Bacteria change so quickly that even the best surveillance system for early detection of resistant germs would find the problem too late, epidemiologists say.
"Our analysis suggests that animal antibiotic use hastens the appearance of antibiotic resistant bacteria in humans," says a report in the April 30 issue of the Proceedings of the National Academy of Sciences. "Our model indicates that the greatest impact occurs very early in the emergence of resistance, when antibiotic resistant bacteria are rare, possibly below the detection limits of current surveillance methods."Critics say animal farms use more than 20 million pounds a year of antibiotics, almost all of them to stimulate growth rather than treat disease. The fear is that bacteria in animals given antibiotics will develop resistance, and that the genes for resistance or resistant bacteria could infect humans, making infectious diseases harder to treat.
The mathematical model, developed by researchers in the University of Maryland department of epidemiology and preventive medicine, starts with the knowledge that people are constantly exposed to bacteria from animals. Most of these bacterial colonies disappear quickly, but some are persistent, and they can spread from person to person.
Some complex mathematics are needed to determine what happens in people, says the report, because "the epidemic spread of antibiotic resistant bacteria is a nonlinear process, and intuition about nonlinear processes is notoriously bad." But applying estimates of incidence and spread in differential equations produces a chilling conclusion:
"Data collected through ordinary surveillance would probably not have sufficient statistical power to detect an increase in prevalence caused by animal antibiotic use. Once an increase was detectable, the spread of antibiotic resistant bacteria would be irreversible. ... Small increases in prevalence when antibiotic resistance is rare have dramatic effects, like sparks that start forest fires. The consequences might be due to rare events, which are notoriously difficult to study."
David L. Smith, lead author of the paper, calls it "a pretty robust conclusion." And he says that no surveillance system could eliminate the menace, because "by the time you find the first case, you have just found the tail of the tiger. The problem is there."
"Trying to lay out a surveillance network so you would allow antibiotic use in animals until you detect a resistant strain in humans would be the wrong model," Smith says. "What we propose is an opposing framework, presenting antibiotic use in animals. That would avoid most of the damage. It would have a desirable result, but whether you want to do it is up to the policy makers, not to us."
An accompanying commentary by Marc Lipsitch, an epidemiologist at the Harvard School of Public Health, calls the model "simple but realistic," and says it "supports the adage that once the horse has fled the barn, it is too late to close the door."
Lipsitch calls the model "a good description of a phenomenon that constantly occurs, the transmission of bacteria from animals to humans.
"It makes that valid point that the potential of harm to human health from the use of antibiotics may be greatest when significant evidence of clinical harm has not yet appeared," says Lipsitch, who wrote an accompanying commentary.
Still, the model applies only to some species of bacteria, Lipsitch says. "There is good reason to reduce the use of certain antibiotics, in animals, but you have to take it case by case," he says The biology of the organism matters."
One predictable skeptic is Richard Lobb, spokesman for the National Chicken Council. Antibiotics are routinely used in chicken feed.
"Obviously, this is highly theoretical," says Lobb. "This is not real-world research, this is a mathematical model that very much remains to be demonstrated by real-world findings."
The poultry industry does not use antibiotics indiscriminately, Lobb says: "They are always used in a responsible manner in the chicken industry."
A spokesperson for the Food and Drug Administration, which regulates antibiotic use, says its experts will not comment on the report until they have time to analyze it in depth.
What To Do
You can reduce the risk of picking up germs from food by washing you hands, utensils and cutting surfaces diligently, and by cooking meat thoroughly.
To learn about antibiotic resistance, go to the Food and Drug Administration or the National Antimicrobial Resistance Monitoring System.