Abstract
The objective of this negatively controlled, randomized clinical trial was to examine clinical outcomes of 2-d or 8-d treatment using an approved intramammary (IMM) product containing ceftiofur hydrochloride compared with no antimicrobial treatment of nonsevere, gram-negative cases of clinical mastitis (CM). Additionally, we contrasted clinical outcomes of cases caused by Escherichia coli (n = 56) or Klebsiella pneumoniae (n = 54). Cases (n = 168) of nonsevere (abnormal milk or abnormal milk and udder) CM were randomly assigned to receive 2 d (n = 56) or 8 d (n = 56) of IMM ceftiofur or assigned to a negative control group (n = 56). At enrollment, quarter milk samples were collected and used for on-farm culture, somatic cell count (SCC), and confirmatory microbiological analysis. Quarter milk samples were collected weekly from 7 to 28 d after enrollment for microbiological and SCC analysis. Clinical outcomes were followed for 90 d or until the end of lactation (follow-up period, FUP). Overall, no significant differences in quarter-level recurrence of CM (32% for negative control, 34% for the 2-d treatment, and 32% for the 8-d treatment), culling (18% for negative control, 12% for 2-d treatment, and 11% for 8-d treatment), voluntary dry-off of affected quarters (20% for negative control, 30% for 2-d treatment, and 27% for 8-d treatment), days until return to normal milk (4.2 days for negative control, 4.8 days for 2-d treatment, 4.5 days for 8-d treatment), weekly quarter-SCC during the FUP (6.1, 6.3, and 6.0 log10SCC for the negative control, 2-d, and 8-d treatments, respectively), or daily milk yield during the FUP (37.1, 36.3, and 37.6 kg/cow per day for the negative control, 2-d, and 8-d treatments, respectively) were observed among experimental groups. Days of discarded milk were greater for cows assigned to 8-d IMM ceftiofur (11.1 d) than for cows assigned to 2-d (6.9 d) or cows assigned to negative control (5.6 d). Bacteriological cure (BC) at 14 and 21 d after enrollment was greater in cows assigned to 8-d (89%) and 2-d (84%) treatment than in cows assigned to negative control (67%), but this outcome was confounded by pathogen. For CM caused by Kleb. pneumoniae, BC was greater for quarters assigned to receive treatment (combined 2-d and 8-d groups; 74% BC) than for quarters assigned to negative control (18%). In contrast, no differences in BC were observed for CM caused by E. coli (97-98%). Culling and voluntary dry-off of affected quarters were significantly greater for cows with quarters affected by Kleb. pneumoniae (22% culled, 39% voluntary dry-off of quarters) than for cows with quarters affected with E. coli (7% culled, 11% voluntary dry-off of quarters). Overall, use of IMM ceftiofur did not result in improvement of most clinical outcomes, but differences between E. coli and Kleb. pneumoniae were evident. In contrast to E. coli, Kleb. pneumoniae caused chronic intramammary infection and induced worse clinical outcomes. Intramammary antibiotic treatment of most mild and moderate cases of CM caused by E. coli is not necessary, but more research is needed to identify which quarters affected by Kleb. pneumoniae may benefit from antimicrobial therapy.
Highlights
Clinical mastitis (CM) is the most common reason that dairy cows receive intramammary (IMM) antibiotics (USDA, 2014), and the majority of IMM treatments are not targeted based on etiology (Oliveira et al, 2013)
We demonstrated that nonsevere clinical mastitis (CM) caused by culture-negative pathogens was associated with infrequent occurrence of culling (Fuenzalida and Ruegg, 2019), but we observed that overall risk of culling was greater for cows affected with gram-negative mastitis
Use of IMM ceftiofur resulted in increased Bacteriological cure (BC) and increased number of days of discarded milk but did not result in improved daily milk production, reduced culling, or improvements in somatic cell count (SCC)
Summary
Clinical mastitis (CM) is the most common reason that dairy cows receive intramammary (IMM) antibiotics (USDA, 2014), and the majority of IMM treatments are not targeted based on etiology (Oliveira et al, 2013). Nonspecific use of antibiotics on farms has led some to believe that antibiotics are misused, and consumers and regulators often associate use of antimicrobials on farms with increasing antimicrobial resistance (Sifferlin, 2017). An association between increased risk of antimicrobial resistance of non-aureus staphylococci and systemic administration of some antimicrobials has been recently described (Nobrega et al, 2018). Selective treatment protocols for CM can reduce use of antimicrobials for cases that are culture negative or gram negative (Lago et al, 2011 a,b; Vasquez et al, 2017; Fuenzalida and Ruegg, 2019). Among gram-negative bacteria causing IMI, severity of the inflammatory response and outcomes of antimicrobial treatment may vary (Oliveira et al, 2013)
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