For gram-positive mastitis, antimicrobial choice hinges on whether the pathogen is a staphylococcus or streptococcus; gram-negative mastitis often resolves without antimicrobials.

Staphylococcus aureus is best treated with penicillin-based or cephalosporin-based intramammary therapy when susceptibility is confirmed. In North America, resistance among S. aureus isolates remains very low across all commonly used intramammary drugs: penicillin-novobiocin (0% resistance), ceftiofur (0.1%), oxacillin (0.2%), erythromycin (0.7%), cefoperazone (1.2%), and pirlimycin (2.8%).Veterinary Micr… In southeastern Australia, antimicrobial resistance among Staph. aureus is similarly uncommon despite decades of use.Journal of Dair… The exception is large Chinese dairy herds, where Staph. aureus resistance to penicillin reaches 66% and to tetracycline 59%, making empirical penicillin unreliable in that context.Journal of Dair… Subclinical Staph. aureus isolates carry a higher probability of penicillin resistance than clinical isolates — 5.16 times higher for penicillin and 4.70 times higher for amoxicillin — so culture and susceptibility testing is especially important before treating subclinical cases.Journal of Dair… Methicillin-resistant Staph. aureus (MRSA) is rare in North American dairy cattle (0.2% of isolates with oxacillin MIC ≥4 µg/mL) and has not increased substantially between 2002 and 2022.Veterinary Micr… When MRSA is suspected, oxacillin or cefoxitin disk diffusion is the appropriate phenotypic confirmatory test.Journal of Dair… First-generation cephalosporins or amoxicillin-clavulanate are preferred over third- and fourth-generation cephalosporins for staphylococcal mastitis; use of third-generation (cefoperazone, ceftiofur) and fourth-generation (cefquinome) cephalosporins should be reserved for gram-negative infections resistant to non-critically important antimicrobials.Journal of Dair…

Streptococcal mastitis is reliably treated with beta-lactams in North America and Australia. Streptococcus dysgalactiae resistance to penicillin-novobiocin is 0%, to ceftiofur 0.1%, to erythromycin 3.2%, and to pirlimycin 4.6%.Veterinary Micr… Streptococcus uberis resistance to ampicillin is 0% and to ceftiofur 0.2%, though resistance to erythromycin reaches 11.9% and to pirlimycin 18.4% in North America.Veterinary Micr… Pirlimycin resistance for S. uberis has actually declined from 27.8% (2002–2010 survey) to 18.4% (2011–2022 survey).Veterinary Micr… In Wisconsin, all Streptococcus spp. were phenotypically susceptible to ceftiofur, cephalothin, and oxacillin, though 13–22% showed resistance to pirlimycin and resistance to sulfadimethoxine differed between S. dysgalactiae and S. uberis.Journal of Dair… In large Chinese herds, streptococcal resistance to tetracycline reaches 59%, and the most frequent multidrug-resistant pattern for streptococci is clindamycin-enrofloxacin-tetracycline.Journal of Dair… Penicillin remains the antibiotic of choice for gram-positive streptococci per German veterinary guidelines, though anecdotal therapy-resistant Strep. uberis cases have been reported.Journal of Dair…

Regarding treatment protocol for gram-positive mastitis, a 2-day intramammary amoxicillin protocol (3 infusions) achieves clinical cure rates above 84% and does not differ significantly from a 5-day ceftiofur protocol for overall clinical cure, bacteriological cure, recurrence, or culling/death at 90 days.Journal of Dair… Because amoxicillin is a narrower-spectrum agent and requires fewer administrations, it is the more antimicrobial-stewardship-appropriate choice for confirmed gram-positive mastitis on farms using culture-guided treatment.Journal of Dair… This equivalence does not apply to Staph. aureus or Trueperella pyogenes mastitis, where cure rates with standard protocols are expected to be lower.Journal of Dair…

Gram-negative mastitis (primarily E. coli, Klebsiella spp., Serratia spp.) commonly resolves without antimicrobial therapy, and antimicrobial treatment — particularly with critically important antibiotics — provides little to no advantage in recovery while increasing resistance risk.Journal of Dair… When treatment is pursued, ceftiofur and cefoperazone show low resistance rates in North America: E. coli resistance to ceftiofur is 2.8% and to cefoperazone 3.4%, while ampicillin resistance is moderate at 9.2%.Veterinary Micr… In large Chinese herds, both E. coli and Klebsiella spp. show high resistance to amoxicillin-clavulanate (81% and 38%, respectively), making this combination unreliable empirically in that population.Journal of Dair…

Would you like guidance on when to use culture-guided versus empirical treatment protocols for clinical mastitis on farm?