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CORVALLIS, Ore. - A study by researchers at Oregon State University has found that Salmonella, E. coli and other bacteria identified from samples taken at Oregon dairy farms have a disturbingly high rate of resistance to antibiotics.

Though there is no evidence to suggest that these resistant strains of bacteria found in the dairy cattle's feed directly affect human health, the high level of resistance may be cause for concern, researchers say.

The findings were announced this week in the Journal of Environmental Health.

"There is a growing concern in the medical community about the ability of these bacteria to develop resistance to antibiotics and this is another piece of evidence that says that such a concern is warranted," said Annette Rossignol, an epidemiologist and professor of public health at OSU. "Their resistance is building faster than the medical community's ability to develop new antibiotics."

Lead author on the study was Riam S. Kidd, a former graduate student in OSU's Department of Public Health and now a microbiologist with the Willow Lake Treatment Plant lab for the City of Salem. Other researchers included Rossignol and Mike Gamroth, a professor of animal sciences and a dairy specialist with the OSU Extension Service.

As part of the study, Kidd took samples of feed ingredients from 50 feed piles located on 12 different dairy farms in western Oregon in 1998. For 10 of those piles, she took repeated samples until the feed pile was depleted.

What she found was that 42 percent of all the feed piles tested positive for Salmonella, and an even higher rate (60 percent) of the bacteria was found in the piles subjected to repeated sampling.

That isn't necessarily unusual, the researchers say, but further laboratory testing found that all of the samples of Enterobacteriaceae - a bacterial family that includes Salmonella and E. coli - were resistant either to ampicillin, tetracycline and/or streptomycin.

The antibiotic that was least effective in killing the bacteria was ampicillin. Sixty-two percent of the samples were resistant to the commonly used antibiotic.

"We know that Salmonella is a common bacterium in the dairy environment, so the prevalence didn't surprise me," Gamroth said. "But the level of resistance did. Salmonella gets to the dairies from a variety of sources, such as birds, rodents, sick animals brought onto the farm, and even from hired employees.

"It is readily killed by pasteurization, so there is no threat to the commercial milk supply," Gamroth added. "But anyone consuming raw milk on the farm could be at risk."

In addition to Salmonella and E. coli, the researchers found some isolates of Klebsiella pneumoniae to be multi-drug resistant.

Gamroth said antibiotics are used in the dairy industry at low levels for the first two to six months of calves' lives. Mixed into calf milk replacer and some calf grain, they help promote growth. There is a concern, he said, that the widespread use of these antibiotics contributes to the bacteria's ability to develop resistance.

"On the other hand, the bacteria showed resistance to ciprofloxacin (or 'Cipro'), that has never been used in dairy veterinary care, and to chloramphenicol, which has been banned for some time," Gamroth said. "So the data could be interpreted that the resistance seen in some of the serotypes found in the study started off the farm."

Kidd said she began the study four years ago after learning from colleagues at the Oregon Department of Agriculture Laboratory Services that some people had suffered severe diarrhea after drinking raw milk that contained Salmonella enteritidis DT104, which is resistant to at least five antibiotics and difficult to treat.

"We hypothesized that the bacterial contaminants were in the feeds," Kidd said. "We knew some of the feeds were pasteurized prior to the farm, but many are not. The few (research) papers out there indicated that there could be bacterial contamination with dairy feed, especially those that contained rendered meats. So I became interested in the possibility of anti-microbial resistance because you hear more and more about the prevalence of drug resistance in pathogenic bacteria.

"Still, I found it surprising to find such a high percentage of resistance in such a short time," she added. "The sampling only lasted a month." Gamroth said the study's findings has prompted him to offer several recommendations to dairy farmers interested in controlling feedstuff contamination. These include:

• Rotate feeds in storage: first feed in, first feed out.
• Thoroughly clean and dry bins when empty before new feed goes in.
• Control rodents and birds, if possible.
• Consider covering feeds in open bins with thin polyethylene.
• Reject any feeds delivered that are damp, or have evidence of mold.
• Keep the feed mixing area and equipment clean. Wash feeding equipment after each use if it becomes contaminated with mud and manure.

The researchers say more studies are necessary to determine where the contamination comes from, and whether these resistant strains of bacteria are transferred to the milk supply - especially in products that are not pasteurized.

"It also is important to look at what extent, if any, the widespread use of antimicrobials on dairy farms and other food-producing farms may contribute to the eventual loss of antimicrobials to successfully treat human bacterial infections," Kidd said.

Rossignol said the danger of losing antibiotics and antimicrobials as an effective medical tool would equate to losing one of our few remaining "magic bullets" from the past half-century.

"When penicillin was first developed, it was used strictly for saving life and limb - literally, in cases of gangrene or life-threatening bacterial infections," Rossignol said. "Now it's used for treating colds, and other antibiotics are used to promote growth in livestock. How long will it be before bacteria develop complete resistance to them, rendering them useless as a medical tool?

"We just don't have that many magic bullets in the medical world."