Tests commonly used to screen for antibiotics in the milk of commercial dairy cows often falsely indicate the presence of antibiotic residues, according to a study by a veterinary researcher at the University of California, Davis. These tests are frequently unable to differentiate between disease-fighting agents produced naturally by the cow's own immune system and antibiotics administered to the cow for medical treatment, according to Dr. James S. Cullor, associate professor of pathology at the UC Davis School of Veterinary Medicine. Cullor will present his findings Wednesday, Feb. 12, during the annual meeting of the National Mastitis Council in Arlington, Va. Such false positive tests could unduly alarm the public, result in unwarranted discard of milk, prove costly to the dairy producer who is unnecessarily keeping cows out of the milking string, and cast a shadow of suspicion on the actions of the producer, veterinarian and antibiotic manufacturer, Cullor said. Antibiotics are widely used to treat a variety of diseases in dairy cattle including respiratory illnesses, diarrhea and hoof infection. Such medicines are also used for mastitis, an inflammation of the cow's mammary gland. Residues of these antibiotics in the commercial milk supply are undesirable both from a food safety standpoint and for processing reasons, Cullor said. "There is a minor concern that the presence of antibiotics in milk may cause allergic reactions as well as create drug-resistant bacteria in humans," Cullor explained. "And, antibiotics in the milk could kill the beneficial bacteria necessary for making dairy products such as cheese." While antibiotics above certain levels in milk are undesirable, the cow's naturally produced disease-fighting agents pose no threat to humans or to dairy processing, Cullor stressed. Federal regulations have established a random system for testing milk in bulk tanks for antibiotic residues, and, in certain situations, individual cows must also be monitored. After a cow receives medical treatment, she is removed from the milking string for a few days to allow the medicine to pass from her system. Before the cow is returned to the milking string, her milk must test free of antibiotics. Using commercially produced drug-residue tests called assays, Cullor and his colleagues recently ran more than 4,000 tests on approximately 600 cows at commercial dairies in Northern California. Depending on the test used, they found that the accuracy rates of the assays, when applied to individual cows, ranged as high as 100 percent false positives -- results that indicate the presence of an antibiotic residue in the milk when none is actually there. "We ran tests at the UC Davis dairy on 68 animals that we knew had received no medication for more than 30 days," said Cullor. "One test indicated that 38 of those animals had antibiotic residues in their milk." There are three basic types of tests used to identify antibiotic residues. One relies on the inhibition of bacterial growth, the other is a liquid carrying an enzyme that would be involved in a chemical reaction and resulting color change if the antibiotic is present, and the third is much like a home pregnancy kit where a solid compound changes color in the presence of the antibody. While the tests are efficient monitors of milk safety at the bulk tank level, they are not appropriate indicators for antibiotic residues in the milk of individual cows, Cullor's study concluded. The chemical tests are accurate at the bulk tank level, because the influence of any one cow's immune system is so diluted that the tests do not pick it up. "Technology simply is not ready to do what the regulations require," Cullor said. He suggested that a four-phase method be used to determine whether a chemical test is specific enough to use on an individual cow, rather than in the bulk tank. He also recommended that a regulatory agency be made responsible for quality control of the milk-testing assays, that a review panel be established to consider approval of licensing such assays, that proposed assays be compared to existing identification procedures and, finally, that the license for the antibiotic residue test take into consideration the situation in which it will be used, the need for such a test and its reliability. Dairy producers nationwide currently use a 10-point Quality Assurance Program, developed by the American Veterinary Medical Association and the National Milk Producers Federation, to promote and document responsible use of antibiotics at the dairy. Cullor's work was done through the Dairy Food Safety Laboratory in the UC Davis School of Veterinary Medicine. It was partially funded by the California Milk Advisory Board, the Livestock Disease Research Laboratory at UC Davis and the Upjohn Co.