The objective of the present study was to compare the efficacy of conventional and extended intramammary (IMM) therapy of persistent subclinical mastitis in lactating dairy cattle using nafcillinpenicillin- dihydrostreptomycin combination (NPD). Sixty-five dairy cows with 126 infected quarters were enrolled in the study. Infected cows were allocated randomly to 1 of 3 different treatment regimens: (1) conventional group: NPD administered IMM 3 times at 24-h intervals (20 infected cows, 43 intramammary infections [IMI]), (2) extended group: NPD administered IMM 6 times at 24-h intervals (23 cows, 43 IMI), and (3) untreated control group (22 cows, 40 IMI). The overall bacteriological cure (BC) rates for subclinical IMI were 86.04%, 100%, and 20% for the conventional, extended and the control groups, respectively; indicating a higher BC rate (P<0.0001) for the treated groups than the control group. Significant difference (P=0.029) concerning the BC rate was also observed between the extended and the conventional groups. Significant difference (P=0.0021) in somatic cell count (SCC) was detected between the extended and the control group. Fat percentage increased in the conventional (P=0.029) and in the extended (P<0.0001) groups, and protein percentage increased only in the extended group (P=0.0016). There was no significant difference in posttreatment milk production between the groups (P>0.05). Results of this study indicate that NPD therapy was effective in eliminating subclinical IMI in lactating dairy cows, and that extended therapy enhanced BC rate and reduced SCC.

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