Where does antimicrobial resistance come from?

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It seems logical that antimicrobial resistance (AMR) originates in the places where antimicrobials are used. It seems logical, that is, until one starts to think about all the places antimicrobials are used and the results of survey studies that have found the presence of bacteria with genetic indicators of resistance to be widespread. In this third part of a four-part series on AMR we look at who is at fault for the increase in resistance.

Many people are pointing the finger at animal agriculture. Certainly, antibiotics are used in animal agriculture, maybe even a greater amount by volume than is used in human medicine. In animal agriculture, antibiotic use is governed by label requirements that indicate the dose, route of administration, and length of treatment. Most commonly, antibiotics are given to control infectious diseases. In some cases, antibiotics may also be given to prevent infectious diseases. For example, in dairy cows, it has been the standard recommendation to infuse broad-spectrum antibiotics into all four teats of all cows at the time they are dried off to help control intramammary infections.

Antibiotics have also been used to enhance performance. For example, in swine production, antimicrobials are fed to change the gut microbe population with the result that growth is increased.

Antibiotic use in animal agriculture should be done in consultation with a veterinarian. Dairy producers are encouraged to have a valid veterinary-client-patient relationship (VCPR) through which all issues of herd health and treatment are discussed. Veterinarian involvement has been shown to reduce the incidence of antibiotic residues in animals that leave the farm.

But animal agriculture is only one user of antimicrobials. Veterinary drugs are limited by label, however antibiotic use in human medicine does not have label limits. Individual doctors specify the drug use, dose, length of treatment and more. Doctors sometimes prescribe antibiotics when antibiotics are not indicated.

I represented Michigan State University Extension at a recent National Institute of Animal Agriculture (NIAA) conference dedicated to antimicrobial resistance.  At the NIAA conference, Dr. Kurt Stevenson of The Ohio State University Medical Center, reported a study that found that 25-40 percent of hospitalized patients receive antibiotics and 10-70 percent are unnecessary or sub-optimal in dosage. Inappropriate use of antibiotics is a risk factor for the development of antimicrobial resistance.

Not only are antibiotics used in humans and animals, antibiotics may be used in the production of fruits and vegetables and in aquaspecies. Though it represents only a small percentage of total antibiotic use in the U.S., Dr. George Sundin of Michigan State University reported on antibiotic use on apple, pear and peach trees and on vegetable crops for various bacterial diseases. In addition, farmed fish are often raised with medicated feed dispersed in the water.

Indirect dissemination of antibiotics in the environment may occur through excretion of waste by humans and animals that receive antibiotics. These direct applications and indirect dispersals of antibiotics in the environment are another risk factor for the development of resistance.

Seeking to determine where resistance most likely originated, Dr. Stuart Reid led a group in Scotland that looked for the precursors of resistance to Salmonella bacteria. Salmonella are often associated with animal agriculture and the question was whether these bacteria were gaining resistance through veterinary antibiotic use. Dr. Reid reported at the NIAA conference that his work indicates that animals were unlikely to be the major source of resistance by Salmonella for humans.

This study was of one bacteria and in one country, yet it raises important questions and reminds us that solving the problem of antimicrobial resistance will take a “one health” approach rather than each side pointing fingers at the other. All antibiotic use should be examined, and the consequences of use, non-use or alternatives should be fully explored.

This article is part three of a four-part series. Click here for part one and here for part two.


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