London - A clear link between the use of antibiotics in animal feed and the emergence of "superbugs" in hospitals has been established for the first time. Doctors have repeatedly warned of the danger, but proving it has been more difficult. The emergence of antibiotic-resistant strains of common bacteria is often blamed on excessive use of antibiotics in medicine, rather than in animal feedstuffs.
Now gene tests on bacteria in the gut of people, pigs and chickens have shown that resistance to one particular antibiotic has moved from animals to humans. The new studies, carried out by Henrik Wegener of the Danish Veterinary Laboratory, suggest that a common type of bacterium found in the intestine developed resistance to vancomycin, a widely used antibiotic, when a similar drug was used in animal feed.
Antibiotics are given in animal feed because they typically increase the growth rate by 5 per cent. Dr. Wegener now believes that they should be banned as growth promoters.
Enterococci - bacteria in the gut - became resistant to vancomycin in 1986, and the resistant forms spread throughout Europe and the U.S. They are not usually dangerous except in patients with poor immune systems, so these new strains have not caused as much alarm as vancomycin-resistant Staphylococcis aureus, known as "superstaph," which has since begun to appear.
Dr. Wegener showed that the resistance moved from animals to humans by isolating the gene responsible for vancomycin resistance in enterococci from people, pigs and chickens. He found that the gene - apart from disarming vancomycin - contained a mutation.
Bacteria in poultry from several countries all carried one type of mutation, pigs carried another. Humans carried both.
This means, says Dr. Wegener, that humans must have got the resistance from animals. If the traffic had been in the other direction, animals would show both variants.
Avoparcin, the antibiotic used in animal feeds, was banned in 1997, but animals are now being given another antibiotic, virginiamycin, which is very similar to the new drug, Synercid, used to replace vancomycin in human beings. Studies have already shown that some enterococci in farm animals are resistant to Synercid. "The story about avoparcin and vancomycin is rewriting itself," Dr Wegener told New Scientist.
Roche Products, the company which makes avoparcin, remains unconvinced. "These are interesting data, but I'm not sure you can categorically state from them that it is one-way traffic of resistance," said Dr. Tony Mudd, of Roche.