The number of cases between March 2009 and February 2012 was 30 with a fatality rate of 36.7%.
Cheese was the probable source of infection, found the report in Eurosurveillance.
“The long duration of this outbreak (March 2009 to February 2012) is noteworthy and reinforces the importance of setting up an effective multidisciplinary team able to help ensure rapid notification of cases and the prompt submission of L. monocytogenes isolates for routine laboratory typing,” said the researchers.
A retrospective study involving 25 national hospitals led to the awareness of the outbreak – meaning the time between the presumed onset and its detection was 16 months.
Notifiable disease but no active surveillance
In Portugal, listeriosis has been notifiable since April 2014, but there is no active surveillance programme.
As there is no such programme, outbreak detection is extremely difficult and work on the one reported was due mainly to retrospective investigations, said the researchers.
The outbreak was investigated by the national health (Direção Geral de Saúde and Administração Regional de Saúde de Lisboa e Vale do Tejo) and food safety (Autoridade de Segurança Alimentar e Económica) authorities with the Listeria Research Centre of Escola Superior de Biotecnologia (LRCESB).
Between January and July 2010, a high number of listeriosis cases was observed (40 compared with 20 during all of 2009) particularly in the Lisbon and Vale do Tejo region
Molecular typing of the 40 L. monocytogenes clinical isolates revealed 18 serotype IVb isolates presented the same PFGE type and ribotype observed for five isolates recovered in 2009.
Continued monitoring detected two more cases with the outbreak strain in November 2010, three more in January, February and March 2011 and two in February 2012.
Processing plant investigation
The food safety authority inspected 42 food retailers and collected 103 samples for analysis (51 meat products, 24 dairy products, 13 ready-to-eat foods and 15 environmental swabs).
L. monocytogenes was detected in four samples collected at a retailer: three from queijo fresco and one from a swab from a ham slicing-machine; one queijo fresco sample contained counts greater than 100 colony-forming units/g.
PFGE typing revealed isolates recovered from two queijo fresco samples of different brands from the same retailer showed the same PFGE type as the clinical isolates with the outbreak strain.
The outbreak strain was detected in L. monocytogenes isolates from cheese samples from one of the two processing plants investigated in the Alentejo region.
Although the cheese from another producer was contaminated at retail this was probably a result of cross-contamination since no positive samples were found in the processing plant, said the study.
The food safety authority recalled both products and analysed more samples.
Cheeses made with pasteurised cow and goat milk collected at the plant tested positive for L. monocytogenes and the isolates had the same PFGE pattern as the outbreak strain.
In March 2011 the processing plant voluntarily suspended activities during 15 days. After cleaning and disinfection measures, intensified product and environmental sampling was done. No positive samples were detected so products were allowed to be sold.
Samples were collected monthly by the food safety authority but none were positive.
Researchers said the increasing incidence and distribution of cases is ‘of concern’.
“In European countries with established surveillance programmes, such as France, Germany and the UK, the incidence of listeriosis is reported to be increasing and the distribution of cases is shifting, primarily affecting elderly persons and those with predisposing medical conditions, leading to a high case fatality rate.
“Data gathered from the surveillance of human disease and also from all stages in the food production chain should be continuously collected and analysed to understand the ecology of L. monocytogenes and its routes of transmission.
“This will be crucial for developing enhanced strategies to control this organism and contribute to a decrease in the incidence of food-borne listeriosis in Portugal.”
Source: Eurosurveillance, Volume 20, Issue 17, 30 April 2015
“Cheese-related Listeriosis outbreak, Portugal, March 2009 to February 2012”
Authors: R Magalhães, G Almeida, V Ferreira, I Santos, J Silva, M M Mendes, J Pita, G Mariano, I Mâncio, M M Sousa, J Farber, F Pagotto, P Teixeira