The proportion of Listeria infections where a food source is identified is very small but increasing, according to researchers who looked at a 30-year period in England and Wales.

Between 1981 and 2015, there were 5,252 cases of human listeriosis in the two countries recorded. It is not often a specific exposure is identified because of the long incubation period of Listeria, low attack rate, prolonged colonization at production facilities, and complexity of the food chain, the researchers reported.

The study, published in the journal Epidemiology and Infection, reviewed foodborne listeriosis in England and Wales prior to introduction of whole genome sequencing (WGS) by Public Health England’s (PHE) national reference laboratory in 2015.

The proportion of all cases linked to specific foods was 3 percent between 2002 and 2006 with implementation of amplified fragment length polymorphism (AFLP) and 5 percent between 2007 and 2015 with fluorescent AFLP as the primary discriminatory typing tool. Out of 500 listeriosis infections, WGS has boosted the percentage of detected cases linked to specific foods to around 10 percent.

Prior to 1987, there were between 58 and 136 cases reported per year and from 1987 to 1989 this increased to 237 to 278 infections per year. There was a decline between 1990 and 2001 with annual totals from 90 to 146. The number rose to between 139 and 233 from 2002 to 2015.

Hospital outbreaks due to sandwiches
There was one outbreak with 378 people sick associated with pâté consumption and 112 cases attributed to specific foods in the other incidents. For the 5,252 cases, there were 28 incidents, or 480 cases, where specific foods were associated with transmission of listeriosis.

The 28 incidents included 11 sporadic cases and 17 outbreaks. Ten incidents were in hospitals with 37 cases and the remaining 18 with 443 cases occurred in the community.

The large outbreak between 1987 and 1989 resulted in 51 percent of reported cases attributed to a food during this period. Between 1981 and 2001 and excluding this outbreak, less than 0.8 percent of cases were linked to consumption of specific foods.

The 10 incidents in hospitals were a single sporadic case and nine outbreaks of between two and nine people. All were associated with pre-prepared sandwiches and one also implicated a salad product. The main food type associated with transmission of listeriosis in England and Wales during 1981 to 2015 was pre-prepared sandwiches served in hospitals.

In 2019, six people died after eating chicken sandwiches supplied to hospitals by the Good Food Chain. Meat was produced by North Country Cooked Meats and distributed by North Country Quality Foods.

Duration of all outbreaks ranged from four days to 32 months. All cases occurred within immunocompromised adults who were more than 60 years old except for one outbreak which affected five pregnant women and their unborn infants who ate sandwiches from a retail outlet in hospital whilst attending antenatal clinics.

In nine of the incidents, Listeria monocytogenes of the same type was recovered from patients’ specimens and implicated sandwiches collected either within the hospital or from the point of manufacture.

Sandwiches with various fillings were contaminated with implicated strains: seven had types of cooked meat, two eggs, five cheese, six salad or other plant based materials and five with fish or crustaceans. In eight incidents, the strain was recovered from environmental sites, utensils or equipment within the sandwich production environments and provided proof of cross-contamination at these factories. There was evidence of poor temperature control (higher than 8 degrees C or 46.4 degrees F) of sandwiches at hospitals in five of the incidents.

Missed opportunities for prevention
The 18 community incidents comprised eight outbreaks and 10 sporadic cases: food of animal origin was implicated in 16 events such as sliced or potted meats, pork pies, pâté, liver, chicken, crab-meat, butter and soft cheese and olives and vegetable rennet in the other two.

There was one large outbreak of 378 cases linked to pâté, four with 10 to 17 cases and the remaining three had between three and five cases. Five outbreaks lasted for between one and seven months and the other three over several years. In four outbreaks Listeria was isolated prior to onset of the first case by three months, nine months, 1.75 years or 2.5 years.

Researchers said the considerable lengths of time between recognition of hygiene problems and the onset of the first cases represented missed opportunities to prevent disease.

In all eight outbreaks, Listeria monocytogenes was indistinguishable between patient’s clinical specimens and food samples. Listeria monocytogenes strains associated with cases were recovered from foods collected from: a patient’s domestic refrigerator in one outbreak; the same retailers used by patients in three outbreaks and from foods or environmental sites collected at production in seven outbreaks.

PHE manages a network of Official Control laboratories in England which test 25,000 food and environmental samples for Listeria each year. This generates more than 700 Listeria monocytogenes isolates being sent for characterization to the reference lab.

Researchers said Listeria monocytogenes recovered from unrelated testing of samples from the food chain together with sampling foods from the refrigerators of patients should be considered essential components of surveillance for listeriosis. It is also important to integrate data over many years on a national and international basis.

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