Nearly 60 people fell ill in an outbreak of Salmonella infections in Australia in 2018 and 2019, but the source has not been found.

The outbreak of Salmonella Heidelberg infections affected five states over seven months. Most people were sick from December 2018 to late March 2019 but one case was reported in May, according to a study published in the journal Emerging Infectious Diseases.

Salmonella Heidelberg is uncommon in Australia. An average of 37 cases were reported annually in 2009 to 2017 that were mostly acquired overseas. Six outbreaks have been reported nationally since 1995; one in 1996 had more than 500 cases, but most had less than seven patients.

Overall, 59 outbreak cases were identified in 2018 and 2019 with 18 in New South Wales (NSW), 14 in Victoria, 13 in Queensland, eight in Western Australia, and six in South Australia. Patients were 2 months to 95 years old with a median age of 43. None had a history of international travel.

Of cases for which data were available, 16 were hospitalized with a median duration of four and range of one to 18 days.

Although Salmonella Heidelberg incidents are relatively uncommon in Australia, given this outbreak’s comparatively high hospitalization rate, future cases warrant prompt investigation to assess severity and invasiveness, said researchers.

Timeline of incident
In December 2018, OzFoodNet, Australia’s network for foodborne disease surveillance, noted that the 15 Salmonella Heidelberg cases diagnosed in November were above the national historical 5-year mean. New South Wales and Victoria started separate investigations in December 2018 and February 2019.

In February to March 2019, whole genome sequencing (WGS) analysis of isolates identified 36 related cases, 12 each from Queensland, NSW, and Victoria. Salmonella Heidelberg infections with WGS pending were identified in South Australia and Western Australia. Queensland cases were not initially investigated because Salmonella Heidelberg is more common in that state.

OzFoodNet opened an outbreak investigation in March 2019 but case numbers declined soon after, preventing more rigorous epidemiological investigation.

Thirty-nine patients completed interviews using hypothesis-generating questionnaires.

Laboratory investigations could not identify a source of infection but detected indicators of severity with a hospitalization rate of 36 percent.

Cooked chicken, macadamia nuts, frozen vegetable products and lamb were identified as foods of interest. Past Salmonella Heidelberg outbreaks have been associated with poultry or eggs and raw macadamia nuts were considered because of their popularity during Christmas and previous positive findings in Queensland. However, epidemiological and microbiological evidence was insufficient to develop a strong hypothesis for any food item so sampling was not done.

A platform for the real-time exchange of sequence data in Australia and use of routine WGS for salmonellosis cases, including comparison with local and international strain data, may enable more timely detection of outbreaks, said researchers.

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