It may seem as though wild boars are suddenly everywhere, but that impression is not mistaken.
Feral pig numbers are rising rapidly across North America, particularly Texas and Arizona, as well as Europe and South America.
Beyond wrecking crops and turning up in suburban gardens, wild boars may also be contributing to the spread of antibiotic-resistant bacteria, according to new research from Brazil.
Brazilian public-health officials have worried that commercially farmed wild boars, bred there for meat, could harbour zoonotic bacteria and bacteria resistant to antimicrobials.
Until now, though, that concern was largely an informed suspicion rather than something backed by robust data.
Wild boars and antibiotic-resistant bacteria in Brazil
To test whether those concerns were justified, scientists took rectal swabs from 100 healthy European wild boars (Sus scrofa scrofa) kept on two commercial farms in Brazil's Goiás State.
A wild boar (Sus scrofa) in the Lainzer Tiergarten nature reserve in south-west Vienna, Austria. (Valentin Panzirsch/Wikimedia Commons/CC BY-SA 3.0 AT)
The team tested the swabs for Salmonella and E. coli, assessed the bacteria against a range of widely used antibiotics, and looked for genes associated with virulence - characteristics that can improve a bacterium's capacity to infect a host.
There was little sign of Salmonella, which was the encouraging result. However, more than half the boars carried E. coli, and almost all of those bacterial strains resisted several antibiotics.
The single Salmonella sample detected was resistant to several antibiotics. Yet it did not contain the virulence or resistance genes assessed by the researchers, indicating that the immediate Salmonella threat was comparatively low.
The findings for E. coli were far more troubling.
E. coli was recovered from 56 of the 100 wild boars. Of those, 55 - a remarkable 98.2 percent - met the definition of multidrug resistant, meaning they could survive antibiotics from several drug classes.
Resistance was especially prevalent to sulfonamides, tetracycline, amoxicillin, ampicillin and doxycycline. Some of these names may be familiar because they are among the most common and affordable first-line antibiotics.
A young wild boar (Sus scrofa) in a wildlife park in the Netherlands. (Sander van der Wel/Wikimedia Commons/CC BY-SA 2.0)
Almost every isolate was still susceptible to ceftriaxone, an antibiotic of critical importance in human medicine. Nevertheless, the researchers say that even the small level of resistance observed should be carefully monitored.
In all, the researchers described the widespread drug-resistant E. coli as a "striking finding", which is about as near as a scientist comes to saying 'yikes'.
They also screened for genes linked with bacterial virulence. Nearly 30 percent of the E. coli isolates contained the tsh gene, while smaller numbers carried papC and iss - genes associated with adhesion, survival in the bloodstream and effective colonisation of hosts.
Some bacteria contained more than one of these genes, combinations that could potentially make them better able to cause disease.
How resistance may spread among wild boars
Farm managers said the wild boars were not given antibiotics as a routine matter. This poses an obvious question: what is the source of so much resistance?
The researchers believe the surrounding environment may hold the explanation.
Genes conferring antibiotic resistance can remain in water and soil, circulate between microbes in the environment, and transfer from one bacterium to another through mobile fragments of DNA.
Wild boars are also mobile, opportunistic creatures that regularly encounter livestock, wildlife and landscapes modified by people. These interactions create numerous routes through which resistant bacteria can spread, even where antibiotics are not used directly.
Taking a broad view of the environment around animals, including farmed animals, forms part of Brazil's One Health approach. It acknowledges the "intersectoral health challenges" shared by human, animal and environmental health.
Rather than treating antibiotic resistance solely as a medical concern, One Health examines how resistant bacteria may travel between ecosystems - from farms into forests, from wildlife to livestock, and ultimately to humans.
That systems-based perspective is the reason researchers are examining the links between wild boars and antibiotics.
As commercial wild boar farming grows in Brazil, while escapees continue to establish feral populations, these invasive pigs may become an increasingly significant reservoir of antimicrobial-resistant bacteria.
The researchers therefore argue that captive wild boars should be included in health-surveillance networks.
An invasive species with important caveats
European wild boars have not always belonged to South America's landscapes.
Domestic pigs were brought over by European settlers centuries ago, whereas an Argentine rancher introduced wild boars for hunting in the early 1900s. Since then, escaped animals have spread throughout the continent, interbred with domestic pigs and formed flourishing populations.
In Brazil, they have thrived in a landscape offering both shelter and food: fragments of forest provide cover, while farm fields offer energy-rich crops such as maize and sugar cane.
Their ongoing expansion has made them one of Brazil's most successful - and most contentious - invasive mammals.
The research does, however, have significant limitations. Samples were gathered from 2014 to 2017, meaning the extent of the problem could have shifted in the intervening years.
The scientists also depended on farm managers' statements that antibiotics were not routinely administered, instead of independently checking the records, and sampled animals from only two farms.
Related: Drug-Resistant Bacteria May Be Jumping Between Pets And Humans, Scientists Warn
Even with these caveats, the study offers some of the earliest detailed information on antibiotic resistance in Brazilian wild boars - and a timely reminder that antimicrobial resistance will not be defeated in hospitals alone.
At times, the first step is noticing what is moving through the woodland.
Read the full report in Archives of Microbiology.
This article was fact-checked by Fiona MacDonald and edited by Peter Dockrill. Although we take pride in our process, we are only human. If you spot an error, please let us know.
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