The first antibiotic, penicillin, became widely available after World War II, a decade or two before the beginning of the allergy epidemic. By 1992, the apogee of the allergy epidemic, children and teens were averaging nearly one course of antibiotics per year, often for minor conditions, such as otitis media, an ear infection.
That was half again as many antibiotics as the average child took in 1980. In that same period, the number of broad-spectrum antibiotics, such as cephalosporins and fluoroquinolones — drugs that functioned like sledgehammers rather than scalpels on microbial communities — increased.
By the turn of the millennium, worry over antibiotics resistance had lessened the prescription rate by half. (Doctors also realized that otitis media was a self-limiting condition, and didn’t always require antibiotics.) It’s perhaps no accident that the allergy epidemic leveled off by the new millennium as well.
The secret in plain sight, however, was that most antibiotics weren’t given to people at all. Roughly 70 percent in the U.S. went to animals, not humans. They were given to check infections in some cases, but mostly, low-dose antibiotics were used to accelerate animal growth. How often they made their way into the food supply remained a largely unexplored question.
Protocols existed to ensure that meat and dairy remained antibiotic-free. Chief among them, livestock producers were to discontinue antibiotic administration an adequate amount of time before slaughter. The problem was, the USDA’s Food Safety and Inspection Service — the agency tasked with enforcing these rules — had, in its own words, “serious shortcomings.”
A scathing 2010 report by the Office of the Inspector General cited a litany of failures: carcasses known to be tainted that entered the food supply anyway; repeat offenders ignoring multiple warnings; and an inability to discern where, in the complicated web of livestock raisers, feedlot owners, and meat resellers, tainted meat originated. The National Residue Program was “not accomplishing its mission of monitoring the food supply for harmful residues,” the report concluded.
Even vegetarianism couldn’t save you. Between three-quarters and half of all antibiotics consumed by animals were, after all, excreted in feces and urine. That manure went to fertilize crops. First, it usually sat in pits where, apologists argued, microbes made quick work of any residues. But scientists looking into the question found that antibiotics endured, arriving intact in farmland. And food crops readily absorbed them.
In experiments, lettuces took up florfenicol, levamisole, and trimethoprim from tainted soil. Carrots absorbed diazinon, enrofloxacin, and florfenicol. Staple crops, such as wheat, corn, barley, and potatoes, soaked up veterinary antibiotics when present. And that was hardly the extent of the problem.
Treated human sewage became fertilizer as well. Just over half of the sludge removed from U.S. wastewater-treatment plants ended up in fields. Human-derived slurries contained an even wider array of pharmaceuticals than your average animal manure — antidepressants, antimicrobials, compounds from fragrances and soaps, hormones from birth-control pills — in short, everything people took.
Food crops sopped up these molecules as well. Soybeans fertilized with human biosolids readily took up the antimicrobials triclosan and triclocarban. Hydroponically grown cabbage absorbed the antimicrobials sulfamethoxazole and trimethoprim.
According to the USDA, however, fertilizer derived from manure goes onto just 5 percent of U.S. farmland, mostly corn. And human biosolids are used on just 1 percent of our farmland, according to the EPA.
But even waterways distant from manure pits and sewer outfalls can contain traces of drugs. Of 139 streams tested across 30 states, over one-quarter contained a little trimethoprim. One-fifth, although not the same fifth, contained erythromycin, lincomycin, and sulfamethoxazole.
A more recent analysis of drinking-water plants found 10 different antimicrobial drugs were common enough. Meanwhile, sediments laid down in the 1970s in waterways across the country were full of the antimicrobial triclosan, a common ingredient in hand sanitizers and some toothpastes. The molecule, which, aside from affecting microbial communities directly, degrades into dioxin, a potentially toxic substance that binds to hormone receptors, accumulated in snails, frogs, fish, and earthworms wherever it was present.
To make a long and depressing story short, the world was full of human-made, microbiome-altering substances. The only boon was that concentrations tended to be minute. In the carrot experiment, for example, the skins contained 10 percent of the official daily maximum intake amounts. And in those drinking-water plants, the amounts were even more minuscule, several orders of magnitude below therapeutic levels. But the drugs nonetheless had far-reaching impact. Antibiotic-resistant bacteria showed up in wild seagulls and sharks, among other wildlife.
The concern over antibiotics administered to livestock had traditionally centered on antibiotics resistance, but given that some members of the human microflora were more sensitive to them than others, alteration of our internal microbial communities presented a new reason to worry. A scenario whereby a few crucial allies were knocked out, leaving behind an imbalanced, destabilized ecosystem, wasn’t completely farfetched.
Ilseung Cho, a scientist at NYU, found that antibiotics given to mice at one-tenth the therapeutic dose measurably altered the microbiota and increased body fat by 10 to 15 percent. Antibiotics given at concentrations seen in carrots fertilized by animal slurries, in other words, could change your vulnerability to, among other conditions, metabolic syndrome.
And, according to a 2012 Danish study on sausages, antibiotic residues can make their way into pepperoni and salami, where they slow down the natural fermentation process and thereby encourage the growth of dangerous bacteria, such as E. coli.
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