A new strain of antibiotic-resistant bacteria circulating among pigs is ringing alarm bells over its ability to jump to humans.
A University of Cambridge study found the "superbug" staphylococcus aureus – resistant to the best frontline antibiotics currently available to medicine – has been evolving over the past 50 years. And it's because industrial-scale battery pig farms have been dosing their livestock to overcome the spread of disease in overcrowded conditions.
The CC398 strain is the dominant Methicillin-resistant staphylococcus aureus (MRSA) bacteria in European piggeries.
It's also starting to appear in human infections.
And that's alarming evidence supporting World Health Organisation's fears that drug-resistant infections are set to explode.
MRSA first appeared in humans in 1960. Antibiotic-resistant infections now kill about 750,000 people annually. By 2050, the WHO expects this to soar to 10 million.
Just as the Covid-19 pandemic fulfilled the WHO's worst "Disease X" fears, the global medical community is on the watch for an almost inevitable antibiotic-resistant plague.
CC398 isn't that.
But it may contain the building blocks for one.
"Cases of livestock-associated MRSA in humans are still only a small fraction of all MRSA cases in human populations, but the fact that they're increasing is a worrying sign," says senior study author Dr Luxy Weinert.
For decades, scientists have warned industries ranging from chicken factories to fish farms of the dangers of dosing their animals with human-grade antibiotics.
"We found that the antibiotic resistance in this livestock-associated MRSA is extremely stable - it has persisted over several decades, and also as the bacteria has spread across different livestock species," adds Dr Gemma Murray.
The findings were published in the journal eLife.
CC398 is most commonly found in pigs. Sties in Denmark recently reported that 90 per cent of their stock was infected.
Pigs don't develop symptoms.
But it is now showing a capacity to thrive in humans.
And once it jumps the species gap, it has been observed to retain its ability to shrug off medication.
A similar capability was recently found in another livestock superbug – clostridioides.
It also resides among pigs and shows signs of developing the ability to jump to humans.
"This alarming discovery suggests that resistance to antibiotics can spread more widely than previously thought, and confirms links in the resistance chain leading from farm animals to humans," says Copenhagen University researcher Semeh Bejaoui.
Efficiency versus resiliency
High-intensity farming is a contentious issue.
On one hand, it is an efficient way to mass-produce common meat and animal product foods.
On the other, there have long been scandals involving animal welfare and the spread of disease.
It's now become a major international concern as a "reservoir" of antibiotic and viral infections.
And international supply lines combined with narrowing genetic diversity is only adding to the risk of a lightning-fast outbreak.
The exact genetic mechanism behind such bacterial adaptation is beginning to be understood.
The Cambridge researchers identified three "mobile" genetic elements within CC398 carrying chunks of genetic data detailing antibiotic resistance. Being "mobile" means these genetic packets can be exchanged between bacteria in different species.
Two genetic elements – Tn916 and SCCmec – have instructions on how to evade antibiotics.
One – φSa3 – gives CC398 the plans to dodge the human immune system.
And φSa3 has repeatedly appeared and disappeared over time – suggesting the bacteria can adapt rapidly to human hosts.
"Understanding the emergence and success of CC398 in European livestock – and its capacity to infect humans – is vitally important in managing the risk it poses to public health," says Dr Weinert.
Europe this month banned the use of zinc oxide in piggeries. It was commonly used to treat diarrhoea in piglets.
But this is too little, too late, researchers warn: "This ban may not help reduce the prevalence of CC398 because the genes conferring antibiotic resistance are not always linked to the genes that confer resistance to zinc treatment."
An outbreak of disease between 2018 and 2020 killed half the pigs in China.
That's millions of animals.
The African swine flu induced fever, loss of appetite and internal bleeding. Eventually, the pigs' internal organs collapse. An attempt to contain the outbreak saw an industry-wide cull, with millions of pigs slaughtered and buried.
Pork prices soared as shortages became common. International analysts believe the total economic impact of African swine flu was more than 1 per cent of China's GDP.
Its original spread from Africa to Lisbon in 1957 was found to have been caused by discarded airline ham sandwiches being fed to pigs.
The virus has since hitchhiked across the globe, carried in live trade exports and frozen goods.
It's now entrenched in Europe and Asia. It has recently been found in Haiti and the Dominican Republic in the Caribbean.
Australia has declared it to be a significant biosecurity threat to the local pig industry.
So far, the virus has shown no sign of crossing to humans. But pig diseases – like bird diseases – have a long history of finding ways to make the jump.
• Jamie Seidel is a freelance writer.