Dye Marking of Antibiotics Used in Treatment of Mastitis

The National Mastitis Council, Inc. defines mastitis as the infection of udder tissues (74). Mastitis is the most costly disease affecting dairy cows; many people do not realize the considerable losses due to mastitis (6). A reasonable estimate of the prevalence of mastitis is 50% of cows infected at any time (62); cows in the average herd contract clinical mastitis 1.5 times per year (74). The minimum antibiotic treatments per case of mastitis is two; with approximately 11 million dairy cows in the United States, the annual minimum number of antibiotic treatments is 33 million. In South Dakota, with nearly 160 thousand dairy cows, the number of antibiotic treatments is about 500,000 per year. A problem with the large number of treatments used to control mastitis is the potential for the drugs to enter the food chain. The Food, Drug, and Cosmetic Act established that milk and milk products containing antibiotics are considered adulterated, this makes monitoring milk for antibiotics required. Despite these regulations, a 1977 study of the US milk supply showed that 10.3% of bulk milk was contaminated with some type of antibiotic (74). Consumption of antibiotic residues in milk can cause allergic reactions in people. Welch et al. (82) estimated there are 17 to 20 million people in the US who are allergic to penicillin and other chemotherapeutic medications; however, the exact number of hypersensitive people is unknown (38). Kerne and Wimberley (44) reported that 15% of the US population is allergic to penicillin. Reactions attributed· to ingestion of penicillin in milk and milk products have been of the urticarial type. There is no documented evidence that consumption of milk or milk products containing penicillin can alone induce hypersensitivity (56). Another problem with antibiotic contaminated milk is that on prolonged exposure to sublethal concentrations of antibiotics, some bacteria can develop resistant strains. As little as two parts per million (ppm) daily has resulted in mutant strains of drug resistant gram-negative enteric bacteria (38). It is difficult, if not impossible, for the dairy industry to manufacture cultured dairy products with milk contaminated with antibiotics (19, 22, 31, 32, 76). Inhibition of lactic cultures in milk containing 0.01 units of penicillin per ml has been reported (1). However, penicillin inhibits Streptococcus thermophiles at 0.01 to 0.004 IU per ml, and Lactobacillus bulgaricus at 0.100 to 0.02 IU per ml (72). Detection of antibiotic residues in milk can be accomplished by direct and indirect methods. Direct control methods are conventional microbiological assays. Problems associated with direct control methods are: a) they are slow, the Food and Drug Administration (FDA) approved cylinder plate assay requires 16 to 18 h incubation time; b) they are tedious; and c) they require trained laboratory personnel and must be conducted in an appropriately equipped laboratory. The second form of antibiotic residue detection is by indirect methods. Mastitis remedies containing marker dyes are infused into a cow\u27s udder. For effective indirect control, the dye is excreted in milk with endpoints equal to those of the antibiotic. The potential advantage of dye marking is that it immediately alerts the dairy farmer to antibiotic contamination by visually discoloring milk. The milk would not be added to the bulk tank, thus, minimizing the chance of sending contaminated milk to a processing plant (75). Indirect controls cannot replace the conventional microbiological methods of monitoring milk for the presence of antibiotic residues, because intramammary infusion is not the sole source of antibiotic contamination in milk. Antibiotics can be excreted in milk through intravenous, intramuscular, and intrauterine therapy, and through transfer from treated to untreated quarters of the udder (75). Also, it is unlikely that perfectly parallel, unit-for-unit excretion of dye and antibiotic(s) can be achieved. For practical purposes, researchers can only show conclusively that the concentration of dye in milk from treated quarters does not fall below the visual endpoint, about 0.25 mg FD&C Blue No. 1 or FD&C Blue No. 2 per liter milk, until the antibiotic has fallen below detectable limits. Dye marking is a potential on farm supplement to conventional antibiotic testing and withdrawal times. The objectives of this research were to determine: a) the excretion endpoints of six dye-antibiotic intramammary infusion products; b) the effectiveness of each dye type/level combination as an indicator of antibiotic(s) in producer milk; and c) if the 250 mg levels of Food, Drug and Cosmetics (FD&C) Blue No. 1 (Color Index 42090) and FD&C Blue No. 2 (Color Index 73015) were irritating to healthy udder tissue