Unchecked use of antibiotics in Chinese farms had led to widespread antibiotic resistance, a new study has found, with researchers warning the problem may spread worldwide through manure and fertiliser run-off.

China is the world’s biggest producer and consumer of antibiotics in the world, with at least 46.1% being used in livestock, as much as four times the amount used by livestock industry in the US.

Some bacteria have genes that make them resistant to antibiotics, while others can get such resistance from another bacteria. Overuse of antibiotics kills the germs with no resistance but grows the proportion of resistant bacteria in the environment.

In a study published today in the journal PNAS, researchers from the Chinese Academy of Sciences and Michigan State University analysed manure from three large scale commercial pig farms in China, looking for antibiotic resistant genes (ARGs) in bacteria.

They also tested a control sample of manure from pigs never fed antibiotics and soil from a pristine forest in Putian, China.

“We detected 149 unique ARGs among all of the [commercial farm] samples, which is three times more types of ARGs than were found in the control samples,” the researchers said in their paper, warning that the antibiotic resistant genes could spread globally and pose a human health risk.

“Microbes from manure, compost, or soil containing the ARGs are subject to dispersal via runoff into rivers, leaching to subsurface waters, air dispersal via dust, human travel, and distribution of agricultural products, including compost for gardening, which could expand a local contamination to regional and even global scales.”

The researchers also found high levels of metals such as arsenic and copper in the commercial farm samples, and theorise that the metals may be contributing to the development and persistence of antibiotic resistance.

“Policies and management tools to facilitate prudent use of antibiotics and heavy metals, including their combined use, in animal industries and animal waste management are needed,” the researchers said.

A co-author of the study, graduate student Timothy Johnson from Michigan State University, said the researchers did not test for superbugs, which are germs that are resistant to a combination of antibiotics.

“We do see that there is extensive and alarming enrichment for antibiotic resistance genes, but these genes may be spread out in many different bacteria so that individual bacteria may not be resistant to many antibiotics, or it may be the case that they are superbugs. With the data we have we just cannot say if they are superbugs or not,” he said.

Professor Matthew Cooper from the University of Queensland’s Institute for Molecular Bioscience said this type of resistance evolution had been seen before but this paper quantified it in Chinese pigs for the first time.

“If you look at the reports and many studies, there is a recognised and clear statistical link between the use of antibiotics as growth promoters and the incidence of superbugs in animals and their transfer to the human population,” said Professor Cooper, who was not involved in the study.

“That drove the European Union to ban antibiotics as growth promoters, then more recently the US has begun to ban more and more classes of antibiotics and Australia have done the same.”

The scale of the problem means that a collective approach is needed, he said.

“Let’s take the hypothesis that a country was the only country in the world that used antibiotics — that doesn’t help us at all. We need some type of framework whereby all countries standardised the practice.”

Professor Cooper called on authorities in China and India to bring in more transparent reporting measures for antibiotic use, saying that people should be very concerned by the results of this new study.

“Animals get transported between countries, as do people. Infectious disease is no longer a national issue, it’s a global issue.”