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Shortly before Americans fired up their grills for Independence Day, researchers announced that industrial farm workers have been contaminated with “pig MRSA,” an antibiotic resistant bacteria that is increasingly found in American hogs. According to a new study, workers at factory hog farms that use antibiotics are far more likely to contract the drug-resistant bacteria from the pigs than workers at antibiotic-free operations.

This finding puts a huge dent in the meat industry’s argument that blanket antibiotic use in animals is perfectly safe. Conventional factory farms regularly dose their livestock with antibiotics, regardless of whether or not they are sick. More than 70 percent of all antibiotics sold in the U.S. go to pigs, cows, chickens, turkey, and other meat animals. As a result, drug-resistant superbugs are on the rise; earlier this year, the FDA determined that more than half the meat in the U.S. contains antibiotic-resistant strains of bacteria including salmonella and Campylobacteron.

But the new study, which focused on North Carolinian hog farms, makes the first clear link between the bacteria’s spread and exposure to animals pumped full of antibiotics. Farm workers are not only at risk themselves, but give the bugs a convenient way to escape the farm and spread to more humans. Researchers note that this particular bug, MRSA, “is a major cause of morbidity and mortality in the United States and globally.” Pig MRSA was studied because it has a genetic signature that allowed scientists to trace its path from animal to human. It is likely that other drug-resistant bugs are spread in the same way.