Outbreak of staphylococcal food poisoning among children and staff at a Swiss boarding school due to soft cheese made from raw milk

On October 1, 2014, children and staff members at a Swiss boarding school consumed Tomme, a soft cheese produced from raw cow milk. Within the following 7 h, all 14 persons who ingested the cheese fell ill, including 10 children and 4 staff members. Symptoms included abdominal pain and violent vomiting, followed by severe diarrhea and fever. We aim to present this food poisoning outbreak and characterize the causative agent. The duration of the incubation period was dependent of the age of the patient: 2.5 h in children under 10 yr of age, 3.5 h in older children and teenagers, and 7 h in adults. The soft cheese exhibited low levels of staphylococcal enterotoxin (SE) A (> 6 ng of SEA/g of cheese) and high levels of staphylococcal enterotoxin D (> 200 ng of SED/g of cheese). Counts of 107 cfu of coagulase-positive staphylococci per gram of cheese were detected, with 3 different Staphylococcus aureus strains being present at levels > 106 cfu/g. The 3 strains were characterized using spa typing and a DNA microarray. An enterotoxinproducing strain exhibiting sea and sed was identified as the source of the outbreak. The strain was assigned to spa type t711 and clonal complex 8, and it exhibited genetic criteria consistent with the characteristics of a genotype B strain. This genotype comprises bovine Staph. aureus strains exclusively associated with very high within-herd prevalence of mastitis and has been described as a major contaminant in Swiss raw milk cheese. It is therefore highly likely that the raw milk used for Tomme production was heavily contaminated with Staph. aureus and that levels further increased due to growth of the organism and physical concentration effects during the cheese-making process. Only a few staphylococcal food poisoning outbreaks involving raw milk products have been described. Still, in view of this outbreak and the possible occurrence of other foodborne pathogens in bovine milk, consumption of raw milk and soft cheese produced from raw milk constitutes a health risk, particularly when young children or other members of sensitive populations are involved.