Efficacy of a lactoferrin-penicillin combination to treat {beta}-lactam-resistant Staphylococcus aureus mastitis

The efficacy of intramammary (IM) treatments containing penicillin G (PG) alone or a combination of PG and bovine lactoferrin (bLF) was evaluated using a model of experimentally induced chronic bovine mastitis caused by a clinical isolate of Staphylococcus aureus highly resistant to beta-lactam antibiotics. First, we confirmed that this strain could cause mastitis and infection could not be cured with PG alone. In a second trial, chronic mastitis was induced in 19 late-lactating cows by injecting a low dose of Staph. aureus through the teat canal of all quarters. After 15 d, cows with stable infections in their 4 quarters had their mammary quarters randomly assigned, within cow, to 1 of 4 IM treatments as follows: 1) citrate buffer, 2) 100,000 IU of PG, (3) 1 g of bLF, or 4) 1 g of bLF + 100,000 IU of PG. Treatments were repeated twice a day for 5 d. A third trial was undertaken to investigate the effect of an extended therapy on chronic mastitis acquired in a previous lactation. One month before dry-off, 20 gravid cows regrouped by dates of calving were infected in their 4 quarters. Once infections were established, cows were dried off abruptly. After calving, aseptic milk samples were collected separately from all quarters for 4 wk to monitor infection. Mammary quarters from enrolled cows were then randomly assigned, within cow, to 1 of 2 treatments as follows: 1) 100,000 IU of PG or 2) 250 mg of bLF + 100,000 IU of PG. Treatments were administered IM twice a day for 7 d. For all trials, milk samples were taken to monitor bacterial concentration and somatic cell count. Bacteriological cure rate was determined using milk samples taken 3 and 4 wk after initiation of treatments. For the second trial, cure rate was null for control quarters, 11.1% for bLF, 9.1% for PG, and 45.5% for the bLF + PG combination. For cows infected in their previous lactation, cure rate was higher for the bLF + PG combination (33.3%) compared with PG alone (12.5%). In conclusion, bLF added to PG is an effective combination (i.e., 3- to 5-times higher cure rate) for the treatment of stable Staph. aureu infections highly resistant to beta-lactam antibiotics.