The overuse of antibiotics among humans has contributed to a growing resistance to their medications. This trend is, in turn, a major health concern in hospitals. But another of our habits is creating the same problem in farm animals: Eating meat.
New research shows that resistance to antimicrobials — the broader category of medication that includes antibiotics — has increased in livestock and “hotspots” of resistance are now appearing globally. Scientists reported their findings Thursday in the journal Science.
The new research focuses on low- and middle-income countries, building on previous studies that examined resistance more thoroughly in high-income countries.
From 2000 to 2018, the researchers behind this new study observed that the proportion of antimicrobials that failed more than 50 percent of the time had increased in both chickens and pigs. In 2000, one in 10 drugs failed more than 50 percent of the time in both animals. That number grew to more than three in 10 for pigs and about four in 10 for chickens over the 18-year span.
As more drugs fail, veterinarians will probably stop using them, study co-author João Pires, Ph.D., tells Inverse.
“In brief, we observe a clear reduction of the number of treatments available to treat infections in animals which occurred in a short period of time,” says Pires, a researcher at ETH Zurich.
Antimicrobials are generally used “to treat animals and as surrogates for good hygiene on farms,” Pires and his team write in the paper, but the factors affecting antimicrobial resistance differ across countries as income varies.
In low- and middle-income countries, they write, lower biosecurity, less nutritious feed, and looser veterinary drug regulations exacerbate conditions compared to those with higher incomes. At the same time, lower meat consumption and limited access to veterinary drugs in rural areas can temper the effects.
In addition to the broad trend, the researchers also found evidence of concentrated areas of antimicrobial resistance within certain countries. China and India have the largest areas, with new hot spots emerging in Brazil and Kenya.
As humans have gotten hungrier for meat, farmers have ramped up their use of antimicrobials to support the growing demand. The related increase in antimicrobial resistance is “both an enabler and a consequence of the global scale-up in demand for animal protein,” write the researchers.
They hope this new research can be used to better inform global antimicrobial policies, given the already international nature of livestock trade.
In the animal trade, antimicrobial resistance “is an important issue because trade can be suspended based on the presence of drug-resistant pathogens,” Pires says.
But trade also provides an opportunity to legislate antimicrobial use, he points out. Countries with more strict antimicrobial use laws, like the United States and the European Union, can insist that any livestock they import follow those same laws.
Monitoring antimicrobial resistance in animals has potential implications for human health, too. Methicillin-resistant Staphylococcus aureus (MRSA), a potentially deadly (and very antibiotic-resistant) infection can be spread between people and livestock. A study on pig farms in the Midwestern United States found that “agricultural animals could become an important reservoir” for the bacteria.
And chickens can transfer to humans a strain of Salmonella that’s resistant to ceftiofur, an antibiotic used to treat infections in livestock. Canadian researchers found evidence that using the antibiotic in chickens increased resistance to the drug in both chickens and humans.
Following that study and others, a ban on using ceftiofur in chicken led to a decrease in resistance levels for both E. coli and Salmonella, Pires says. It also brought a drop in ceftiofur-resistant Salmonella infections in humans.
“Therefore, increases in resistance in livestock will likely have an effect on human health and the transfer of resistant bacteria that can result in untreatable infections,” Pires writes.
The global scale-up in demand for animal protein is the most notable dietary trend of our time. Antimicrobial consumption in animals is threefold that of humans and has enabled large-scale animal protein production. The consequences for the development of antimicrobial resistance in animals have received comparatively less attention than in humans. We analyzed 901 point prevalence surveys of pathogens in developing countries to map resistance in animals. China and India represented the largest hotspots of resistance, with new hotspots emerging in Brazil and Kenya. From 2000 to 2018, the proportion of antimicrobials showing resistance above 50% increased from 0.15 to 0.41 in chickens and from 0.13 to 0.34 in pigs. Escalating resistance in animals is anticipated to have important consequences for animal health and, eventually, for human health.