For decades, livestock producers have routinely administered antibiotics to animals to make them grow faster or as a preventive treatment against disease. This may be contributing to the increasing number of humans being infected by antibiotic-resistant bacteria. Researchers are now searching for viable alternatives.
Antibiotics are a mainstay in agriculture to help producers raise healthy livestock. Although the exact effect that the antibiotics administered to livestock have on microbes’ resistance is under debate, there is evidence that transmission of resistant strains to humans does occur through food. Now, scientists are looking at alternatives — such as biologically modified probiotics — to reduce the spread of pathogens in animals and reduce the risk of transmission of antibiotic-resistant bacteria to humans.
The challenge of antibiotic-resistant bacteria in livestock production
Foodborne gastrointestinal infections exact a vast toll on humans (1–4). The most common cause of death from diarrheal disease globally is non-typhoidal, foodborne and waterborne Salmonella (3, 4). In the U.S., Salmonella enterica serotypes Typhimurium and Enteritidis are the leading causes of foodborne disease outbreaks, with over one million infections annually (4) and healthcare and lost productivity costs that exceed $3 billion every year (5).
Of growing concern is the continuing emergence of microbial resistance to first-line antibiotics (6–9). More than one million people are sickened in the U.S. by multidrug-resistant infections and over 20,000 die per year. This statistic is disconcerting. In January 2017, the Centers for Disease Control and Prevention (CDC) announced the death of a woman in Nevada by a pneumonia bacterial strain that is pan-resistant, i.e., resistant to all antibiotics available in the U.S.
Numerous cases have been reported of antibiotic-resistant foodborne pathogens, including Salmonella and Campylobacter spp. (6–9). For example, in 2006, the CDC detected clusters of human infection by multidrug-resistant Salmonella serotype Newport (9). This serotype is the third most common one in the U.S., and it is resistant to ampicillin, cephalothin, cefoxitin, amoxicillin, ceftriaxone, chloramphenicol, tetracycline, and streptomycin, among other first-line antibiotics. The CDC has designated non-typhoidal Salmonella and Campylobacter jejuni strains serious threats (9).
One likely significant source of increasing drug resistance is the widespread use of antibiotics in farm animal production (10, 11). An estimated 14,000 tons of antibiotics, approximately 70% of all antibiotics produced in the U.S., were administered to cattle, pigs, and poultry in 2015.
For decades, antibiotics have helped producers to raise healthy livestock. Antibiotics are also often used to promote growth and improve feed efficiency, even in the absence of infection (11). Arguably, antibiotics have facilitated and sustained (along with major advances in animal breeding and production processes) significant enhancements in the production of livestock.
The problem of resistance may stem from the unregulated use of antibiotics in agriculture. An estimated 97% of antibiotics administered in livestock are over-the-counter (OTC) and used as growth promoters. This pretherapeutic administration of antibiotics to animals likely creates a vast reservoir of drug-resistant bacteria that can infect humans through food.
Alarmingly, there is substantial overlap between classes of antibiotics listed as critically important for human health by the World Health Organization and those antibiotics listed as critically important in agriculture by the World Organization for Animal Health (12). For example, three classes of antibiotics, including quinolones, third- and fourth-generation cephalosporins, and macrolides, are reportedly used in agriculture, even though they are among the few viable therapeutic solutions against certain serious infections in humans (12).
It is important to note again that the precise contribution of antibiotics in animal feed to infections in humans by antibiotic-resistant microbes is under debate (13–16). In complex systems such as food production, it is difficult to establish causal relationships. Nevertheless, there is undisputed evidence that transmission of resistant strains to humans does occur through food (17–20).
Because of these concerns, the European Union banned the use of antibiotics in food animal production in 2006 (21). Similar legislation was introduced in the U.S. House and Senate. That law, entitled the “Preservation of Antibiotics for Medical Treatment Act” (H.R. 965/S. 1211), would have phased out the nontherapeutic use in animals of certain antimicrobial drugs that are important for human health.
The attempts to pass that law were met with resistance. There are, indeed, many important and demonstrated benefits of using antibiotics in livestock production (14–16). A widespread ban of antibiotics may jeopardize the global supply of abundant, high-quality, nutritious, safe, and relatively inexpensive food. In the U.S., a ban on the use of antibiotics, in the absence of alternative antibiotic technologies, could substantially increase food prices (22). That, in turn, could diminish the enormous positive impact of the U.S. animal agriculture sector on the economy.
Curtailing antibiotic use in livestock production is a problem with global dimensions. The earth’s population is projected to reach more than nine billion within three decades. The growing human population and...
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