"We are about to reach the point where more antibiotics will be consumed by farm animals worldwide than by humans," says Mark Woolhouse, at the University of Edinburgh, UK.
Researchers calculate that farmers globally feed 63,000 tonnes of antibiotics to chickens, pigs and cattle every year -- and that will climb by 67 per cent, to 106,000 tonnes, by 2030.
This will mean more resistant bacteria, which could be a big threat. The livestock industry has long played down any risk to human health caused by using antibiotics in farming, but the danger is now accepted, according to the UN Food and Agriculture Organization (FAO).
Most of the increase in antibiotic use is expected to be in middle-income countries, but once resistant bacteria appear, they can spread round the world. The problem is getting worse as people become more prosperous and eat more meat and dairy. For example, Tim Robinson of the International Livestock Research Institute in Nairobi, Kenya, and his colleagues calculate that the total biomass of livestock around the world now outstrips that of people, illustrating the size of the demand.
Colistin, a drug that is used more often in animals than people, is one example. It is now the only antibiotic left that works against some human infections, yet colistin resistance has developed, and spread worldwide in 2015. The European Medicines Agency says bacteria resistant to colistin probably arose in livestock, and that some EU countries could easily cut their use of this antibiotic 25-fold.
Antibiotic resistance is also being introduced artificially into microorganisms through genetically modified organisms (GMOs) and laboratory protocols, sometimes used as a selectable marker to examine the mechanisms of gene transfer or to identify individuals that absorbed a piece of DNA that included the resistance gene and another gene of interest. A recent study demonstrated that the extent of horizontal gene transfer among Staphylococcus is much greater than previously expected--and encompasses genes with functions beyond antibiotic resistance and virulence, and beyond genes residing within the mobile genetic elements.
The UN General Assembly has called for countries to encourage the best use of antibiotics. But it hasn't yet called for specific measures, such as banning their use to assist livestock growth -- rather than fight infections -- which can promote resistance.
At least agencies like the FAO are calling for change, says Woolhouse, as is China, where growing demand for meat has lead to soaring livestock production and resistance. But progress will require finding other ways to keep animals healthy, especially in poor countries where production is growing fastest and there are few alternatives.