Source remains unknown in Salmonella outbreak in Australia

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Nearly 60 people fell ill during 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 for seven months. Most people were sick from December 2018 to the end of March 2019, but one case was reported in May, according to a study published in the journal Emerging infectious diseases.

Salmonella Heidelberg is rare in Australia. On average, 37 cases were reported each year from 2009 to 2017, mostly acquired abroad. Six outbreaks have been reported nationally since 1995; one in 1996 had over 500 cases, but most had fewer than seven patients.

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

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

Although incidents of Salmonella Heidelberg are relatively rare in Australia, given the relatively high hospitalization rate of this outbreak, future cases warrant rapid investigation to assess severity and invasiveness, the researchers said.

Chronology of the incident
In December 2018, OzFoodNet, Australia’s Foodborne Illness Surveillance Network, noted that the 15 cases of Salmonella Heidelberg diagnosed in November were above the national historic 5-year average. New South Wales and Victoria opened separate investigations in December 2018 and February 2019.

From February to March 2019, whole genome sequencing (WGS) analysis of the isolates identified 36 linked cases, 12 each from Queensland, New South Wales and Victoria. Salmonella Heidelberg infections with pending WGS have been identified in South Australia and Western Australia. The Queensland cases were not investigated initially as Salmonella Heidelberg is more common in that state.

OzFoodNet opened an investigation into the outbreak in March 2019, but the number of cases declined soon after, preventing a more rigorous epidemiological investigation.

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

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

Cooked chicken, macadamia nuts, frozen plant products, and lamb have been identified as foods of interest. Past outbreaks of Salmonella Heidelberg have been linked to poultry or eggs and raw macadamia nuts have been taken into account due to their popularity over Christmas and previous positive results in Queensland. However, there was insufficient epidemiological and microbiological evidence to develop a strong hypothesis for any food, so sampling was not carried out.

A platform for the real-time exchange of sequence data in Australia and the use of routine WGS for salmonellosis cases, including comparison with data from local and international strains, could enable faster detection epidemics, the researchers said.

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