Antibiotics given to healthy animals to help them grow, such as these piglets in Frankenstein, Mo., are creating strains of drug-resistant infections, scientists say.

Frankenstein, Mo. ? Editor’s note: Once-curable diseases such as tuberculosis and malaria are coming back, as germs rapidly mutate to form aggressive strains that resist drugs. The reason: The misuse of the very drugs that were supposed to save us has built up drug resistance worldwide. One in a series.

The mystery started the day farmer Russ Kremer got between a jealous boar and a sow in heat.

The boar gored Kremer in the knee with a razor-sharp tusk. The burly pig farmer shrugged it off, figuring: “You pour the blood out of your boot and go on.”

But Kremer’s red-hot leg ballooned to double its size. A strep infection spread, threatening his life and baffling doctors. Two months of multiple antibiotics did virtually nothing.

The answer was flowing in the veins of the boar. The animal had been fed low doses of penicillin, spawning a strain of strep that was resistant to other antibiotics. That drug-resistant germ passed to Kremer.

Like Kremer, more and more Americans — many of them living far from barns and pastures — are at risk from the widespread practice of feeding livestock antibiotics. These animals grow faster, but they can also develop drug-resistant infections that are passed on to people. The issue is now gaining attention because of interest from a new White House administration and a flurry of new research tying antibiotic use in animals to drug resistance in people.

Researchers say the overuse of antibiotics in humans and animals has led to a plague of drug-resistant infections that killed more than 65,000 people in the U.S. last year — more than prostate and breast cancer combined. And in a nation that used about 35 million pounds of antibiotics last year, 70 percent of the drugs — 28 million pounds — went to pigs, chickens and cows. Worldwide, it’s 50 percent.

“This is a living, breathing problem. It’s the big bad wolf, and it’s knocking at our door,” said Dr. Vance Fowler, an infectious disease specialist at Duke University. “It’s here. It’s arrived.”

The rise in the use of antibiotics is part of a growing problem of soaring drug resistance worldwide, The Associated Press found in a six-month look at the issue. As a result, killer diseases like malaria, tuberculosis and staph are resurging in new and more deadly forms.

In response, the pressure against the use of antibiotics in agriculture is rising. The World Health Organization concluded this year that surging antibiotic resistance is one of the leading threats to human health, and the White House last month said the problem is “urgent.”

“If we’re not careful with antibiotics and the programs to administer them, we’re going to be in a post-antibiotic era,” said Dr. Thomas Frieden, who was tapped to lead the Centers for Disease Control and Prevention this year.

Also this year, the three federal agencies tasked with protecting public health — the Food and Drug Administration, CDC and U.S. Department of Agriculture — declared drug-resistant diseases stemming from antibiotic use in animals a “serious emerging concern.” And FDA deputy commissioner Dr. Joshua Sharfstein told Congress this summer that farmers need to stop feeding antibiotics to healthy farm animals.

Farm groups and pharmaceutical companies argue that drugs keep animals healthy and meat costs low, and have defeated a series of proposed limits on their use.

Use in healthy animals

America’s farmers give their pigs, cows and chickens about 8 percent more antibiotics each year, usually to heal lung, skin or blood infections. However, 13 percent of the antibiotics administered on farms last year were fed to healthy animals to make them grow faster. Antibiotics also save as much as 30 percent in feed costs among young swine, although the savings fade as pigs get older, according to a new USDA study.

However, these animals can develop germs that are immune to the antibiotics. The germs then rub into scratches on farmworkers’ arms, causing oozing infections. They blow into neighboring communities in dust clouds, run off into lakes and rivers during heavy rains, and are sliced into roasts, chops and hocks and sent to our dinner tables.

More than 20 percent of all human cases of a deadly drug-resistant staph infection in the Netherlands could be traced to an animal strain, according to a study published online in a CDC journal. Federal food safety studies routinely find drug-resistant bacteria in beef, chicken and pork sold in supermarkets, and 20 percent of people who get salmonella have a drug-resistant strain, according to the CDC.

Here’s how it happens: In the early ’90s, farmers in several countries, including the U.S., started feeding animals fluoroquinolones, a family of antibiotics that includes drugs such as ciprofloxacin. In following years, the once powerful antibiotic Cipro stopped working 80 percent of the time on some of the deadliest human infections it used to wipe out. Twelve years later, the New England Journal of Medicine published a study linking people infected with a Cipro-resistant bacteria to pork they had eaten.