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.