October 14, 2009
by Amy Peterson, DVM, and Meghan Davis, DVM, MPH
In a Sept. 29th prepared floor statement, Senator Chuck Grassley spoke in response to an August 21st Time magazine article by Bryan Walsh. An important point raised by Mr. Walsh concerned the non-therapeutic use of antimicrobials in food animals and the impact of use of antimicrobials on the emergence of drug-resistant strains of bacteria. The PEW commission report on industrial food animal production (IFAP) cites several studies supporting a connection between the use of antimicrobials and development of drug resistance in both pathogenic (disease-causing) and non-pathogenic bacteria on and around industrial animal farms. A major component of Senator Grassley argument is captured in his quote of a response to the PEW report released by the American Veterinary Medical Association (AVMA) on August 17th. The response states that “[a] scientific human/animal nexus, connecting antimicrobial treatments in animals with foodborne or environmentally-contracted human disease, has not been proven.”
Sen. Grassley claims that the PEW commission report on IFAP states that animals treated with antibiotics produce meat that is unsafe to eat. Animals treated at therapeutic levels with antimicrobials are not the issue-the very word treatment suggests that there is a relationship with a veterinarian and that the antimicrobial agents are being given with a therapeutic intent for the treatment of a specific need or condition, either in an individual animal or in a specific group of animals. The main claim in the PEW report is that non-therapeutic use of antibiotics in animals such as for growth promotion (AGP-antibiotic growth promoters) is dangerous because it leads to an increase in antibiotic resistance in both pathogenic as well as in other bacteria.
This concept of a “reservoir” or pool of resistance helps explain how non-therapeutic antibiotic use in animals, e.g. feed additives for growth promotion, threatens human health. Animals and humans carry some bacteria commensally- that is, bacteria that live on or in humans without causing disease. In addition, non-pathogenic bacteria may survive in the environment. Although they generally will not cause disease, these commensal and other non-pathogenic bacteria can acquire genes-blueprints for resistance to antibiotics-from other bacteria they encounter. They are more likely to do so when they are under pressure from antibiotics, such as AGP, that is used in high volumes in some animal production settings. Resistant bacteria then may spread these genes to fellow bacteria, including those which cause disease in humans. Furthermore, some of the groups of genes they share will provide resistance to multiple drugs, and a single encounter between bacteria may give a pathogen the ability to resist many classes of antibiotics, leading to emergence of a multi-drug resistant (MDR) “super-bug.”
The AVMA’s statement that antimicrobial treatments in animals is not linked to foodborne or environmentally-contracted disease is disingenuous. While the use or lack of use of AGP has not been linked to presence of or increase in levels of pathogens such as Salmonella, E. coli or Campylobacter, it has been linked to increased rates of drug resistance in those bacteria. This has led to a decrease in the ability of doctors to treat infections successfully when humans come into contact with meat contaminated by drug-resistant or MDR pathogens.