Etiological and Epidemiological Characteristics of Severe Mastitis and the Outcomes Treatment Following a Single Dose of Fluoroquinolones Administered During On-Farm Veterinary Interventions

Background/objectives: severe mastitis is one of the leading causes of mortality in dairy cows. Its primary complication is shock, predominantly associated with systemic inflammatory response syndrome, which remains extremely challenging for practitioners to manage. The average mortality rate is estimated at approximately 25%. Many authors recommend the use of fluoroquinolones for this indication. However, these antibiotics are classified as critically important for human health, and their use requires strict compliance with specific guidelines (bacteriological analysis and antimicrobial susceptibility testing). In addition, some practitioners remain reluctant to use this class of antibiotics in field conditions. Therefore, the present study aimed to evaluate the outcomes of systematic antibiotic therapy using fluoroquinolones in cases of severe mastitis and to identify factors that may influence treatment success. Methods: a total of 323 cows with severe mastitis were enrolled by eight participating veterinary clinics located across different regions of France. The study design included: (i) clinical scoring based on a standardized grid developed by practitioners routinely managing this condition, (ii) bacteriological analysis of milk samples (with antimicrobial susceptibility testing performed when Gram-negative bacteria were isolated), and (iii) post-treatment follow-up consisting of telephone interviews conducted at 5 and 15 days after inclusion. Cows presenting with a clinical score ≥3 (scale 0–36) in association with local signs of mastitis were classified as having severe mastitis and received an injection of 10 mg/kg marbofloxacin along with 2.2 mg/kg flunixin (unless another NSAID had been administered within the previous 24 h). When the clinical score was ≥6, cows additionally received intravenous fluid therapy consisting of 3 L of 7.2% NaCl, supplemented by oral drenching if spontaneous water intake was insufficient. Results: a total of 43 cows died or were euthanized during the study period, corresponding to a mortality rate of 13.3%. The mean clinical score at inclusion was 12.6. The clinical signs most strongly associated with mortality were decubitus and hypothermia at admission. Escherichia coli was isolated in 67.0% of severe mastitis cases, either as a single pathogen (82.9%) or in mixed infections (17.1%). Overall, Gram-negative bacteria (Escherichia coli, Klebsiella spp., Pseudomonas aeruginosa, other Gram-negative organisms) were identified in 79.0% of cases. A total of 188 coliform isolates were tested for antimicrobial susceptibility. All isolates (100%) were susceptible to marbofloxacin, as were all tested Gram-negative strains, whereas only 79.9% of E. coli isolates were susceptible to sulfonamide/trimethoprim. Compared with previously published data, the observed mortality rate was lower despite the poor clinical condition of cows at admission. Conclusion: the timeliness of initiating effective antimicrobial therapy appears to be a critical determinant of survival in cows with severe mastitis.