The Staphylococcus aureus outbreak in a buffet restaurant at an international show-jumping event in June 2014 in Roeser hospitalised 31 people.
While 150-200 people were estimated to have eaten the buffet lunch in the VIP restaurant on both days and 31 were admitted to hospital emergency departments over the two days, the number of affected persons is unknown, said researchers in Eurosurveillance.
Genotypically identical strains were isolated in 10 patients, shiitake mushrooms, cured ham, and in three staff.
Case–control investigation dependence
The case–control study strongly suggested pasta salad with pesto as the vehicle of infection, but this could not be tested, because there were no leftovers, said the researchers.
Primary ingredients used to make the pesto sauce for the pasta salad (fresh basil, hard cheese, and pine nuts) were all negative for S. aureus.
Isolates of S. aureus from patients, food and catering staff were characterised and compared using traditional typing methods (PCR, spa-typing, and multilocus variable-number tandem repeat analysis (MLVA)) and whole genome sequencing (WGS).
Isolates of S. aureus from patients, food, and catering employees were confirmed by MALDI-TOF mass spectrometry.
Results from the analytical epidemiological case–control study implicated pasta salad with pesto as the most likely vehicle of SFP.
Eighteen of 22 cases reported eating this food item compared to three of 21 controls. All 14 interviewed cases who had been hospitalised reported eating the pasta salad with pesto.
“Our report shows that, even in the era of whole genome sequencing, public health investigations of foodborne outbreaks remain very dependent on classical case–control investigations for interpretation of events,” said the researchers.
“Whereas initial microbiological typing results suggested cured ham as the main vehicle for the intoxication, the case–control study clearly identified the pasta salad with pesto as the most likely source, which was no longer available for microbiological testing.
“Because three catering employees were colonised by a strain with the same genotype, it is likely that at least one of them may represent the source of food contamination, either via manual contact or through respiratory secretions.
“However, because catering employees were screened a week after the outbreak, it cannot also be ruled out that some staff members became colonised only during or after the event.”
S. aureus isolation
Isolates of S. aureus with a genotype identical to patient isolates were detected in cured ham samples (range <40–5,200 colony-forming units (CFU)/g and shiitake mushrooms (<40 CFU/g) sampled at the event site and in cured ham samples (enumeration range <40–120 CFU/g) from the offsite catering kitchen where the ham was sliced and stored.
Non-enterotoxigenic isolates of S. aureus with a different genotype to patient isolates were found in cooked asparagus (<40 CFU/g), the floating island dessert (<40 CFU/g) and several samples of cooked ham (range 50–320 CFU/g).
Unsliced complete legs of cured and cooked hams from the supplying butcher were negative for S. aureus.
All 18 food items sampled from the event buffet were negative for Salmonella and E. coli. One food item (cooked asparagus) was positive for presumptive Bacillus cereus (840 CFU/g).
The fact S. aureus was detected in several dishes including cured and cooked ham, at concentrations up to 5,200 CFU/g, suggests the cold chain before or during the event was interrupted to allow sufficient microbial growth during or following food manipulation, said the researchers.
In France staphylococcal food poisoning (SFP) has ranked as the first cause of foodborne outbreaks with 1,288 reported in 2012, 300 (23%) were due to SFP.
SFPs are thought to be under-reported due to the short duration of symptoms, only an estimated 10% of patients visit a hospital.
If a stool analysis is performed, the microbiological routine procedures often do not include testing for enterotoxigenic S. aureus unless specifically requested by the physician.
Staphylococcal enterotoxin (SE) is also highly stable and heat-resistant. Although the bacteria may have been inactivated by heating food prior to consumption and cannot be isolated from food or stool of the patient, the highly stable enterotoxins preformed by S. aureus may still be active.
“Investigation of a staphylococcal food poisoning outbreak combining case–control, traditional typing and whole genome sequencing methods, Luxembourg, June 2014”
Authors: J Mossong, F Decruyenaere, G Moris, C Ragimbeau, CM Olinger, S Johler, M Perrin, P Hau, P Weicherding