Cross contamination and temperature abuse are factors that played a role in an outbreak of Salmonella in Hong Kong earlier this year.

In May, the Centre for Health Protection (CHP) of the Department of Health recorded 236 illnesses linked to consumption of sandwiches produced in a food factory with more than 50 people needing hospital treatment.

Cross contamination of raw eggs by food handlers at the factory, prolonged storage of sandwiches at improper temperatures during delivery and at retail were suspected to be contributing factors to the outbreak.

The most commonly consumed items included a ham, cheese and egg sandwich by 166 cases, ham and egg sandwich by 50 cases and a cheese and egg sandwich by 29 patients.

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

The 236 infections included 87 males and 149 females and ages ranged from 10 months to 68 years old. Symptoms included diarrhea, abdominal pain, fever, nausea and vomiting.

Sandwiches had been purchased from nine different retail outlets and one food factory and were eaten immediately to 48 hours later. The incubation period ranged from 0.5 to 87.5 hours.

Fifty-one people needed hospitalization including one person who required intensive care; 149 patients sought treatment from private practitioners or outpatient clinics; and 38 did not seek medical attention. All hospitalized patients have now been discharged.

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

Preparation problems
The Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department (FEHD) investigation revealed the implicated sandwiches were supplied from the same factory in Kwun Tong, which provided sandwiches to 12 retail outlets.

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

Whole genome sequencing on isolates from the sandwich samples, sandwich leftovers and stool samples of 38 affected people revealed they all belonged to ST (sequence type)-11.

Review of the food preparation process at the factory in Kwun Tong found fried egg sheets were prepared from an egg mixture of unpasteurized eggs, and were cut and stacked before being stored in the fridge for later use. The same working table was used for preparing raw and cooked egg sheets. It was noted a food handler was not wearing gloves during preparation of egg sheets.

All food handlers shared the same towel for drying after hand washing, and it would only be washed after close of business. Ten food handlers reported they did not have gastroenteritis symptoms and stool samples were all negative for Salmonella.

Packed sandwiches were delivered from the food factory to retail outlets throughout the day using one vehicle without temperature control equipment. Unsold sandwiches were sometimes moved between retail outlets for replenishment of stock.

All retailers stopped selling affected products and the factory was told to halt production of the food items on May 21. All patients ate sandwiches before control measures were put in place.

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