Cross-contamination and temperature abuse are factors that contributed to the Salmonella outbreak in Hong Kong earlier this year.

In May, the Ministry of Health’s Center for Health Protection (CHP) recorded 236 illnesses related to consumption of sandwiches produced in food factories with more than 50 people who needed hospital treatment.

Cross-contamination of raw eggs by food handlers at the factory, storing of sandwiches for too long at inappropriate temperatures during shipping and in retail is thought to be a contributing factor to the outbreak.

The most frequently consumed items included 166 ham cases, cheese and eggs, ham and egg sandwiches by 50 cases and cheese and egg sandwiches by 29 patients.

Details of those affected
CHP was told about first two groups on May 21. The sandwiches involved in both clusters were purchased at the same retail outlet in Tsuen Wan on the same day. CHP identified 99 groups of suspected food poisoning related to sandwiches. Their sizes range from two to five cases.

236 infections including 87 men and 149 women and ages ranging from 10 months to 68 years. Symptoms include diarrhea, abdominal pain, fever, nausea and vomiting.

Sandwiches have been purchased from nine retail outlets and one food factory and eaten immediately up to 48 hours later. The incubation period ranges from 0.5 to 87.5 hours.

Fifty-one people need to be hospitalized including one who needs intensive care; 149 patients sought treatment from private practitioners or outpatient clinics; and 38 did not seek medical attention. All patients treated at the hospital have now been discharged.

A total of 53 stool specimens were collected for culture, of which 37 yielded Salmonella and five species of Salmonella grew. Salmonella is cultured from blood specimens of people who need intensive care. Leftover food was provided by one patient for testing, from which Salmonella was isolated.

Problems with preparation
The Food Safety Center (CFS) investigation from the Department of Hygiene and Food (FEHD) revealed that the sandwiches involved were supplied from the same factory in Kwun Tong, which provides sandwiches to 12 retail outlets.

A question by CFS FEHD at a retail outlet in Tsuen Wan revealed a refrigerator for storing sandwiches recorded 19 degrees C to 20.7 degrees C (66 degrees F to 69 degrees F), which is much higher than the recommended storage temperature at or below 4 level. C (39 degrees F). No expiration or production date was found on the sandwich package. Two positive sandwich samples for Salmonella.

Intact genome sequencing of isolates from sandwich samples, remaining sandwiches and fecal samples from 38 affected people revealed that they all belonged to ST (sequence type) -11.

An overview of the food preparation process at the factory in Kwun Tong found sheets of fried eggs prepared from egg mixes that were not pasteurized, and cut and stacked before being stored in the refrigerator for later use. The same workbench is used to prepare raw and cooked eggs. It is noted that a food handler does not wear gloves when preparing egg sheets.

All food handlers share the same towel for drying after washing hands, and that will only be washed after business closure. Ten food handlers reported they had no symptoms of gastroenteritis and fecal samples were all negative for Salmonella.

Sandwich packets are sent from food factories to retail outlets all day using one vehicle without temperature control equipment. Unsold sandwiches are sometimes transferred between retail outlets for stock replenishment.

All retailers stopped selling affected products and factories were told to stop producing food items on May 21. All patients ate sandwiches before the control steps were taken.

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