Drugs and Bugs: The Widespread Use of Antibiotics in Agriculture May Build Resistant Bacteria and Affect Our Ability to Treat Human Disease

By McEwen, Scott A. | Alternatives Journal, August 2005 | Go to article overview
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Drugs and Bugs: The Widespread Use of Antibiotics in Agriculture May Build Resistant Bacteria and Affect Our Ability to Treat Human Disease


McEwen, Scott A., Alternatives Journal


"SUPERBUGS", bacteria resistant to all or nearly all of the antibiotics available to doctors, have received a lot of press lately. It's scary to think that common infections that we are used to treating with antibiotics may someday be untreatable, but in some cases that may soon happen. Some important bacteria, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci are already resistant to all but a very few antibiotics. And development of new classes of antibiotics by the pharmaceutical industry has slowed to a trickle.

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It's been shown in many countries that antibiotic-prescribing practices in hospitals and the community can often be directly linked to emergence of resistance. (1) But what about non-human uses of antibiotics? We use antibiotics to treat bacterial infections in animals, including fish, farm animals and pets. But antibiotics are also extensively used to prevent disease and to promote growth in food animals. In some countries, antibiotics are also used for prevention of bacterial infections of fruit.

How does the human health impact of these non-human uses stack up? The evidence suggests that antibiotic use and abuse in humans drives resistance among the foremost respiratory, genital, urinary and skin infections. (2) On the other hand, antibiotic use in agriculture is thought to have an impact largely on the human intestinal infections for which animals are important sources (for example zoonoses--diseases that are passed from animals to humans--such as Salmonella and Campylobacter). (3) Since plants and farmed fish are infrequent sources of intestinal bacteria for humans, most of the attention has focused on antibiotic use in terrestrial animals.

In Canada and many other countries, thousands of intestinal infections are passed from animals to humans through contaminated food and water. These agents are pathogenic to humans whether or not they are resistant to antibiotics. Therefore to assess the impact of resistance on human health it is necessary to estimate any extra burden of illness attributable to resistance.

In food-borne infections, antibiotic resistance can have multiple effects on human health. Resistance can render infections more difficult or expensive to treat, or produce more severe or longer-lasting disease through enhanced virulence or pathogenicity. And because antibiotics also kill healthy flora in the gut that protect against illness, resistance can also increase the risk of infection (of Salmonella in particular) in people taking antibiotics for other reasons.

Furthermore, evolutionary processes of selection and adaptation can increase the pool of resistant genes in our ecosystem, and promote their spread among different species of bacteria. And when animals undergo antibiotic therapy, increased resistance to antibiotics can enhance the spread of zoonotic infections among the animals themselves, making them more likely to spread to humans by direct or indirect means. (4)

While there is qualitative evidence that antibiotic use in agriculture contributes to resistance in human infections, only a few estimates of the magnitude of this contribution have been published. Many national bodies that regulate veterinary drugs, as well as the World Health Organization and other international agencies, are currently grappling with how best to assess the risks in order to be able to make reasonable regulatory decisions that address those risks. It is also important to consider the benefits of antibiotics to animal health, animal welfare and animal production.

Because of the complexity of the farm-to-fork continuum, it is difficult to study the public health risks associated with non-human uses of antibiotics directly. Therefore, indirect means of studying these risks must be found.

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