New data from the Food and Drug Administration shows that the sale of antibiotics for farm animals is on the rise, amid concerns that their use is contributing to increasing drug-resistant infections in humans.

The FDA reported Thursday that sales of antibiotics for agriculture climbed 16 percent in the United States between 2009 and 2012. More than 32 million pounds of antibiotics intended for use on American farms were sold in 2012 alone — a nearly 8 percent rise over the previous year.

The new data comes at a time when the FDA is trying to reduce antibiotic use in food animals. In December 2013, the agency began a voluntary three-year phase out of the practice of using antibiotics to make farm animals grow faster.

Antibiotics have long been considered vital to human health, but in recent decades, many have been rendered less effective or useless as bacteria have become increasing resistant. Human overuse and abuse of the drugs has contributed to antibiotic resistance, but for more than four decades, scientists and public health officials have warned that their widespread use in agriculture may also play a role.

Up to 70 percent of all antibiotics sold in the U.S. are used in food animals.

For years, farmers and ranchers have used antibiotics to help food animals grow faster on less feed. This practice, known as “growth promotion,” is the target of the FDA’s voluntary phase-out program. The program will also require that veterinarians administer any antibiotics sold over the counter to treat or prevent disease.

According to the new FDA data, nearly all medically important antibiotics used on the farm were sold over the counter in 2012.

Historically, the farm lobby and Congress have blocked efforts to restrict the use of antibiotics in agriculture, and industry has maintained that farmers are not adding significantly to the risks for human health.

The debate has been clouded by a lack of information. While the new FDA report provides data — gathered from pharmaceutical manufacturers — about antibiotics sales and distribution, it does not show how the drugs are used. Manufacturers are not required to report this to the FDA, and the agency says it cannot break down sales data by specific drugs and indications because it is considered confidential business information. Attempts by a few legislators in Congress to require more specific data reporting have gone nowhere.

The new report shows that by far the largest class of antibiotics given to farm animals — about two-thirds of the total — are tetracyclines. While not widely used in humans, doctors do prescribe them to treat certain conditions like acne. The sales and distribution of tetracyclines increased by 13 percent in 2012, the agency reported.

The report also shows a surge in the use of another class of antibiotics, cephalosporins, which are more widely prescribed for human use, for example to treat severe salmonella poisoning in children and pregnant women. Cephalosporins sales rose 37 percent between 2009 and 2012, according to FDA’s new data.

David E. Hoffman

Emma Schwartz, Producer & Reporter

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