Effects of teat dipping on intramammary infections by staphylococci other than Staphylococcus aureus

Effects of intramammary infections in herds 1) either not teat dipping or postmilking teat dipping with either 2) linear dodecyl benzene sulfonic acid, 3) chlorhexidine, or 4) iodophor containing products were determined. Duplicate quarter foremilk samples were collected once from all lactating cows in 16 herds, four for each practice. Mean percentage of quarters infected with Staphylococcus species other than Staph. aureus was 11.0 in herds not teat dipping and 7.2 in herds teat dipping. Prevalence of Staphylococcus species intramammary infections in herds teat dipping with linear dodecyl benzene sulfonic acid was comparable to herds not teat dipping and greater than in herds using chlorhexidine and iodophor. The predominant Staphylococcus species in herds not teat dipping was Staph. epidermidis (37.1%). Staphylococcus hyicus constituted 48.5% of total Staphylococcus species intramammary infections in herds teat dipping and 12.4% in herds not teat dipping. Differences were not observed among SCC for quarters infected with different Staphylococcus species. Application of germicidal teat dips appeared to have selectively altered both prevalence and distribution of Staphylococcus species intramammary infections.     

Increased susceptibility to intramammary infection following removal of teat canal keratin.

Influence of teat canal keratin on susceptibility to intramammary infection was investigated in lactating Jersey cows. In each of two replicate trials, keratin was removed from the left teats of 20 cows immediately before milking. Immediately after milking, all teats were exposed to bacterial challenge by immersion in a suspension of Streptococcus agalactiae (5 x 10(7) cfu/ml). Bacterial challenge was repeated after the next four milkings. Foremilk samples were obtained for 8 d after keratin removal to determine infection status. A mammary quarter was classified as infected based solely upon the bacteriological criteria outlined by the National Mastitis Council. The rate of infection in quarters from which keratin was removed was greater than that in control quarters. Infection rates were 26.3% for keratin-removed quarters and 8.3% for control quarters in trial 1 and 13.5 and 0%, respectively, in trial 2. When more stringent criteria (recovery of greater than 100 cfu of S. agalactiae/ml in three or more successive milk samples and a SCC of greater than 10(6)) were used to identify a subset of infections that were clearly intramammary, infection rates were 9.3% for keratin-removed quarters and 1.4% for control quarters. Thus, partial removal of keratin from the teat canal compromised the ability of the teat to prevent passage of bacterial pathogens from the external environment into the mammary gland.     

Effects of a 1.94% sulfonic acid teat dip and a 1% iodophor teat dip on teat canal infections in lactating dairy cows

Sixty-three cows were used in a natural exposure trial for 1 yr to determine the effects of a 1.94% linear dodecyl benzene sulfonic acid and a 1% iodophor teat dip on teat canal and intramammary infections. Overall, total teat canal infections increased 28.9% and total intramammary infections increased 30% in quarters dipped with the linear dodecyl benzene sulfonic acid compared with a 14.3% increase in teat canal infections and a 17.4% decrease in intramammary infections for the iodophor group. Higher frequencies were observed for teat canal and intramammary infections caused by Staphylococcus epidermidis in the linear dodecyl benzene sulfonic acid group. Staphylococcus hyicus was the predominant organism isolated from teat canal and intramammary infections in the iodophor group. No difference was observed between the two teat dips in the persistence of Staphylococcus aureus teat canal infections. A greater number of S. hyicus teat canal infections persisted in the iodophor group, whereas teat canal infections produced by S. epidermidis and Staphylococcus xylosus persisted longer in the sulfonic acid group. Results of this study indicated that use of a linear dodecyl benzene sulfonic acid teat dip for 1 yr was not associated with increased S. aureus teat canal infections and intramammary infections. However, linear dodecyl benzene sulfonic acid appeared to affect the distribution of coagulase-negative staphylococci isolated from teat canal infections and intramammary infections.     

Effect of postmilking teat antisepsis on teat canal infections in lactating dairy cows

Effects of a .18% iodophor teat dip and a fatty acid plus lactic acid teat dip on teat canal infections were studied in two separate trials under experimental challenge procedures using Staphylococcus aureus and Streptococcus agalactiae. The iodophor teat dip was 90% effective in preventing new S. aureus teat canal infections and 95.6% effective in reducing progression of S. aureus teat canal infections to intramammary infections. The dip also reduced persistence of S. aureus teat canal colonizations from week to week by 94.4%. The fatty acid plus lactic acid dip had no effect on preventing S. aureus teat canal infections but reduced progression of S. aureus teat canal infections to intramammary infections by 43.8%. The fatty acid plus lactic acid dip also reduced the persistence of S. aureus teat canal infections by 39%. An insufficient number of Strep. agalactiae teat canal infections was detected in both trials for valid analyses. Neither dip had any effect on prevalence of naturally occurring, coagulase-negative staphylococcal teat canal infections or on teat canal infections by other organisms.     

Antimicrobial susceptibilities of staphylococcal species isolated from mammary glands of unbred and primigravid dairy heifers

Staphylococcal isolates from teat canal keratin and mammary secretion samples of unbred and primigravid Jersey heifers were tested in vitro for susceptibility to 12 antimicrobial agents. More than 92% of the 311 isolates were susceptible to all antimicrobial agents tested. Staphylococci other than Staphylococcus aureus demonstrated an overall susceptibility of 98.3% to all antibiotics, and Staphylococcus aureus demonstrated a 97% susceptibility. Across all staphylococcal species, susceptibility of isolates from secretion samples was 98.1%, and susceptibility of isolates from teat canal keratin samples was 93.1%. Differences in susceptibilities were observed among herds.     

Effects of Corynebacterium bovis infection on susceptibility to major mastitis pathogens

Experimental challenge procedures were used to study infectivity and virulence of Corynebacterium bovis. Challenge procedures using Staphylococcus aureus (Newbould 305) and Streptococcus agalactiae (McDonald 44) were used to study effects of Corynebacterium bovis infections on superinfection with major pathogens. Rate of infection under experimental challenge conditions was significantly greater with Corynebacterium bovis than previously observed for Staphylococcus aureus or Streptococcus agalactiae. Principal location of Corynebacterium bovis colonization appeared to the teat canal region, although the organism was isolated from 75% of the teat cisterns by puncture technique. Quarters with Corynebacterium bovis infections were more resistant to infection by Staphylococcus aureus than bacteriologically negative quarters. Quarters infected with Corynebacterium bacterium bovis were approximately 8.5-fold more susceptible to Streptococcus agalactiae infection than negative quarters. Somatic cell counts were doubled in negative quarters that developed Corynebacterium bovis infections; the geometric mean was 2.4 X 10(5).     

Relationship between the level of N-acetyl-beta-D-glucosaminidase (NAGase) in bovine milk and the presence of mastitis pathogens

Changes in the level of the tissue damage marker enzyme, N-acetyl-beta-D-glucosaminidase (NAGase) in quarter fore milks were found to be related to the presence and types of pathogenic bacteria present and to somatic cell counts (SCC). Minor pathogens (coagulase-negative staphylococci, Corynebacterium bovis) elicited a mild SCC increase (from a mean of 243 X 10(3)/ml in healthy quarters to 504 X 10(3)/ml in infected quarters) with marginal tissue damage (mean NAGase activity increased from 21 in healthy quarters to 28 in infected quarters). Major pathogens (i.e. Staphylococcus aureus, Streptococcus agalactiae, Str. dysgalactiae and Str. uberis) caused more severe tissue damage (mean NAGase of 48) and SCC increases (mean, 2803 X 10(3)/ml). The NAGase test could also be used effectively on composite milk samples where regular monthly NAGase analysis was able to identify correctly 74% of animals having infected quarters. The possibility of combining SCC and NAGase data in order to give a more definite diagnosis of bovine mastitis is discussed.     

Effect of intramammary device on new infection rate, milk yield, and milk somatic cell counts in maryland dairy herds

Effectiveness of a polyethylene intramammary device against naturally occurring infections was evaluated in three Maryland herds over 2 yr. Treated cows [62] were fitted with intramammary devices in all quarters of udders. Control cows [62] were sham treated. Rates of new intramammary infection over single lactation in treated and control quarters of primiparous cows averaged 18 and 27%. Reduction of infection rate was due primarily to fewer Corynebacterium bovis infections. Infection rate between multiparous cow treatments were similar. In uninfected quarters cell counts in strippings averaged .11 to .13 X 10(6)/ml and in quarters fitted with intramammary devices concentrations were only .22 to .31 X 10(6) cells/ml. But in infected quarters with intramammary devices, cell counts of strippings were 1.38 to 1.48 X 10(6)/ml. Concentrations of somatic cells of strippings in infected quarters without devices averaged .48 to .63 X 10(6)/ml. Dairy herd improvement cell counts for primiparous and multiparous cows with and without intramammary devices were similar and averaged .2 X 10(6)/ml. Neither milk nor fat production differed. The intramammary device as currently designed is incapable of stimulating a leukocytosis sufficient in stripping milk to prevent intramammary infection.     

Efficacy of a cephapirin dry cow product for treatment of experimentally induced Staphylococcus aureus mastitis in heifers.

A dry cow product containing 300 mg of cephapirin benzathine was evaluated in pregnant and nonpregnant Jersey heifers for its effect on experimentally induced Staphylococcus aureus mastitis. Cephapirin was detectable in mammary secretion of nonpregnant heifers for up to 5 wk and in tissue for 1 wk after intramammary infusion. Staphylococcus aureus was not detectable in tissue and secretion of treated quarters at 1 and 3 wk but was not eliminated from two quarters of one heifer tested at 6 wk posttreatment. Histologic evaluation of mammary tissue from nonpregnant heifers revealed significant differences in leukocytosis between uninfected and S. aureus-infected quarters but no differences in epithelium, lumen, and stroma, indicating no difference in secretion potential or glandular development. Twenty-five pregnant Jersey heifers were experimentally infected in two quarters with S. aureus 12 to 14 wk prepartum. After 1 to 3 wk, 13 heifers were infused in 21 S. aureus-infected quarters with a commercial cephapirin dry cow product. Nine infected quarters were left untreated. All treated quarters were bacteriologically negative both at calving and through 2 mo after calving. Of the 9 infected quarters not treated prepartum, 1 spontaneously cured and 2 became non-functional. The remaining quarters were treated at calving with a commercial cephapirin lactating cow product. Of these, 3 cured and 3 failed to resolve.     

Influence of prepartum pirlimycin hydrochloride or penicillin-novobiocin therapy on mastitis in heifers during early lactation

A study was conducted in 2 dairy research herds to determine whether prepartum therapy of heifer mammary glands with penicillin-novobiocin or pirlimycin hydrochloride was effective for reducing the percentage of heifers and mammary quarters infected with mastitis pathogens during early lactation. Almost 96% of Jersey heifers (67 of 70) and 71.3% of quarters (199 of 279) were infected 14 d before expected calving. Of the quarters infected at 14 d before expected parturition, 75% (54 of 72) were uninfected following treatment with penicillin-novobiocin; 87% (61 of 70) were uninfected following treatment with pirlimycin, and 56% (32 of 57) were uninfected in the untreated negative control group. The majority of intramammary infections in Jersey heifers were due to coagulase-negative staphylococci (61%), Streptococcus species, primarily Streptococcus uberis (19%), and Staphylococcus aureus (8%). Almost 73% of Holstein heifers (40 of 55) and 34.3% of mammary quarters (73 of 213) were infected 14 d before expected calving. Of the quarters infected at 14 d before expected parturition, 76% (19 of 25) were uninfected following treatment with penicillin-novobiocin; 59% (17 of 29) were uninfected following treatment with pirlimycin, and 26% (5 of 19) were uninfected in the untreated negative control group. The majority of intramammary infections in Holstein heifers were due to coagulase-negative staphylococci (44%) and Staph. aureus (30%). In both herds, the bacteriological cure rate was significantly higher in heifer mammary glands treated with penicillin-novobiocin or pirlimycin hydrochloride than in untreated controls. Prepartum therapy of heifer mammary glands with penicillin-novobiocin or pirlimycin hydrochloride significantly reduced the percentage of heifers and quarters infected with mastitis pathogens during early lactation.     

Molecular characterization of non-aureus Staphylococcus spp. from heifer intramammary infections and body sites

The purpose of this study was to investigate non-aureus Staphylococcus spp. intramammary infections (IMI) in periparturient heifers and determine the relationship of precalving body site isolation with precalving IMI and postcalving IMI using molecular speciation and strain-typing methods. Primiparous heifers were enrolled at approximately 14 d before expected calving date. Precalving mammary quarter secretions and body site swabbing samples (teat skin, inguinal skin, muzzle, and perineum) were collected. Postcalving, mammary quarter milk samples were collected for culture and somatic cell counting. Precalving body site samples were cultured, and up to 10 staphylococcal colonies were saved for characterization. Staphylococcal isolates were speciated using matrix-assisted laser/desorption ionization time-of-flight mass spectrometry or sequencing of rpoB or tuf. Pulsed-field gel electrophoresis was used to strain type a subset of isolates. Overall, Staphylococcus chromogenes, Staphylococcus agnetis, and Staphylococcus simulans were the most common species identified in precalving mammary secretions, whereas S. chromogenes, Staphylococcus xylosus, and S. agnetis were the most common species found in postcalving milk samples. The most common species identified from body site samples were S. chromogenes, S. xylosus, and Staphylococcus haemolyticus. Mammary quarters that had a precalving mammary secretion that was culture positive for S. agnetis, S. chromogenes, or Staphylococcus devriesei had increased odds of having an IMI with the same species postcalving. A S. chromogenes IMI postcalving was associated with higher somatic cell count when compared with postcalving culture-negative quarters. Among heifers identified with a non-aureus Staphylococcus spp. IMI either precalving or postcalving, heifers that had S. agnetis or S. chromogenes isolated from their teat skin had increased odds of having the same species found in their precalving mammary secretions, and heifers with S. chromogenes, S. simulans, and S. xylosus isolated from their teat skin precalving were at increased odds of having an IMI with the same species postcalving. Overall, 44% of all heifers with a S. chromogenes IMI around the time of parturition had the same strain isolated from a body site. Based on pulsed-field gel electrophoresis, a high level of strain diversity was found.     

Efficacies of chlorine dioxide and lodophor teat dips during experimental challenge with Staphylococcus aureus and Streptococcus agalactiae

We tested two postmilking teat dips for efficacy against Staphylococcus aureus and Streptococcus agalactiae using experimental challenge procedures recommended by the National Mastitis Council. The chlorine dioxide teat dip that contained 0.7% sodium chlorite reduced the number of new intramammary infections (IMI) caused by Staph. aureus by 86.6% and reduced new IMI caused by Strep. agalactiae by 88.4%. The 0.5% iodophor teat dip reduced the number of new IMI caused by Staph. aureus by 92.9% and reduced the number of new IMI caused by Strep. agalactiae by 43.4%. Teat skin and teat end conditions were evaluated before and after the study, and no deleterious effects were noted among dipped quarters compared with undipped control quarters for either teat dip.     

Rate of environmental mastitis in quarters infected with Corynebacterium bovis and Staphylococcus species

Rates of environmental streptococcal and coliform intramammary infections were compared among quarters uninfected and infected with either Corynebacterium bovis or Staphylococcus species. Rate of environmental streptococcal intramammary infections was 3.9 times greater in C. bovis-infected quarters than in uninfected quarters. Rate of environmental streptococcal infections was 2.6 times greater in quarters infected with Staphylococcus species than in uninfected quarters. Rate of coliform intramammary infections did not differ among quarters with differing bacteriological infection statuses. Quarters infected with either C. bovis or Staphylococcus species had higher milk SCC than did uninfected quarters. Intramammary infection with either Corynebacterium bovis or Staphylococcus species did not protect quarters against coliform infection. Rate of environmental streptococcal infections was enhanced in quarters infected with either C. bovis or Staphylococcus species.     

Experimental Staphylococcus aureus intramammary challenge in late lactation dairy cows: quarter and cow effects determining the probability of infection

The purpose of this study was to identify factors at the quarter and cow level that determine whether a quarter remains infected after an intramammary challenge with Staphylococcus aureus Newbould 305. A total of 135 cows were studied. Information on animal characteristics, cow-conformation, cow somatic cell count (SCC), and bacteriology, blood vitamin E levels, serology for retro-viral infections, bovine leukocyte adhesion deficiency-carrier status, and the presence of bovine lymphocyte antigens class I alleles was collected on each animal. All quarters of all cows were then challenged with Staphylococcus aureus Newbould 305. The challenge with S. aureus Newbould 305 resulted in 28 cows (20.7%) that did not establish infection in any of the quarters, 21 (15.6%) cows had 1 quarter infected, 35 (25.9%) had 2 quarters infected, 24 (17.8%) had 3 quarters infected, and 27 (20.0%) had all quarters infected. A higher prechallenge SCC decreased the risk of infection. An infection with Corynebacterium bovis prior to challenge decreased the risk of S.aureus infection. Of the bovine lymphocyte antigen alleles, the presence of the W20A allele proved to be significantly associated with a decreased risk of infection. No other factors proved to be significant.     

Effects of prepartum intramammary antibiotic therapy on udder health, milk production, and reproductive performance in dairy heifers

Preparturient heifers (n = 561) from 9 herds in 6 US states and 1 Canadian province were enrolled in a study to test the hypothesis that prepartum intramammary therapy would cure existing intramammary infections (IMI) and lead to increased milk production, reduced linear somatic cell count (LSCC), and improved reproductive performance. Mammary secretions were collected 10 to 21 d before expected calving from each quarter. Heifers were then assigned by identification number to receive intramammary therapy consisting of infusion of one tube per mammary quarter of a lactating cow commercial antibiotic preparation containing cephapirin or to a nontreated control group. Overall, 34.1% of mammary quarters were infected with a mastitis pathogen before parturition and 63.4% of heifers had at least one mammary quarter infected. The coagulase-negative staphylococci (CNS) caused the majority (74.8%) of prepartum IMI. Coagulase-positive staphylococci, environmental streptococci, and coliforms accounted for 24.5% of prepartum infections. Treatment had a significant effect on the cure rate of infected mammary quarters. Mammary quarters that were infected prepartum and treated with antibiotics had a 59.5% efficacy of cure rate and the percentage reduction in heifers with IMI was 51.9. Control quarters had a spontaneous cure rate of 31.7%. Treatment did not significantly affect milk production or LSCC in the first 200 d of lactation; however, there was a significant treatment by herd interaction for milk production. Quarters cured of either CNS or major pathogens had a lower LSCC in the first 200 d of lactation. No significant effect on services per conception or days open between treatment and control groups was observed. This trial demonstrated that prepartum intramammary antibiotic therapy did reduce the number of heifer IMI postpartum. Milk production, LSCC, and reproductive performance during the first 200 d of the first lactation were not significantly affected by treatment. Given these results, use of prepartum intramammary antibiotic therapy in heifers as a universal strategy to increase milk production in first-lactation dairy cows may not be warranted.     

Local immunostimulation of the bovine mammary gland with interleukin-2

One day after drying off, miniosmotic pumps releasing interleukin-2 were implanted into teat cisterns of six Jersey cows. Each cow had two quarters with (sensitized) and without (unsensitized) chronic Staphylococcus aureus infection. At 1 and 3 wk after implantation, three cows were sacrificed and mammary tissues processed to examine the cytological response to interleukin-2. Concentrations of lymphoid cells were determined in tissues lining teat and gland cistern linings, and plasma cells were characterized by Ig class. For total lymphoid cell and IgG1 and IgG2 plasma cell concentrations, sensitized placebo quarters and unsensitized interleukin-2 quarters were similar, indicating interleukin-2 stimulated a cellular immune response equal to that of quarters infected with S. aureus. Greatest response was observed in sensitized interleukin-2 quarters, suggesting that the cytokine was more effective in quarters with prior exposure to S. aureus antigens. Greater numbers of lymphocytes and macrophages were found in secretions of interleukin-2 quarters than in secretion of control quarters. Local interleukin-2 immunostimulation may supplement immunization procedures and enhance protection against natural S. aureus challenge.     

Evaluation of a postmilking teat disinfectant containing a phenolic combination for the prevention of mastitis in lactating dairy cows

A trial was conducted for 12 months in a herd of 120 Holstein cows in order to determine the efficacy of a teat disinfectant, which contained a phenolic combination, for the prevention of bovine intramammary infections during lactation. Postmilking teat disinfection was compared to a negative control using a split-udder experimental design. The percentage of quarters newly infected by mastitis pathogens was 45% lower in mammary glands with teats that had been dipped in the experimental teat disinfectant after milking than it was in undipped controls. New infections caused by Streptococcus uberis, Staphylococcus aureus, coagulase-negative Staphylococcus species, and Corynebacterium bovis were significantly lower in mammary glands with teats that had been dipped in the experimental teat disinfectant than in undipped controls. No statistical differences in the incidence of clinical mastitis between treatment groups were observed. No irritation or chapping of teats dipped in the experimental teat disinfectant were observed. The results of this study suggest that the experimental teat disinfectant containing a phenolic combination is an effective postmilking teat disinfectant for use in the prevention of new intramammary infections by both contagious and environmental mastitis pathogens.     

Lysostaphin: use of a recombinant bactericidal enzyme as a mastitis therapeutic.

A recombinant mucolytic protein, lysostaphin, was evaluated as a potential intramammary therapeutic for Staphylococcus aureus mastitis in dairy cattle. Lysostaphin, a product of Staphylococcus simulans, enzymatically degrades the cell wall of Staphylococcus aureus and is bactericidal. Thirty Holstein-Freisian dairy cattle in their first lactation were infected with Staphylococcus aureus (Newbould 305, ATCC 29740) in all quarters. Infections were established and monitored for somatic cell counts and Staphylococcus aureus colony-forming units 3 wk prior to subsequent treatment. Infected animals were injected through the teat canal with a single dose of recombinant lysostaphin (dose response 1 to 500 mg) or after three successive p.m. milkings with 100 mg of recombinant lysostaphin in 60 ml of sterile phosphate-buffered saline. Animals were considered cured if the milk remained free of Staphylococcus aureus for a total of 28 milkings after last treatment. Kinetic analysis of immunologically active recombinant lysostaphin demonstrated that a minimum bactericidal concentration was maintained in the milk for up to 36 to 48 h after a single infusion of 100 mg of recombinant lysostaphin. The cure rate of quarters receiving recombinant lysostaphin (100 mg in sterile phosphate-buffered saline, administered over three consecutive p.m. milkings) was 20% compared with 29% for sodium cephapirin in saline and 57% for a commercial antibiotic formulation, respectively. An improved formulation of recombinant lysostaphin may prove to be an effective alternative to antibiotic therapy for bovine mastitis.     

Use of latex teat dip with germicide during the prepartum period

The efficacy of an acrylic latex barrier teat dip with germicide on new infections at parturition was tested on 113 cows and heifers during the prepartum period. A split udder design was used in which right quarters were undipped controls and left quarters were dipped with latex dip once daily for approximately 14 d prior to parturition. Distal streak canal swabs were taken from all quarters prior to the beginning of dipping, and all cows were quarter sampled in duplicate at drying off, parturition, and the first 5 consecutive wk of lactation. All quarters received antibiotic therapy at drying off. Excluding infections detected by distal streak canal swabs, the number of new infections at parturition in control and dipped quarters was 32 and 36. New infections at parturition in control and dipped quarters were Staphylococcus aureus, 2 and 2; coagulase-negative staphylococci, 8 and 9; stretptococci, 10 and 12; coliforms, 8 and 8; and others, 4 and 5. Differences were not significant. These results suggest no benefit to use of prepartum latex teat dip with germicide on new infections at parturition.     

Comparison of tilmicosin and cephapirin as therapeutics for Staphylococcus aureus mastitis at dry-off

Forty-four cows (26 Jerseys and 18 Holsteins) that had at least 1 mammary quarter that was naturally (n = 12) or experimentally (n = 84) infected with Staphylococcus aureus were allotted to three treatment groups of approximately equal number at the end of lactation. Cows were dried off by abrupt cessation of milking, and dry cow therapy was administered as an intramammary infusion of cephapirin benzathine at 10 ml per quarter, an intramammary infusion of tilmicosin (solution containing 300 mg/ml) at 5 ml per quarter, or a subcutaneous injection of tilmicosin at 5 mg/kg of body weight on the day of drying off and another injection 4 d later. Mammary secretions were monitored during the dry period and postpartum for antimicrobial residues, intramammary infection (IMI) status, and somatic cell counts. Results demonstrated the following percentage cures for IMI caused by Staph. aureus at 28 d postcalving based on individual mammary quarters: cephapirin benzathine, 78.1%; tilmicosin infused, 74.2%; and tilmicosin injected, 9.1%. During the first 4 wk after drying off, the mean concentration of tilmicosin in mammary secretions from cows infused with the antibiotic remained approximately 10-fold higher than that in secretions from cows injected with the antibiotic (3.43 vs. 0.32 ppm), and, by the time of calving, concentrations for cows treated with both methods were below the dilution limit of the assay (< 0.1 ppm). Results demonstrated that intramammary infusion of tilmicosin was equally as effective as cephapirin benzathine in curing IMI caused by Staph. aureus at drying off; however, the subcutaneous injection of tilmicosin at the dose used was not effective as a dry cow therapeutic against Staph. aureus.