As much as 70 percent of the bacterial antibiotics produced in the U.S. is fed to healthy livestock for growth promotion. At the same time, pathogenic bacteria that infect humans are increasingly lashing back with drug resistance to the antibiotics prescribed against them.

Is there a man/beast connection?

Epidemiologist David Smith, at the University of Maryland School of Medicine in Baltimore, explained: “Farmers discovered early on probably in the 1950s that their animals would grow faster if fed antibiotics. Ever since, they have been adding antibiotics to their feed. The animals grow faster, so they can come to market a few days earlier. That means an economic benefit for farmers. But antibiotic resistance has become common on the farm.”

To connect those two dots animal and human Smith narrated this scenario:

“Most poultry harbor large numbers of microbes in their guts notably Enterococci, which inhabit the intestinal tract. These microbes get on the outside of the chickens when the fowl are transported to the slaughterhouse. These bacteria are present in the abattoir environment when the birds are eviscerated. Sometimes the gut gets perforated, and then the bacteria contaminate the outside of the chicken. They persist at the supermarket where you buy it and bring it home.

“Most people cook their foods appropriately, but they’re not completely hygienic about the way they handle raw meat. They’ll get it on their hands, slosh it around on the kitchen countertop where they cut up some lettuce or vegetables. So raw produce becomes contaminated with antibiotic-resistant bacteria that accompany the chicken’s juices. These bacteria end up in your salad and in your gut. Then, say, someone in the community who’s been colonized by antibiotic-resistant bacteria enters a hospital and receives an antibiotic. That individual begins to shed large numbers of antibiotic-resistant bacteria, and then you have an epidemic on your hands because those bacteria spread from patient to patient.”

From Animal Farm To Hospital Epidemic

“These antibiotic-resistant bacteria then come in contact with immuno-compromised hospital patients, who are difficult to treat. The two main factors,” Smith summed up, “are contamination of food, and medical use of antibiotics in hospitals leading to epidemics.”

Smith, an assistant professor of epidemiology and preventive medicine, is lead author of a paper in the Proceedings of the National Academy of Sciences (PNAS), dated April 23, 2002. Its title: “Animal antibiotic use has an early but important impact on the emergence of antibiotic resistance in human commensal bacteria.”

“The problem,” he told BioWorld Today, “is that antibiotic use leads to antibiotic resistance. In hospitals, eventually resistance to that antibiotic emerges and spreads. On a farm, it does the same. The question that’s always been in contention is how antibiotics on farms affect humans in hospitals. That’s what we built our mathematical model to describe.

“What we found and reported in PNAS was that medical antibiotic use and animal antibiotic uses play different roles. Antibiotic use in humans,” Smith analogized, “is the hot breeze that fans the flame. And animal antibiotic use is the spark that starts the forest fire. Our main finding is that to figure out what the impact of using an antibiotic in animals has been, one must investigate the historical processes that led to the evolution of resistance. To my knowledge,” Smith observed, “this is the first model of its kind that links antibiotic use in animals to the emergence of antibiotic resistance in the commensal bacteria of humans.”

Family doctors are frequently blamed for giving their patients an antibacterial drug when they really have a viral infection, such as a common cold. Smith said, “Some doctors do imprudently prescribe antibiotics like candy. They should be made aware that they’re doing great harm. But even if you could tell all of those doctors to prescribe an antibiotic only when necessary, and only for indications of bacterial infection, physicians still face a fair amount of uncertainty. When a patient presents to a doctor with certain symptoms, and the practitioner decides to prescribe antibiotics, it’s entirely possible that they will do no good. But it’s very important to put the patient on antibiotics immediately. At that point, testing should continue, to decide whether it’s the right treatment.

“Most microbes out there are becoming resistant to antibiotics that were previously working against them,” Smith observed. “We’re not in the post-antibiotic era that people talk about not yet. We’re somewhere in the middle of the ball game. But we do seem to be moving in that direction, and we’re trying to find ways right now for scientists and physicians to recommend how to use antibiotics so we can stop that inexorable slide toward a post-antibiotic era.

“My role as a mathematical epidemiologist,” Smith pointed out, “was to build the math model we report in PNAS. Its physical form is a set of three differential equations. The effect of using antibiotics in animals is to shift the sigmoidal curve we derived,” he noted. “During that phase antibiotic resistance is increasing, but exponentially at a very slow rate. That’s called a honeymoon phase when antibiotics have their maximum period of utility because resistance is so rare. What antibiotic use in animals can do is shorten that honeymoon period or eliminate it altogether.”

Sigmoidal Curve Plots Resistance Honeymoon

“So that once you start using the antibiotic in humans,” he recalled, “resistance is immediately available because you’ve just been contaminated from chickens. So conditions are ripe for it to spread in the hospital immediately instead of taking several years. The reason I think this is so critical,” Smith pointed out, “is because the way antibiotic crisis in this country is going, there’s a balance between discovery chemical drug companies designing new antibiotic agents and putting them through the very expensive approval process, on the one hand; and on the other, the wise management of existing antibiotics. We are urging reserving these antibiotics for humans to spend and maximize the honeymoon period not using them in animals first.

“There still might be reasons not to use them in animals,” he added. “There is the concern that by heavily dosing them in livestock, multidrug resistance may evolve and spread to humans. Once clinically significant resistance has appeared and spread, you can have a debate as to whether to use it in animals. Otherwise,” he concluded, “it would be wise to reserve these antibiotics for humans.”