By Lisa Baertlein

(Reuters) - A McDonald's Corp <MCD.N> shareholder is redoubling efforts to convince the fast-food chain to stop all of its global restaurants from serving the meat of animals raised with antibiotics that are vital for fighting human infections.

More than 70 percent of medically important antibiotics in the United States are sold for livestock use. Scientists have warned that the routine use of antibiotics to promote growth and prevent illness in healthy farms animals contributes to the rise of dangerous, antibiotic-resistant "superbug" infections, which kill at least 23,000 Americans each year and pose a significant threat to global health.

The Congregation of Benedictine Sisters of Boerne, Texas, asked directors at McDonald's to prohibit the use of medically important antibiotics in its global poultry supply chain. McDonald's already has adopted that policy for the chicken served in its U.S. restaurants.

The group also asking the fast-food chain to set global targets and timelines for switching to pork and beef raised without the non-therapeutic use of medically important antibiotics.

Just over 20 percent of McDonald's shareholders voted in favor of a similar proposal at this year's annual meeting. The sisters pulled a prior resolution after McDonald's announced its plans to change its policies on chicken in the United States.

The sisters aim to have shareholders vote on the new proposal at McDonald's 2017 annual meeting.

McDonald's told Reuters it continues "to engage with key experts, including some who serve as advisors to the World Health Organization (WHO), to advance progress across the industry."

The company said its current policy "provides guidance to our suppliers in parts of the world where the industry does not yet have systems in place that would allow them to verify compliance throughout the supply chain."

Sister Susan Mika said the resolution is part of an ongoing process.

"I said we will be back," Mika said in a phone interview. "We want them to set goals and to be moving in a certain direction. We expect them to be a leader in taking on these questions of our time."

The sisters are part of the Interfaith Center on Corporate Responsibility (ICCR), which along with ShareAction, Farm Animal Investment Risk & Return (FAIRR) Initiative and As You Sow, also are targeting companies such as Sanderson Farms <SAFM.O> and Yum Brands Inc <YUM.N> with similar campaigns aimed at preserving the efficacy of antibiotics.

(Reporting by Lisa Baertlein in Los Angeles; Editing by Bernadette Baum and Andrew Hay)