In a review study, researchers from Tufts University School of Medicine zero in on the controversial, non-therapeutic use of antibiotics in food animals and fish farming as a cause of antibiotic resistance. They report that the preponderance of evidence argues for stricter regulation of the practice. Stuart Levy, a world-renowned expert in antibiotic resistance, notes that a guiding tenet of public health, the precautionary principle, requires that steps be taken to avoid harm.

“The United States lags behind its European counterparts in establishing a ban on the use of antibiotics for growth promotion. For years it was believed that giving low-dose antibiotics via feed to promote growth in cows, swine, chickens and the use of antibiotics in fish farming had no negative consequences. Today, there is overwhelming evidence that non-therapeutic use of antibiotics contributes to antibiotic resistance, even if we do not understand all the mechanisms in the genetic transmission chain,” says Levy, MD, professor of molecular biology and microbiology and director of the Center for Adaptation Genetics and Drug Resistance at Tufts University School of Medicine.

For the past 70 years, humans have relied on antibiotics to combat bacterial infections such as streptococcus, meningitis, tuberculosis and urinary tract infections. The misuse and overuse of antibiotics, however, has contributed to antibiotic resistance, making antibiotics less effective at saving lives. Levy and co-author Bonnie Marshall summarize and synthesize the findings of a large number of studies assessing the link between antibiotic resistance and the use of non-therapeutic antibiotics in livestock and fish farming. Highlights include the following.

The use of non-therapeutic antibiotics is widespread
  According to estimates, antibiotics are eight times more likely to be used for non-therapeutic purposes than for treating a sick animal.

Current practices set the stage for the rapid spread of antibiotic-resistant bacteria
-    The long-term administration of antibiotics in animal feed creates an optimal environment for antibiotic resistance genes to multiply. Essentially, treated animals become “factories” for the production and distribution of antibiotic-resistant bacteria such as Salmonella and Methicillin-resistant Staphylococcus aureus (MRSA), a troubling infection that is resistant to common antibiotics.

-    Bacteria can transfer antibiotic resistance to other bacteria, and multiple different resistance genes can be linked together in this process. Thus, even if farmers turn to antibiotics that are not commonly used to treat people, these drugs – given over long periods of time – can also promote resistance. Several studies demonstrated that antibiotic-resistant bacteria can easily spread from animals to people in close contact with animals, such as veterinarians, slaughterhouse workers, farmers, and the families of farmers.

-  As much as 90 percent of antibiotics given to livestock are excreted into the environment. Resistance spreads directly by contact and indirectly through the food chain, water, air, and manured and sludge-fertilized soils.

-  The broad use of antibiotics in fish food in farm fishing, particularly overseas, leads to leaching where it can be washed to other sites, exposing wild fish to trace amounts of antibiotics.