UPDATE 2-Parent of Burger King, Tim Hortons to curb antibiotics in chicken

(Adds details on plans for Popeyes and context on Sanderson Farms, paragraphs 3, 9-12)

LOS ANGELES, June 22 (Reuters) - The parent of Burger King and Tim Hortons on Thursday vowed to cut the use of antibiotics in its chicken supply, joining other major fast-food chain operators in the battle against the rise of dangerous antibiotic-resistant bacteria known as superbugs.

Restaurant Brands International Inc said it intends to switch its Burger King and Tim Hortons chains in the United States and Canada to chicken raised without the use of antibiotics important to human medicine by the end of 2018.

The company, which bought Popeyes Louisiana Chicken this year, said it intends to apply the new policy to all brands over time. It did not share additional details about the new policy.

Human infections from antibiotic-resistant bacteria pose a grave threat to global health and are estimated to kill at least 23,000 Americans each year, although a recent Reuters investigation found that many infection-related deaths are going uncounted.

Some 70 percent of antibiotics vital for fighting infections in humans are sold for use in meat and dairy production. Medical researchers have concerns that overuse of those drugs on farms may diminish their effectiveness in fighting disease in humans.

As You Sow, a non-profit shareholder advocacy group, withdrew a shareholder resolution calling for reduced antibiotic use in Restaurant Brands' meat supply following its commitment on chicken.

"This is great news for modern medicine and for long-term shareholder value," said Austin Wilson, Environmental Health Program Manager at As You Sow.

McDonald's Corp, Wendy's Co, KFC and Chick-fil-A are among the companies that have made commitments to reduce the use of antibiotics in the poultry they buy.

Activists on Thursday also stepped up pressure on Sanderson Farms Inc, the only major producer that has not committed to limit use of medically important antibiotics in its chicken production.

The Organic Consumers Association, Friends of the Earth and the Center for Food Safety sued Sanderson Farms in U.S. District Court for the Northern District of California, over its use of the claim that its chicken products are "100 percent natural."

Among other things, the lawsuit alleged that government tests of finished Sanderson products found residues of drugs such as ketamine, an animal anesthetic dubbed "Special K" by recreational drug users, and human antibiotics including ciprofloxacin and chloramphenicol, the latter of which is not approved for use in animals raised to become food.

Sanderson Farms did not immediately respond to a request for comment. (Addional reporting by Tom Polansek in Chicago; Editing by Phil Berlowitz and David Gregorio)