Identifying high-risk foods in a case–control study with sporadic cases can be difficult due to the ubiquity of the microorganism in the environment and fluctuating rates of contamination in food, but also because some risky products are frequently eaten in the control population and the varying incubation period, said the researchers in Eurosurveillance.
Food products could be shown to be high-risk by subtyping and comparing Listeria isolates found in humans and food. Although finding similar strains does not prove causality, it could provide information about possible sources of infection and help understanding of trends in human cases.
Outbreaks are often associated with errors during food production, such as contaminated slicing machines followed by opportunities for pathogen growth.
Case-control study
Researchers performed a case–control study to determine host- and food-related risk factors for non-perinatal listeriosis in the Netherlands.
Firstly, underlying diseases and use of medicines were analysed, adjusted for sex, age group, season and level of urbanisation.
In the second phase of the logistic analyses, the food consumption of cases and controls was analysed including only highly susceptible cases and controls (with underlying diseases or taking immunosuppressants).
Food consumption consisted of 10 variables of meat, eight fish and seafood and five dairy products.
Patients with non-perinatal listeriosis between July 2008 and December 2013 were compared with controls from a periodic control survey who completed a questionnaire in the same period.
Higher age, male sex, underlying disease, especially cancer and kidney disease, and use of immunosuppressive medicine were strong risk factors for acquiring non-perinatal listeriosis.
Between July 2008 and December 2013, 406 patients with non-perinatal listeriosis were reported, 241 men (59%), 163 women (40%) and two of unknown sex (0.5%).
Incidence increased with age, but it was more pronounced in men than in women.
Forty-one of 345 cases with known status died (12%), for 61 this was unknown.
A total of 2,363 controls completed the questionnaire.
Infection with L. monocytogenes serotype 4b was most common (35%), followed by serotype 1/2a (24%) and serotype 1/2b (12%); serotype 1/2c (2%) and 3b (< 1%) were rarely seen, and for 75 patients (27%) no serotype was available.
Thirty-one of the 273 listeriosis cases included in the case–control analyses died (11%), for six the outcome was unknown.
No high risk food identified
Analysis of food in the group of cases and controls with underlying diseases did not reveal any high-risk products.
Eight of the 10 meat products, two of eight fish and seafood and all five dairy items were eaten significantly less often by the cases than by the controls. The remaining eight products were eaten as often.
“In our analyses, some food products generally recognised as risk foods for Listeria were significantly less often eaten by cases than by controls with similar host susceptibility. This probably indicates that persons with underlying diseases (cases and controls) have some knowledge about high-risk food and to some extent avoid eating such products,” said the researchers
“Because of the ubiquity of the microorganism in the environment and the psychrotrophic nature of the bacterium, a wide variety of food products can become contaminated with L. monocytogenes and, when prepared without heating just before consumption, can infect a susceptible person.”
The disease burden of 14 foodborne pathogens in the Netherlands was assessed in 2009.
Listeriosis on population level was low (12th place; 96 disability adjusted life years (DALY) per year) but ranked second on individual level (1,220 DALY per 1,000 cases of illness).
Voluntary surveillance for human listeriosis was started in 2005 in the Netherlands and it became a notifiable disease in December 2008.
Measures to minimise contamination are use of high-quality ingredients, hygienic manufacturing practices, indication of appropriate shelf-life, correct refrigerated storage, education of food handlers and food service managers, and monitoring of food industry, catering and retail.
Source: Eurosurveillance, Volume 20, Issue 31, 6 August 2015
“Risk factors for sporadic Listeriosis in the Netherlands, 2008 to 2013”
Authors: I H Friesema, S Kuiling, A van der Ende, M E Heck, L Spanjaard, W van Pelt