You've reached the Virginia Cooperative Extension Newsletter Archive. These files cover more than ten years of newsletters posted on our old website (through April/May 2009), and are provided for historical purposes only. As such, they may contain out-of-date references and broken links.

To see our latest newsletters and current information, visit our website at

Newsletter Archive index:

Virginia Cooperative Extension - Knowledge for the CommonWealth

Use of Antibiotics and Development of Drug Resistance

Dairy Pipeline: July 2002

Gerald M. (Jerry) Jones
Professor & Extension Dairy Scientist,
Milk Quality & milking Management

In many countries, more than 50% of the total output of antibiotics is used in food animals for treatment of infections, disease prevention, and growth promotion. Subtherapeutic use causes concern that resistance will develop, which will reduce the ability of these drugs to fight disease in humans, but most of the clinically important resistant pathogens in humans result from inappropriate or over uses of drugs in human medicine. Scientists at Michigan State University summarized results of susceptibility testing on 2,778 milk samples from cows with clinical or subclinical mastitis that had been submitted to their diagnostic lab from 1994 through 2000. They reported in the Journal of Dairy Science that there were "no consistent trends among mastitis isolates from Michigan dairy herds toward increased resistance to antibacterials commonly used for mastitis therapy." They found that their results were similar to results of 25 years ago. The Michigan State authors suggested that these results may be attributed to development of quality assurance programs, more extensive research and education programs regarding mastitis therapy, and more stringent regulation of antibacterial use, storage, and labeling in the past 10 years. In 1999, John Keeling, from the Animal Health Institute, stated "there has never been a risk assessment that links the use of antibiotics in animals to any specific or quantifiable risk in humans." This is in contrast to opinions by the World Health Organization and the Alliance for the Prudent Use of Antibiotics. Dairy producers should use practices that emphasize healthy herds. Appropriate antibiotics should be selected for treatment of specific bacterial infections. They should use the recommended dose for a time sufficient to control the disease. If not, they should follow the extra-label directions of their herd veterinarian. A valid veterinarian-client-patient relationship is critical in making many management decisions. Herds should follow label or veterinarian instructions in properly administering a drug that has been stored correctly. Milk and meat from treated cows should be withheld from shipment until the risk of residue has passed. The keeping of adequate treatment records is important. All of these measures are included in a quality assurance program.

Visit Virginia Cooperative Extension