Listeria is back

An alarming rise in the rate of the potentially fatal form of food poisoning is baffling public health officials. So far this year the HPA has recorded a 65 percent rise in cases of listeriosis with 89 cases reported up to week 23 compared with 54 over the same period last year. Both men and women over 60 are affected and the increase cannot be explained by individual outbreaks.

Stuart Spears writing in Environmental Health Practitioner explained in their August issue why listeria has the power to provoke panic.

“Between 20 percent and 40 percent of those who contract listeriosis can expect to die. It is this high mortality rate that explains why an upward trend in rates in the last six years, compounded by a sharp increase in reported cases so far this year, has become a source of serious concern”.

Listeria Hysteria’ was one of the many headlines in newspapers in 1988 when pregnant women and their newborn babies were the most likely victims. The advice then was for this vulnerable group to avoid manufactured pate and soft cheeses.

What has changed is that, since 2001, the increase has been predominantly among patients aged over 60 while rates among other vulnerable groups, such as the immuno-compromised, the pregnant and the newborn, have remained relatively constant.

Change in food chain

Dr Jim McLauchlin, director of the Health Protection Agency food safety microbiology laboratory at Colindale has said:

“What we are sure of is that something really has changed and I believe that the cause has something to do with the food chain”.

Listeria food poisoning can happen at any point in the food chain whether on the farm, at the point of production or in the domestic kitchen. Another factor that makes it difficult for the HPA to identify the source of infection is that listeriosis can take up to 3 months to incubate. This makes it hard for the patient to remember what they have eaten. Secondly, the seriousness of the disease does not make this an easy group to study. Some are very ill with the disease and others die, while many are difficult to interview.

Foods linked to cases of listeriosis

Sandwiches have been found to be the culprits in several outbreaks and sliced cooked meat has also been implicated. However, unlike the 1980s, there is no single source of contamination and the majority of cases are now sporadic.

Earlier this year, routine sampling of sandwiches by EHPs from Ashford BC found listeria at Anchor Catering resulting in thousands of sandwiches destined for hospitals, schools and businesses across the South East being recalled and the company voluntarily halting production.

One cluster of listeriosis cases was found to be linked to a different sandwich production facility where investigators traced the source back to contaminated machinery parts. They also discovered that once the sandwiches had reached the catering outlet in a hospital, they had been stored at the wrong temperature thus allowing the low levels of bacteria to multiply.

Advice to caterers

In the light of new information, it is always good advice to review your procedures for high risk foods including sandwich preparation and handling. Always ensure that raw ingredients including cress and lettuce are thoroughly washed as cress in particular has been found to be a source of listeria contamination. Ideally, sandwiches and salads should be prepared in a temperature controlled environment, but if this is not possible then prepare in small batches so that the temperature of ingredients does not rise significantly. Once made, chill salads and sandwiches to below 8°C before putting on display. Check food temperatures often – do not rely of digital read-outs as these only measure air temperatures and are often inaccurate. If the temperature of the sandwiches rises slightly above 8°C keep them for a maximum of 4 hours and then discard.