The lower infection rate among humans almost certainly means that the resistant bacteria passed from animals to humans, the study found.
It said that while mcr-1 was "currently confined to China" it was likely to spread globally.
"This is a worrying report, as polymyxins are often the last resort antibiotic to treat serious infections," said Laura Piddock, a professor of microbiology at the University of Birmingham.
"Equally worrying is that this type of resistance can be easily transferred between bacteria."
Other types of drug resistance -- such as for tuberculosis -- show that "this likely paves the way for it to spread throughout the world," she added.
Some 480,000 people contracted multi-drug resistant tuberculosis in 2014, according to the WHO. The disease killed 190,000 in the same year.