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A new study suggests that regularly dosing animals is a worse idea than was previously thought.

It has long been understood that feeding animals antibiotics can create resistant bacteria - bacteria that can cause problems for human health. That's why the Food and Drug Administration has been concerned for decades over the practice of giving livestock subtherapeutic doses to promote growth. While the agency has yet to do much of anything to curb the problem, save for some voluntary regulations, new research suggests that the steady supply of drugs could make animals sicker - and cause disease to spread more rapidly.

The new study, published this week in the journal PNAS, looked at how salmonella bacteria was spread in a population of mice. When treated with antibiotics, mice that were sick but showed relatively low amounts of salmonella in their droppings started behaving more like "superspreaders," shedding more bacteria and suffering more acute symptoms. Meanwhile, other mice that, before being treated, passed higher amounts of bacteria and showed fewer symptoms did not shed any less salmonella after receiving an antibiotic.

If the results hold true for other animals, such as chicken or cattle - and lead author Denise Monack, associate professor of microbiology and immunology at Stanford University, believes they will - then the amount of antibiotics fed to livestock may be even more troubling than was previously thought.

"We need to think about the possibility that we're not only selecting for antibiotic-resistant microbes, but also impairing the health of our livestock and increasing the spread of contagious pathogens among them and us," Monack told Science Daily.

While the findings aren't quite as dramatic as higher-ups at the Centers for Disease Control and Prevention talking about the post-antibiotic era, they are further ammunition for groups that are pushing for more, and more substantive, federal regulations.

This week also saw new action on that front, with Consumer Reports releasing the results from a survey of doctors. After speaking with 500 family doctors and internal medicine specialists, the groups report that 93 percent of M.D.s said they are concerned with antibiotic use in livestock, while 47 percent said they are "extremely concerned" with the practice.

"The use of antibiotics in animals for non-therapeutic purposes is contributing to their failure in humans, including the youngest of children, who are most prone to complications from bacterial illness," Maria Brown of the Maryland chapter of the American Academy of Pediatrics said in a press release from U.S. Public Interest Research Group, a partner on the survey. "Simply put, sick infants and children and the doctors like myself that care for them need effective antibiotic drugs."