A group of international human microbiologists has concluded that while a theoretical hazard to human health arises from the use of antibiotics in food animals, an examination of the facts shows that the actual risk is extremely small, according to a report from the Animal Health Institute.
The independent expert group - which included risk assessors, veterinarians and animal health experts - evaluated available data on the effects of antibiotics in humans and animals and attempted to confirm or deny a link between antibiotic resistance in animals transferring to humans. They met recently in San Diego, US, prior to the opening of the 42nd Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), the world's premier scientific meeting on infectious diseases and antimicrobial agents.
"In 50 years of antibiotic use in animals and man, the development of resistance in animals has not made a major impact on human and animal health, and such a development seems unlikely to happen overnight now," said Dr Ian Phillips, Emeritus Professor of Medical Microbiology at the medical school of Guy's and St. Thomas' Hospitals, University of London.
Phillips, who is chairing the group of experts, said that evaluating available facts persuaded the group that whereas the use of antibiotics in humans and animals undoubtedly leads to resistance, and while some resistant organisms reach man via the food chain, little additional harm results from resistance, even when infection occurs.
Much of the debate over the issue of antibiotic resistance has centered on the use of antibiotics in animals to promote growth. The case against antibiotic growth promoters has relied very heavily on antibiotic-resistant enterococci, a group of bacterial organisms that cause no disease in animals but can cause disease in man and which might be zoonotic (transmittable from animals to man under natural conditions). However, new surveillance data show that enterococci resistance is increasing in areas where antibiotic growth promoters have been withdrawn.
"Vancomycin-resistant enterococci(VRE) and Synercid-resistant E. faecium are becoming more prevalent as a cause of infections in humans in Europe at a time when these resistant organisms are becoming less prevalent in animals and food products following the antibiotic growth promoter ban," observed group member Ronald Jones, M.D., referring to two commonly used antibiotics that are effective against various bacteria including enterococci. Jones is Principal Investigator of the SENTRY Antimicrobial Resistance Surveillance programme, a global network of healthcare facilities monitoring resistance levels in human bacteria, providing the world's largest database of antibiotic resistance.
"SENTRY Programme data from parts of the world where Synercid was used for patients show an increasing prevalence of resistance in the absence of the genetic mechanisms of resistance commonly found in most animal strains," Dr Jones continued. "We thus conclude that the increasing prevalence of resistance was not due to passive acquisition of resistant strains of animal origin, but due to antibiotic use in humans," he said.
A major topic of the group discussion was the possible adverse effects of antibiotic bans on animal health and well-being. This issue has been studied extensively in Denmark, where the use of antibiotic growth promoters was phased out beginning in 1997 through to 1999.
"Danish farmers have found that banning antibiotic growth promoters has caused pigs to get more cases of diarrhoea, especially baby pigs," said group member John Waddell, D.V.M., a Nebraska veterinarian who has toured several Danish pig farms. "The pigs have slower post-weaning growth rates and increased production costs."
Waddell added that Danish pigs, because of increased prevalence of diarrhoea and other diseases, require more therapeutic antimicrobials, according to DANMAP, the Danish national database that tracks patterns of antibiotic usage and resistance from human and veterinary medicine and food hygiene. While only 48,000 kilograms (kg) of antibiotics were used in Denmark for treatment of food animals in 1996, that amount increased to about 57,000kg in 1997; to 57,300kg in 1998; to 61,900kg in 1999; to 80,600kg in 2000; and to 91,602kg in 2001 - an increase in use of more than 90 per cent since the withdrawal of growth-promoting antibiotics.
"At the same time, human cases of salmonella and campylobacter have reached record levels in Denmark and the proportion of multiple antibiotic-resistant salmonella DT104 has doubled since 1997," Dr Waddell said.
The group of experts concluded that banning any antibiotic usage in animals, in the absence of a full risk assessment, is not useful and could even be harmful to both human and animal health.
"Rather than banning the use of antibiotics in animals, we believe that efforts should focus on reducing the transmission of all food-borne pathogens regardless of their antibiotic susceptibility," commented Phillips. "This can only occur through insistence on good hygienic practices on farms, in abattoirs, during distribution and marketing of food, and in the proper handling and cooking of food, and must be accompanied by consumer vigilance. Considerable progress has been made in the US as demonstrated by the decline over the past five years of food-borne illness reported by the Centers for Disease Control and Prevention (CDC)."
