Researchers have reported on a rare Salmonella serotype that caused an outbreak in Australia from 2017 to 2018.

Salmonella Hessarek was responsible for 25 infections in South Australia between March 2017 to July 2018 linked to one unnamed brand of eggs which are produced and predominantly sold in the state.

The team said any substantial changes in the epidemiology of egg-associated Salmonella needs to be considered in future risk assessments and reviews of regulation.

From Jan. 1, 2012, to Dec. 31, 2016, there were 96 notifications of Salmonella Hessarek nationally. Of these, 52 were for residents of South Australia.

Past clusters of Salmonella Hessarek infection in South Australia involved illness linked to eggs; in 2014, an investigation of eight cases found seven of them consumed chicken and six ate eggs with three having free-range eggs from the same brand as the 2017-2018 outbreak and, in January 2016, a probe of six cases found four ate eggs, with one reporting consumption of these same branded eggs and another purchase from an unknown make in the region where this brand of eggs are produced.

Outbreak investigation

The South Australian Communicable Disease Control Branch (CDCB) saw an increase in Salmonella Hessarek notifications around mid-2014. Between November 2016 and February 2017, two to six cases per month were recorded amongst a total of 136 Salmonella monthly notifications.

In March 2017, CDCB became aware of five Salmonella Hessarek cases in the three weeks since the start of the month and started an investigation to identify any common cause of illness and prevent further cases.

The age of the 25 people involved ranged from 1 to 91 years old with nine cases aged 71 years or older. Fifteen patients were male and ten female. Twenty-one lived in metropolitan Adelaide and four were non-metropolitan residents. Ten were hospitalized and two other infections were in pregnant women.

Twenty-four people reported eating eggs; 17 were known to have consumed one brand of free-range eggs while another type was named by three cases.

Based on responses from seven people interviewed, the SA Health Food and Controlled Drugs Branch (FCDB) did retail sampling of the brand of free-range eggs.

One of four samples was positive for Salmonella Hessarek in contents of the eggs. Salmonella was not present in any whole egg rinses of the egg samples. FCDB found Salmonella Hessarek had been isolated from the content of this brand of eggs but not the whole egg rinse during a retail food survey in 2014.

Salmonella control

Researchers said the proportion of people reporting eating this one brand of free-range eggs and isolation of Salmonella Hessarek from sampling four dozen eggs of the brand was an “unusually strong signal” implicating them as the outbreak source.

“The ongoing nature of this outbreak reflects the difficulty in controlling Salmonella infection in free range laying flocks. Brand X eggs are produced on a free-range farm and birds raised in free range production systems are potentially exposed to different environmental stressors than caged birds, including social stress and aggression, predation, or thermal challenges with stress known to be a determinant of shedding of Salmonella. Additionally, the control of rodents and other potentially infected animals and environments is challenging on free-range farms.”

Egg washing with sanitizers is a common method to reduce eggshell contamination in Australia, Japan, and the United States but is banned in the European Union.

Eggs were cooked in a variety of ways including fried, boiled, scrambled, poached, as omelette and in Béchamel sauce. Seven cases consumed raw egg in smoothies, raw cake batter and one sucked raw eggs.

Salmonella and eggs in Western Australia

Meanwhile, a separate study has looked at the prevalence, serovar diversity, multilocus sequence types, and antimicrobial resistance of Salmonella contamination in retail eggs produced and sold in Western Australia.

“In recent years, the number of human salmonellosis cases in Western Australia (WA) has increased more dramatically than in any other Australian state. In 2017, the number of cases in WA was more than double the five-year average, and eggs had emerged as the key culprit for several Salmonella foodborne disease outbreaks,” according to the study published in the International Journal of Food Microbiology.

A total of 200 visually clean and intact retail samples each containing a dozen eggs were purchased in 2017 and 2018 from supermarkets in Perth.

Salmonella was detected in 23 of them. It was isolated from nine and six eggshells and egg contents, respectively. In eight samples, Salmonella was recovered from eggshell and egg contents. Salmonella Typhimurium and Salmonella Infantis were the only detected serovars.

Only two Salmonella Typhimurium isolates showed resistance to ampicillin. The others were susceptible to all 14 antimicrobials in the minimum inhibitory concentrations (MICs) testing panel.

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