Scientists using DNA fingerprinting have proven that drug-resistant "superbugs" are escaping from hog farms into nearby water supplies and becoming part of bacteria that normally operate in the food chain.
Researchers say the findings prove that drug-resistant microbes developed in U.S. farm animals can spread in the environment - and potentially to humans, where they cause resistance to drugs.
"The implications are big," said Roderick Mackie, a professor of animal sciences at the University of Illinois-Champaign, one of the authors of the study. He said U.S. farmers must reconsider current practices of using antibiotic drugs as growth-promoters and restrict drug use only to treating sick animals.
The Animal Health Institute, a group financed by the drug and feed industries, estimates that 20 million pounds of antibiotics were sold for animal use in 1999, with about 10 percent of that amount used to promote growth or improve the feed of animals.
Those statistics are disputed by the Union of Concerned Scientists, which contends about 70 percent of 26 million pounds of animal drugs sold each year are used for growth promotion.
In the Illinois study, researchers looked at tetracycline, a common antibiotic that is used primarily to promote animal growth on pig farms.
Using DNA fingerprinting, they found bacteria with genes resistant to tetracycline in both groundwater and water from a 20-foot-deep underground well one-ixth of a mile from two hog farms that used the drug. One of the farms spread the pig manure on its fields, which Mackie said could have contaminated the groundwater supplies.
The researchers say the DNA fingerprinting showed the tetracycline-resistant genes in the bacteria came from the hogs.
Their study was published in a recent edition of the scientific journal, Applied and Environmental Microbiology. Mackie said the team now is conducting similar tests for the presence of an erythromycin-class drug used in the animal industry.
Richard Wood, executive director of the Food Animals Concerns Trust, said he was concerned by the study, which is the latest to cast questions over the widespread use of antibiotics as growth promoters in American agriculture.
"This is another indicator that we do need to be concerned with antibiotics," said Wood, who serves on a veterinary medicine advisory panel for the Food and Drug Administration.
This year the FDA's center for veterinary medicine, which approves drugs for animal use, withdrew one class of drugs for poultry use and has taken steps toward restricting use of a second drug. The center also announced increased funding for research into anti-microbial resistance in animals.
The British Medical Association predicts that one of the biggest public health problems of this century will be increased human resistance to drugs. The U.S. Centers for Disease Control and Prevention has urged restrictions on farm use of drugs that have human applications.
But the Animal Health Association contends that the increased human resistance to antibiotics is due to over-prescription by physicians and not animal use. Any drug-resistant "superbugs" in meat would be killed in the process of cooking, the group argues.
Wood said there is sufficient scientific evidence of problems that farmers need to respond. "The industry says we can't show them dead bodies, but why do we need to wait until that happens?" he said.
Wood said any move to restrict farm use of antibiotics would cause major changes in modern farming practices.
Antibiotics reduce animal epidemics and make it possible to raise poultry and pigs close together on factory farms. But he said farmers in Sweden and Denmark, both of which banned the use of antibiotics to promote animal growth, made the transition using new bio-security procedures and modern hygiene practices of cleaning barns and stalls after herds and flocks were sent off to slaughter.