Life Salmonella outbreaks set to become more severe as drug-resistant strains develop

Salmonella outbreaks set to become more severe as drug-resistant strains develop

Australia has more cases of Salmonella per capita than anywhere else in the world. Photo: Getty
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Four major egg recalls so far this year aren’t a fluke – the number of salmonella outbreaks in Australia is on the rise.

Scientists have known this for years. What’s new is even more worrying: computer modelling from the University of Sydney has found that outbreaks are set to become more severe – meaning more people will become ill.

And as strains become more “successful” – more immune to human resistance – the emergence of a drug-resistant super-bug becomes more likely.

Australia, a world leader for infection

Why this is happening has yet to be explained. But the answer might lie with another mystery: the fact that Australia has more outbreaks of salmonella per capita than anywhere else in the world.

According to the CSIRO, there were 73 cases per 100,000 people – or just over 17,000 cases recorded – in 2015. The number of cases has reportedly doubled over the last decade.

Professor Mikhail Prokopenko is Director, Complex Systems Research Group, School of Civil Engineering, University of Sydney. He and a team of researchers developed a model that can predict salmonella outbreaks several months in advance.

If taken up, this could provide public health officials “early warning signs for disease surveillance and enable a more timely and precise allocation of health resources”.

Great news – and just in time

The modelling is based on an analysis of New South Wales outbreak data from 2008 to 2016 – and a new understanding of salmonella genetic diversity, and how interaction with humans is favouring the bacteria’s rampant evolution into more potent strains.

The researchers examined the genetic signatures of 17,000 registered cases that occurred over a decade, and found they were caused by 1600 closely related strains of salmonella.

“People say it’s all salmonella, but salmonella has huge diversity,” Professor Prokopenko told The New Daily.

“We found that some of those strains popped up once, in a single individual sometimes, and then disappeared. But others were actually quite long-lasting and infected a couple of hundred people – and from the bacteria’s point of view, were a success.

“This is what piqued our attention: why do some of those strains disappear, and why do some keep appearing.”

The answer: human beings and salmonella are engaged in an arms race – and the “bug” is pulling ahead.

“People provide the host environment for these bugs, but they also provide some resistance. If those bugs cannot propagate, it’s because our own human response is good enough to defeat them,” Professor Prokopenko said.

But some mutations initially out-gun our immune system, which needs time to develop a successful defensive response.

“And while it’s developing, the strain runs crazy through the population,” he said.

“They find better weaponry, so to speak, to keep infecting us.”

As a statement from the University of Sydney notes: “Evolving continuously, annually there are an estimated 93.8 million global salmonella infections, resulting in approximately 155,000 deaths worldwide.”

According to the CSIRO, the pathogen is responsible for more deaths in Australia than any other food-borne diseases. The study demonstrated that salmonella networks are linked through a short chain of connections or degrees.

Mutations cooking up a superbug

The researchers believe this close degree of pathogen separation could lead to emergence of an aggressive strain, increasing chances for the appearance of a superbug.

“If the mutations are successful, then it increases the chances of a superbug,” Professor Prokopenko said.

“It’s a connection is terms of probabilities.”

He said no one could explain why Australia has more cases of salmonella per capita than elsewhere in the world. He said our unique food distribution system, long distances, coastal population and mixed climate could all play a part.

“The likely reason has something to do with the (Australian) sub-population of the pathogen,” he said.

“They are mutating in a particular way. So it’s less than the social factors or food supply but more on the genetics. But research needs to find out why.”

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