Salmonellosis is the second most commonly reported bacterial gastro-intestinal infection after campylobacteriosis in the European Union (EU) and European Economic Area (EEA). In the EU/EEA, the yearly notification rate of salmonellosis has been around 20 cases per 100,000 individuals in the period from 2015 to 2019 . In 2020, the notification rate decreased to 14 cases per 100,000 individuals; the decline is considered to be an effect of the COVID-19 pandemic . Salmonella enterica serotype Newport was the fifth most common serotype in the EU/EEA in 2019, accounting for 1.1% of reported serotypes from human salmonellosis cases . In Sweden, human salmonellosis is notifiable by law. The average yearly notification rate of salmonellosis in Sweden between 2015 and 2019 was 21.5 cases per 100,000 individuals, with the majority of cases being infected abroad . In 2019, Salmonella Newport was the fifth most common serotype also in Sweden and accounted for 5% of the reported serotypes .
Salmonellosis is a bacterial zoonosis. Humans get infected by contaminated foods, through contact with infected animals or humans or via the environment . In recent years, egg and egg products, bakery products and pork and pork products have been the most common food vehicles in food-borne salmonellosis outbreaks reported in EU countries . Salmonella Newport has previously been linked to outbreaks caused by vehicles of both animal and vegetable origin, for example beef, watermelon and mung bean sprouts [5–8]. Salmonella Newport have also been detected in fish and different shellfish and in fresh herbs [9–11].
In late September 2019, the Public Health Agency of Sweden (PHAS) identified a cluster of nine cases with Salmonella Newport sequence type (ST) 46. The cluster was detected as part of the routine microbial surveillance programme where isolates of Salmonella from domestic infections are sent to PHAS for typing using whole genome sequencing (WGS). The cluster was put under observation; it evolved slowly and on 23 October, the cluster consisted of 25 cases. The cases were geographically spread across the country and all but one case were adults. The onset of disease ranged from 16 August to 12 October 2019. The spread of cases, geographically and in time, indicated that the infection source was a contaminated food that was distributed nation-wide and could still be on the market. A national outbreak was declared and an outbreak investigation was initiated with the objectives to describe the outbreak and identify the source in order to prevent further cases.
The outbreak team included investigators from PHAS, the Swedish Food Agency (SFA) and the affected regional Departments of Communicable Disease Control and Prevention (CDC department).
Here we report an investigation of a national outbreak of Salmonella Newport in Sweden, with the aim of describing the actions that led to the identification and recall of the source of the infection.