Researchers have stressed the need for awareness of long incubation periods as part of an investigation into a Salmonella outbreak in Norway in 2017.

In September 2017, a cluster of monophasic Salmonella Typhimurium isolates was identified at the National Reference Laboratory for Enteropathogenic Bacteria in Norway. The dispersion of cases around Norway initially suggested an outbreak linked to a nationally distributed product, according to the study published in the journal Eurosurveillance.

Incubation periods ranged from zero to 16 days and increased as the outbreak progressed, likely due to increasingly low-dose exposure as control measures were implemented. The incubation period of salmonellosis is typically between six and 72 hours.

Researchers recommended strengthened hygiene in the case of established environmental contamination as control measures, including temporary closure and kitchen refurbishment, failed to eliminate the environmental source.

Joe & The Juice café
The 21 people had consumed food or drinks from the Joe & The Juice café at Oslo Airport, which was outside the security restricted area in the terminal and accessible to anyone visiting the airport. The source of infection was either introduced via a contaminated food item or infected staff.

The café, wher large quantities of fresh ingredients are processed, is part of an international chain with the company operating sites in seven European countries and nations outside Europe.

Thirteen of those ill were women and ages ranged from 17 to 60 years old. Those sick lived in 10 geographically dispersed counties in Norway. International requests revealed no infections in other European countries.

Date of symptom onset was available for 16 cases and ranged from Aug. 23 to Nov. 18, 2017, with most in the first week of the outbreak. For the 15 cases with available date of symptom onset and of exposure, the incubation period ranged between zero and 16 days and increased as the outbreak progressed. The median incubation period in August was 4.5 days (range zero to five) and nine days (range two to 16) from September onwards.

Dates of onset for the first cases between Aug. 22 and 28 suggested the outbreak was caused by a source introduced at the café in mid-August. During the next several weeks it was open, which meant ongoing exposure for customers and staff. Duration of the outbreak from August until November and the increasing incubation period over this time suggested the source was environmental as customers were continuously exposed but the dose of Salmonella decreased.

In mid-September 2017, the National Reference Laboratory (NRL) for Enteropathogenic Bacteria for human matrices at the Norwegian Institute for Public Health reported a cluster of six monophasic Salmonella Typhimurium isolates sharing a rare multiple locus variable-number tandem repeat analysis (MLVA) type. The cases resided in five different municipalities in Norway and no travel abroad was reported in the week before symptom onset.

Four initial cases were interviewed using a standardized 19-page Salmonella-specific trawling questionnaire. Following these interviews, the questionnaire was shortened to focus on categories of interest, which included domestic travel before symptom onset and food items eaten in cafés at Oslo Airport.

All 21 cases reported having consumed food or drink at the café between Aug. 18 and Oct. 13, 2017. They had at least three different types of sandwiches and seven fruit or vegetable juices; no common food or drink was reported by all interviewed cases, who included café staff members.

Café repeatedly closed and reopened
Environmental swabs and food specimens were collected by Mattilsynet during site inspection in September and environmental swabs in November 2017. Six environmental specimens taken from a kitchen drain, water tap and a wall mounted steel shelf were positive for the outbreak strain. All 10 food specimens were negative for Salmonella.

“The inspections identified several weaknesses in the hygiene routines at the café, including separation between clean and unclean kitchen areas, use of appropriate detergents for different cleaning purposes and which basins that should be used to wash hands, food items and dishes, respectively. Furthermore, the washing routines of workwear was not optimal and there was lacking assessment and management of potential risks connected with the work processes and ingredients,” according to the study.

In October, environmental samples collected by the company were analyzed at a private lab, sent to the Norwegian Veterinary Institute for verification and submitted to the NRL at NIPH for typing. The café operator provided documentation on wholesalers wher they had sourced food with some of them outside Norway.

On Sept. 22, the café closed temporarily before reopening on Oct. 13. The next day it closed again as preliminary results found Salmonella before reopening on Oct. 25. On Nov. 7, one specimen with suspected Salmonella was reported and the café closed again. New specimens collected on Nov. 8 and 15 tested negative and environmental specimens by Mattilsynet on Nov. 21 were also negative.

Researchers said the company undertook control measures, including repeated voluntary closure, refurbishment of the kitchen and deep cleaning but eliminating the environmental contamination source proved difficult.

“This was likely due to the environmental contamination with specimens taken from several sites e.g. a kitchen drain, a tap and a steel shelf, all testing positive for the outbreak strain. The environmental inspection revealed several weaknesses in kitchen hygiene and food handling routines, possibly facilitating the cross-contamination of food items and protraction of the outbreak.”