The objective was to describe in vitro ceftiofur minimum inhibitory concentration (MIC) to inhibit growth (INH) of Klebsiella spp. (KLE) and its association with quarter somatic cell count (QSCC) during a 28-d follow-up period (FUP). Cases of clinical mastitis (CM) confirmed as gram-negative were randomly assigned to receive 2-d (short), 8-d (extended) intramammary ceftiofur or to a non-treated (control) group. Only cases confirmed as KLE were included in this analysis (n = 38). The primary outcome was QSCC collected 7, 14, 21, and 28 d after enrollment. The Wisconsin Veterinary Diagnostic Laboratory (WVDL) performed in vitro antimicrobial susceptibility tests according to the Clinical and Laboratory Standards Institute (CLSI). The WVDL categorized isolates based on the MIC at which ceftiofur (test ranged from 0.5 to 4.0 μg/mL) completely inhibited KLE (≤0.5, 1.0 and > 4.0 μg/mL). Survival analysis was performed using PROC LIFETEST to measure the probability of QSCC reduction (<200,000 cells/mL) during the FUP by MIC categories of INH. Repeated measures analysis was performed to analyze QSCC changes using PROC MIXED. Proportion of MIC categories of INH were not different among treatment groups (P = 0.78). The probability of QSCC reduction over the FUP was not different among MIC categories of INH of KLE (P = 0.49). The log₁₀SCC for quarters that received 8-d (5.8, P < 0.01) therapy was less than QSCC of quarters that received 2-d (6.4) or no therapy (6.6). Time (P = 0.24) and ceftiofur MIC (P = 0.48) were not associated with QSCC. However, an interaction between MIC categories, and treatment group with QSCC was observed (P = 0.04). Quarter SCC did not change with time and was associated with ceftiofur therapy. The exposure to ceftiofur intramammary therapy seemed to modify the QSCC response when compared with the control group depending on the MIC category of INH. Table 1 

Treatment	LSM of log10 SCC at MIC (μg/mL)
≤0.5 (n = 17)	1.0 (n = 11)	>4.0 (n = 10)
Control	6.3	6.7	6.8
Short	6.2a	6.2a	6.7b
Extended	6.2a	5.6b	5.6b

^a,bWithin rows, means with different superscripts differ (P < 0.05).