Effects of a selective regimen of dry cow therapy on intramammary infection and on antibiotic sensitivity of surviving pathogens.

Selective dry cow therapy with 1 million U of procaine penicillin G and 1 g of dihydrostreptomycin in a slow-release base was administered to all quarters of cows currently infected or treated for mastitis in the previous lactation. All other cows in the herd that were dried off during the experimental period were left untreated. Milk samples were cultured for detection of intramammary infection at drying off, 1 wk later (at time of treatment), and after next calving from 227 treated and 143 untreated cows. Isolants were tested for sensitivity to 12 antibiotics including penicillin and streptomycin. Although treatment resulted in clearance of Staphylococcus aureus, Staphylococcus epidermidis, streptococci other than agalactiae, and coliforms from 78, 82, 88, and 87% of infected glands, the net effect on udder health of the herd was a gain of 1.4% in infected quarters. Incidences of new infection in the early dry period (1 wk) were 18.1 and 12.3% among treated and untreated cows. Therapy failed to control new infection in the late dry and peripartum periods; incidence was about 18% of quarters among both long and short dry periods. Among 46 instances of apparent survival of an intramammary pathogen through a treated dry period, 7 of 10 strains previously sensitive to streptomycin became resistant and 8 of 10 strains previously sensitive to penicillin became resistant. The surviving pathogen population was not large enough to be considered a threat to herd udder health in the next lactation.