Despite our best efforts to wash our hands and keep raw meat separate from other ingredients while cooking, over 76 million Americans per year become ill from foodborne bacteria or the toxins produced by them []. The bacterial culprits are typically E. coli, Salmonella, or Campylobacter jejuni. While gastrointestinal diseases are unpleasant, they can be treated with antibiotics that kill the offending bacteria. However, scientists and consumers alike worry that antibiotic-resistant “superbugs” may soon contaminate the food supply. One step the Food and Drug Administration (FDA) is taking to prevent such superbugs is to regulate the use of antibiotics in livestock.
How are antibiotics used in livestock?
Human patients are prescribed antibiotics for therapeutic purposes; that is, they only take the drug until the infection clears. However, this is not the way antibiotics are used in cattle, poultry and fish. In the mid-1940s, shortly after the rise of antibiotics in human therapy, it was observed that continuous treatment of healthy livestock animals with antibiotics made them grow faster []. In some cases, this effect could be traced to eliminating harmful bacteria; chicks that were raised in a hygienic environment received no benefit from antibiotic treatment []. For other animals, there is no clear explanation behind growth promoting-antibiotics.
In addition to enhancing growth of the animal, antibiotics prevent infection by bacteria. Farmers use these drugs to maintain the health of their animals. Unfortunately, overuse of antibiotics provides a pressure for antibiotic-resistant variants of bacteria to arise. Antibiotic resistance can arise from random mutations in the bacteria’s DNA. Most mutations are harmful to bacteria, but rarely, a mutation will be beneficial, sometimes making them resistant to antibiotics. Since resistant cells are unaffected by the antibiotic, but other cells are killed, the resistant cells start to make up a large proportion of the bacterial population over time. Biologists would say that presence of the antibiotic selects for the growth of resistant cells. In addition, since animals are often housed in cramped quarters that are part of concentrated animal feeding operations (CAFOs), there is ample opportunity for these bacteria to spread widely. A trough of animal feed can serve as a reservoir for bacteria to infect many cattle, for instance []. The antibiotic-resistant bacteria could then contaminate the meat during processing of the animal carcass, posing a risk of infecting people who ingest the meat. Scientists and farmers have worried about this problem for decades, but is it a real threat?
Connecting drug-resistant pathogens from plate to patient
To understand whether antibiotic use in livestock poses a risk to consumers, scientists have to establish a link between a drug-resistant bacterium isolated from a patient and a contaminated source of meat. Unfortunately, it can be difficult to trace an infection to its source, and there is a severe lack of studies that have made this connection. However, some outbreaks of multidrug-resistant bacteria have been linked to contaminated meat. In Denmark in 1998, an outbreak of a Salmonella typhimurium strain resistant to five commonly-used antibiotics was traced to a contaminated swine herd on one Danish farm []. Despite finding the source of this bacterium, it is almost impossible to prove that resistance evolved due to antibiotic use in the pigs.
Most scientists working in the field are looking for correlation between the introduction of a new antibiotic in animals with an increase in infections caused by bacteria resistant to that drug. However, in 2006 a panel of experts from the Institute of Food Technologists concluded that there is insufficient evidence to make such a correlation, since outbreaks of foodborne illnesses do not correspond with antibiotic use by farmers []. In fact, this panel concluded that there are several factors that contribute to drug-resistant foodborne pathogens in addition to overuse of antibiotics. This is supported by the fact that drug-resistant bacteria are also found on organic farms that do not use antibiotics.
How are people trying to prevent drug-resistant bacteria?
In 1977, the controversy over antibiotic abuse in livestock feed caused the FDA to propose restrictions on the use of the common drugs penicillin and tetracycline for this purpose. However, the FDA has not created any new statutes based on that proposal. In 2011, the National Research Defense Council (NRDC) and other groups sued the FDA to ensure food safety by making it curtail use of these antibiotics. Early in 2012, federal judges ruled in favor of the NRDC, ordering the FDA to stop the use of several antibiotics in livestock [].
It is still unclear what the implications of this ruling are. Scientists and farmers commonly cite the effects of the ban on non-therapeutic antibiotics in livestock that was issued by the Danish government in the late 1990s. In other words, since the ban, antibiotics could only be administered to sick animals, not healthy ones. The data on the ban’s effects are conflicting – decreasing the use of these antibiotics caused a decrease in drug-resistant bacteria sampled from livestock, but drug-resistant infections in humans decreased in some cases and increased in others. Scientists say that this is because the use of antibiotics in Danish patients has increased over the course of the ban, and the greatest risk factor for getting a drug-resistant infection is using the antibiotic itself [].
Enforcing the judicious use of antibiotics is one way to thwart the evolution of a drug-resistant superbug, but farmers are experimenting with different methods of ensuring food safety. One effective way to reduce contamination is to thoroughly dehair and chemically sanitize the hides of animals after slaughter, because the majority of contaminants are present on the hide. Making the environment more sanitary for the animals by clearing manure frequently, disinfecting transport crates, and reducing moisture content in food and bedding material has been shown to reduce spread of disease among herds.
The debate about antibiotic abuse in livestock is a perfect example of a controversial issue where science should not be ignored, but the data are still inadequate. Right now, the connection between antibiotic use and the rise of drug-resistant foodborne infections is inconclusive. Reducing the use of antibiotics could be bad for both farmers’ and drugmakers’ profits, so they oppose what they see as an unnecessary ban. But the stakes are high; if we don’t act, antibiotics that we rely on might be rendered ineffective. Considering these factors, it is crucial to develop a clear answer backed by solid data. By reading primary sources of information, consumers can stay informed about food safety without media bias.
Alex Meeske is a PhD student in Biological and Biomedical Sciences at Harvard University.
References
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