• Posted July 2, 2025

Livestock Manure Could Be Source Of Antibiotic Resistance, Researchers Warn

Antibiotic resistance is an urgent global public health threat, as more microbes gain the ability to thwart essential bacteria-killing drugs.

And there's a hidden means by which antibiotic resistance is likely increasing, researchers say.

Manure from livestock is a major reservoir for antibiotic resistance genes that could threaten human health, researchers reported June 27 in the journal Science Advances.

If those genes transfer to bacteria that infect humans, they could make common illnesses much harder — or even impossible — to treat, researchers said.

“This research shows that what happens on farms doesn’t stay on farms,” researcher James Tiedje, a professor of microbiology at Michigan State University, said in a news release.

“Genes from manure can make their way into the water we drink, the food we eat and the bacteria that make us sick,” he said.

For the study, researchers analyzed more than 4,000 manure samples from pigs, chickens and cattle in 26 countries over a 14-year period.

Concerns have been mounting that overuse of antibiotics in agriculture could be fueling the rise of antibiotic resistance. More than 93,000 tons of antimicrobials are used every year to address the needs of livestock, researchers said in background notes. 

But there’s much left to learn about how antibiotic-resistant germs make their way from farms to infect everyday humans, researchers said.

More than 2.8 million antimicrobial-resistant infections from germs like MRSA occur each year, and more than 35,000 people die from them, researchers said in background notes.

“Some antibiotic resistance genes are now widely known to be in DNA in soil, water and manures, but are they ones that are really risky?” Tiedje said.

“We took this a step further to determine whether those genes can become mobile and move into harmful bacteria, ones that have bad health outcomes. That’s what makes them a real risk,” he said.

Researchers used the manure data to build a global map highlighting regions where dangerous antibiotic resistance genes are most common.

They also developed a new system to rank which genes pose the greatest threat to human health, based on their mobility, difficulty to treat and current uptake among disease-causing bacteria.

Livestock manure might act as an important bellwether for detecting early signs of spread for antibiotic resistance genes, researcher Xun “Shawn” Qian, a professor at Northwest Agriculture and Forestry University in Yangling, China, said in a news release.

“Our findings highlight the need for targeted monitoring and risk management of antibiotic resistance in major livestock-producing countries,” he said.

“For instance, as the world’s leading beef producer, the United States shows significantly higher abundance and diversity of antibiotic resistance genes in cattle manure compared to other countries,” Qian continued. “Likewise, China, which is the world’s largest pig producer, exhibits elevated levels of bacterial abundance, diversity and overall resistance risk in swine manure, surpassing all other nations in our analysis.”

The results also add to the evidence for limiting antibiotic use in livestock, researchers said.

Antibiotics often are used to promote faster growth in farm animals, rather than to treat disease. This increases the likelihood of antibiotic resistance.

“We need antibiotics to control disease,” Tiedje said. “That’s why it’s so important to protect their effectiveness. If resistance spreads too far, these life-saving drugs won’t work when we really need them.”

On an encouraging note, the study found signs that early efforts to reduce farm antibiotic use are paying off.

“There’s been a global push to reduce antibiotics in agriculture,” Tiedje said. “Denmark and other European countries led the way by banning growth-promotion antibiotics years ago and they’ve seen lower resistance levels as a result.”

The U.S. and China also introduced restrictions in the late 2010s, and they and Denmark have shown a decline in the resistance genes found in livestock manure, results show.

That shows the policies work, but more is needed, Tiedje said.

“The problem is so severe that governments worldwide are being asked by the United Nations to develop national action plans to tackle antibiotic resistance,” he said. “The data from this study can help countries decide what’s most important for them to act on and where those efforts can have the biggest impact.”

Qian agreed.

“By identifying the riskiest genes and the countries where they’re most likely to spread, we can start to better target surveillance and policy,” he said. “Reducing risky antibiotic use on farms could make a real difference.”

As shown by the COVID-19 pandemic, virulent germs don’t recognize national borders, researchers noted.

“A resistant pathogen is only a plane flight away from anywhere,” Tiedje said. “By understanding and managing livestock antibiotic resistance, we help protect the effectiveness of antibiotics for everyone.”

More information

The American Society for Microbiology has more on antibiotic resistance genes.

SOURCE: Michigan State University, news release, June 28, 2025