Linking Drugs to the Dinner Table

By Anastasla Toufexis

Antibiotics on the farm may be playing a role in human disease

If antibiotics have proved to be wonder drugs for medicine, they have been nothing short of miracle workers in agriculture over the past quarter-century. Today, about 15 million Ibs. a year, nearly half of U.S. annual production of antibiotics, are fed to farm animals, primarily cattle, poultry and pigs. Although the drugs help check the spread of bacterial infections among closely penned animals, their use is prompted as much by a happy side effect: for reasons not yet understood, they accelerate animal growth. But the lacing of animal feed with antibiotics is being increasingly challenged by scientists who claim it is a major factor in a fast-growing medical problem: the resistance of disease-causing microbes to antibiotics.

Nearly 25% of Salmonella bacteria--organisms that commonly cause food poisoning--are now resistant to many antibiotics. Critics charge that routinely putting antibiotics in feed promotes bacterial resistance by wiping out the less hardy of the vast array of microbes normally present in animals, leaving those that are drug resistant to flourish. If they are transferred to humans through meat and poultry products, these bacteria could then colonize their new hosts or pass on their antibiotic resistance to other bacteria already in residence.

Definitive proof of a link between drug-laced feed and human illness is difficult to obtain. But recently epidemiologists from Minnesota, South Dakota and the federal Centers for Disease Control in Atlanta traced a serious outbreak of gastrointestinal illness that was caused by antibiotic-resistant germs. The source of the bacteria, say the investigators, was hamburger from cattle that had been fed the antibiotic chlortetracycline. Declares Dr. Scott Holmberg of the CDC, who led the disease detectives in the yearlong investigation: "We were able to show for the first time ever how an antibiotic-resistant bacterium can actually make its way from the barnyard to the dinner table."

The elegant piece of sleuthing, reported in the New England Journal of Medicine, began on Feb. 19, 1983, with a call to the CDC from Michael Osterholm of the Minnesota health department. In the preceding four weeks, ten people in the Minneapolis-St. Paul area had been struck by gastrointestinal illness. The malady, marked by diarrhea, stomach cramps, high fever and vomiting, was severe enough to hospitalize six victims. The culprit was Salmonella newport, a strain of bacteria that normally accounts for only a dozen of the salmonellosis cases in the state in a year. Within days, an investigative team had discovered an upsetting but valuable fact: the bacterium was resistant to the antibiotics ampicillin, carbenicillin and tetracycline.

The most plausible explanation for the outbreak, that there was an unusual food to blame, was quickly rejected. According to Holmberg, "These people had the all-American diet: lots of meat, potatoes and bread.'' Seven of the ten victims had been taking antibiotics shortly before their illness, raising the possibility that the drugs were tainted. This too was eliminated.

The big break came when Kenneth Senger, the state epidemiologist in South Dakota, reported that there had been four cases of infection with antibiotic-resistant S. newport in the state in three months. Interviews established that the victims lived on farms six miles apart and that they got their beef from the same nearby feed lot, which routinely added chlortetracycline to the animals' feed. The CDC traced the path of the meat shipments from the feed lot to eight supermarkets patronized by the ten Minnesota victims. All had reported eating hamburgers within a week of the time they became ill.

The report has added new intensity to the debate about antibiotic additives in livestock feed. Since 1977 the Food and Drug Administration has been proposing a ban on the addition to feed of penicillin and tetracycline, two antibiotics widely used to combat human disease. Farmers would be free to substitute antibiotics not commonly used by people. Great Britain limited the addition of some drugs to livestock feed in 1971, and other European Community countries followed in 1973. But heavy lobbying by livestock breeders and pharmaceutical companies, which supplied antibiotics worth $270.9 million to the feed industry in 1983, has blocked a proposed U.S. ban in Congress. In addition, scientists disagree about whether there is a link between human disease and animal antibiotics.

Most scientists do agree that much of the weakened impact of antibiotics can be blamed on doctors who overprescribe antibiotics, ordering them, for example, for virus-caused colds, and on people who use them indiscriminately. Veterinarian Jerry Brunton of the Animal Health Institute, a lobbying group, finds major flaws in the study: "No meat samples were available to indicate that disease-causing organisms were ever present, nor were such organisms isolated in the meat processing and preparation locations or from the farm where the alleged source animals were raised."

Indeed, the conclusions of the CDC report are "inferential," concedes Epidemiologist Reuel Stallones of the University of Texas, who contributed to a 1980 report from the National Academy of Sciences that found the human health hazards of antibiotic feeds "neither proven nor dis-proven." But, he adds, "this is the best evidence I've seen up to this time that human illness is somehow linked to the use of antibiotics in animals for growth promotion. This study draws the net much tighter around the issue, but it is still a net, not a rope.'' --By Anastasia Toufexis. Reported by Leslie Cauley/Atlanta and Patricia Delaney/Washington

With reporting by Leslie Cauley, Patricia Delaney