National outbreak of Salmonella Give linked to a local food manufacturer in Malta, October 2016

Abstract

Salmonella Give is a rare serotype across Europe. In October 2016, a national outbreak of S. Give occurred in Malta. We describe the epidemiological, environmental, microbiological and veterinary investigations. Whole-genome sequencing (WGS) was performed on human, food, environmental and veterinary isolates. Thirty-six human cases were reported between October and November 2016, 10 (28%) of whom required hospitalisation. Twenty-six (72%) cases were linked to four restaurants. S. Give was isolated from ready-to-eat antipasti served by three restaurants which were all supplied by the same local food manufacturer. Food-trace-back investigations identified S. Give in packaged bean dips, ham, pork and an asymptomatic food handler at the manufacturer; inspections found inadequate separation between raw and ready-to-eat food during processing. WGS indicated two genetically distinguishable strains of S. Give with two distinct clusters identified; one cluster linked to the local food manufacturer and a second linked to veterinary samples. Epidemiological, environmental and WGS evidence pointed towards cross-contamination of raw and ready-to-eat foods at the local manufacturer as the likely source of one cluster. Severity of illness indicates a high virulence of this specific serotype. To prevent future cases and outbreaks, adherence to food safety practices at manufacturing level need to be reinforced.

Keywords: Salmonella Give; Salmonella typing; food safety; food-borne infections; outbreaks.

Fig. 1.

Number of suspected and confirmed (blue square inside) Salmonella Give cases by restaurant exposure reported in October 2016 in Malta by onset of disease (n = 36*).

Fig. 2.

Network diagram showing all known epidemiological links between the 26 confirmed (green) and suspected (orange) human cases by gender, four restaurants (A, B, C and D) and positive environmental/food samples originating from the local food manufacturer.

Fig. 3.

Maximum likelihood tree of complete sequences collected from confirmed case isolates (UK and Malta), food and environmental isolates originating from the local manufacturer and veterinary isolates (broiler chickens and swine). Each isolate in the tree is labelled with the date of sample collection. Human isolates in clusters 1 and 2 are labelled with restaurant exposure or as sporadic if known. The bootstrap values are shown.