So, antibiotics. Given to farm animals. (Yeah, that again.) How does that work, anyway? Pills? Injections? Daily massage with specially compounded creams?

Not quite. Farm animals overwhelmingly get antibiotics in their feed. (You knew that.) And a new paper in the journal Environmental Health Perspectives explains what a bad idea that is: Animals that are given "free choice medicated feeds" (FCMF, in the jargon) can overdose or under-dose themselves, leading not only to the creation of antibiotic-resistant bacteria but to the accumulation of antibiotic residues in their flesh that can persist past slaughter.

There are two issues hidden in this. The first is the practice of giving tiny doses of antibiotics as "growth promoters," a use that dates back to the late 1940s. Despite a fair amount of study, there are still competing explanations for how this actually works, but the results are clear: Give animals micro-doses of antibiotics, and they put on weight faster, meaning they get to market size and can be sold — and replaced by another batch to whom the same thing happens — more quickly than if the drugs were not being used. The second is using treatment-sized doses in feed to take care of any illnesses among animals, as well as to protect animals who might pick up those illnesses in the close quarters of confinement agriculture.

If you view animal raising as an industrial-style process, the equivalent of making widgets on a production line, then medicated feed appears to make economic sense, because it offers a substantial return for little forward investment of money or labor. But as this paper picks apart, medicated feeds are not the bargain they seem.

There's little quality control on medicated feeds, so the drug doses they promise may be unpredictable. Of more concern, because of cross-contamination in manufacturing, feeds may contain additional drugs that are not declared on the labels.

Antibiotic-laced feeds can be bought, mixed and given without any veterinarian oversight. If there's an illness, it's farm staff —possibly a veterinarian, probably someone much less expert — making the diagnosis, choosing the feed, and administering it. There's no protection against misdiagnosis or misuse, meaning the feed may be given for conditions that the drug in it can't address — in other words, wasted.

Because animals are, well, animals — with pecking orders and herd behaviors — there is no guarantee that they will all eat the same dose. Some may crowd others out at the feeder or trough. Sick animals, the ones that most need the drugs, paradoxically may be the ones least likely to get them — and because of their illness, may be less likely to absorb the drugs if they do get an adequate dose. Plus, farms where wildlife have access — not so much chicken batteries, but hog and cattle-feeding operations — may lose some of their investment in feed because it is eaten by wildlife as well.

From the paper:

Given the limited oversight, the availability of FCMF without a veterinary prescription, the potential for undeclared drugs, and variability in drug concentrations within and between feeds, unintended (and therefore inappropriate) drug delivery is likely. At a minimum, these factors make predicting the actual delivered dose to any given individual animal nearly impossible and predicting herd averages for drug delivery complicated. Worse, inappropriate or imprecise drug dosing may drive selection for resistant microorganisms that affect both veterinary and human medicine.

Here's the authors' very nice breakdown of the possibilities, and the potential results:

So, to sum up: It's always been clear that indiscriminate use of antibiotics in agriculture leads to costs outside farms, in environmental contamination and in development of drug-resistant bacteria. What this analysis shows is that antibiotic over-use has costs on the farm as well. It's inefficient, it wastes drugs and money, and it doesn't necessarily do what it is intended to do.

So why keep doing it?

Cite: Love DC, Davis MF, Bassett A, Gunther A, Nachman KE, 2010 Dose Imprecision and Resistance: Free-Choice Medicated Feeds in Industrial Food Animal Production in the United States. Environ Health Perspect 119(3): doi:10.1289/ehp.1002625

Flickr/SalimVirji/CC