Influence of oxytetracycline susceptibility as a first-line antibiotic on the clinical outcome in dairy cattle with acute Escherichia coli mastitis

Abstract

The purpose of this study was to clarify the therapeutic effects of oxytetracycline(OTC) as a first-line antibiotic in cattle with acute Escherichia coli mastitis and systemic signs. Drugsusceptibility was determined by the minimum inhibitory concentration (MIC) of E. coli isolatedfrom cows with acute E. coli mastitis (n=38). Cattle were divided into OTC-susceptible (S, n=30)and OTC-resistant (R, n=8) groups. They were further subdivided according to susceptibility tothe antibiotic used as a second treatment, into susceptible-susceptible (SS, n=30), resistantsusceptible(RS, n=5), and resistant-resistant (RR, n=3) groups. Clinical signs on the day after initialtreatment were compared between S and R groups as short-term indicators of treatment effects.The 28-day survival rate of cattle was then compared among SS, RS, and RR groups as a long-termindicator of treatment effects. There were no differences in clinical signs between S and R groupson the day after the first dose, but the 28-day survival rate was significantly greater in the SS groupthan in the RR group (P=0.04). The results demonstrated that an effective drug is essential forfirst-line treatment of acute coliform mastitis. However, anticipating the effectiveness of a first-lineantibiotic based on clinical symptoms at the second day of treatment is impossible. It is importantto build a picture of drug resistance trends in cattle herds for empirical selection of antibiotics tobe administered.