a concern that many Listeria outbreaks are reported from hospitals. Therefore, it is vitally important that foods (especially cooked and chilled) delivered to hospitals and residential homes for senior citizens and elderly people are reheated to at least 72°C: cold food, such as turkey deli meat and cold-smoked and gravad salmon should be free from L. monocytogenes. Several countries have zero tolerance for RTE foods that support the growth of Listeria.
Since 1986, 68% of the Listeria monocytogenes isolates from human cases of invasive listeriosis in Sweden are available for retrospective studies. The aim of the present study was to characterize 601 human invasive isolates of L. monocytogenes in Sweden from 1986 to 2007 by using serotyping and pulsed-field gel electrophoresis. Since 1996, serovar 4b was permanently reduced to the second or third most common serovar in human cases in Sweden. During the latter period, 2000-2007, only 13% belonged to serovar 4b and 71% to 1/2a. The dendrogram, based on pulsovars, reveals two clusters with different serovars. Cluster 1 exhibits serovars 4b and 1/2b, whereas cluster 2 consists of serovar 1/2a. Serovar 1/2a seems to be more heterogeneous than serovar 4b.
Isolates of Listeria monocytogenes (n = 932) isolated in Sweden during 1958-2010 from human patients with invasive listeriosis were characterized by serotyping and pulsed-field gel electrophoresis (PFGE) (AscI). Of the 932 isolates, 183 different PFGE types were identified, of which 83 were each represented by only one isolate. In all, 483 serovar 1/2a isolates were distributed over 114 PFGE types; 90 serovar 1/2b isolates gave 32 PFGE types; 21 serovar 1/2c isolates gave nine PFGE types; three serovar 3b isolates gave one PFGE type; and, 335 serovar 4b isolates gave 31 PFGE types. During the 1980s in Sweden, several serovar 4b cases were associated with the consumption of European raw soft cheese. However, as cheese-production hygiene has improved, the number of 4b cases has decreased. Since 1996, serovar 1/2a has been the dominant L. monocytogenes serovar in human listeriosis in Sweden. Therefore, based on current serovars and PFGE types, an association between human cases of listeriosis and the consumption of vacuum-packed gravad and cold-smoked salmon is suggested.
Gravad (Gravlax) and cold-smoked salmon are associated with human listeriosis in Sweden. The present investigation of Listeria monocytogenes in salmon was a follow-up of a similar study in the middle of the 1990s. Since our first study, there has been an increasing focus on food hygiene in general and specially on self-inspection and Hazard Analysis and Critical Control Points from the authorities and food producers. L. monocytogenes was isolated from 11 (three manufacturers) of the 56 products analyzed. The highest level of L. monocytogenes was 1500 cfu/g from a cold-smoked salmon product; however, the level was low (<100 cfu/g) in most products. Serovar 1/2a was predominant, followed by 4b. restriction enzyme analysis/ pulsed-field gel electrophoresis typing of the 56 salmon isolates identified five types of L. monocytogenes. One type was identical to a human type, whereas two other were closely related. These findings suggest that gravad and coldsmoked salmon are still possible sources of listeriosis in Sweden.
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