Effect of Antibiotic Administration on Mastitis Screening Tests and Milk Composition

Intramammary infusion of a commercial antibiotic mixture or aqueous penicillin resulted in a marked inflammatory reaction characterized by increases in leucocyte numbers and degree of reaction to mastitis screening tests.Milk lactose content was decreased significantly.Penicillin infusion resulted in an apparent decrease in streptococcus organisms, but had little effect on staphylococci.Intramuscular injection of streptmnycin had only minor effects on leucocytes, screening tests, milk composition, or bacteriological status of the individual quarters.Intramuscular penicillin injection resulted in significant decreases in leucocytes and reactions to the mastitis screening tests.Protein and lactose content of the milk were increased.There was a reduction in the number of quarters shedding staphylococcus as well as streptococcus organisms.Increased use of mastitis screening tests on bulk milk by milk plants and regulatory agencies has resulted in increased use of the California Mastitis Test (CMT) on individual quarters.There is a tendency to consider antibiotic treatment of quarters positive to the CMT, even though these positive quarters may not show visibly abnormal milk or clinical mastitis.Garrison and Turner (4) reported changes in milk composition and increased leucocytes as a result of infusing up to 900 ml of distilled water per quarter; however, when the level was decreased to 250 ml, there was little change in milk composition.Several authors (5, 6, 10) have shown that the use of intramuscular or intramammary treatment reduced the number of quarters shedding streptococcus organisms.These reports did not indicate what effect the treatments had on the mastitis screening test reaction or milk composition.The purpose of this study was to determine the effect of various antibiotic treatments, administered by intramammary or intramuscular Received for publication May 19, 1966.Published with the approval of the Director of the Wisconsin Agricultural Experiment Station.routes, upon mastitis screening test reactions as well as milk composition.The intent was to study cows with varying initial reactions to the screening tests, but not showing visibly abnormal milk or clinical mastitis.