Efficacy of parenteral administration of three antimicrobial agents in treatment of clinical mastitis in lactating cows: 487 cases (1989-1995).

OBJECTIVE: To evaluate the efficacy of parenteral administration of procaine penicillin G, spiramycin, or enrofloxacin in the treatment of clinical mastitis in lactating cows.

DESIGN: Noncontrolled, clinical retrospective study.

ANIMALS: 487 cows with mastitis involving 543 quarters.

PROCEDURE: Clinical signs, histories, and results of bacteriologic examination, somatic cell count, and N-acetyl-beta-D-glucosaminidase activity of milk samples taken before and 3 to 4 weeks after treatment were retrieved from hospital records. Cows treated parenterally with procaine penicillin G, spiramycin, or enrofloxacin for 3 to 5 days were included. Supportive treatment alone was given to 35 cows infected with Escherichia coli. Factors possibly affecting outcome were analyzed, using ANOVA, correlation analyses, and the Mann-Whitney test. chi 2 Test was used to compare bacteriologic cure rates.

RESULTS: Bacteriologic cure rates for mastitis caused by Staphylococcus aureus, coagulase-negative staphylococci, and streptococci were 34, 76, and 65%, respectively. Cure rates in cows in their first lactation and infected with S aureus and coagulase-negative staphylococci were significantly higher than those for older cows. In cows with mastitis caused by E coli, the cure rate was 74% for those treated with penicillin G and 71% for those not treated with antimicrobials. High N-acetyl-beta-D-glucosaminidase activity in milk samples obtained at initial examination indicated a poor outcome in S aureus and streptococcal mastitis. Cows infected in the early lactation period had more severe inflammatory responses and clinical signs if infected with coagulase-negative staphylococci and coliforms.

CLINICAL IMPLICATIONS: 3 to 5 days of treatment with parenterally administered penicillin G for clinical mastitis caused by penicillin-susceptible S aureus strains is efficacious in young cows. Parenteral administration of spiramycin or enrofloxacin does not give satisfactory results in mastitis caused by penicillin-resistant S aureus. Use of antimicrobials in the treatment of mastitis caused by coliform bacteria is questionable.