Effect of naturally occurring coagulase-negative staphylococcal infections on experimental challenge with major mastitis pathogens.

The influence of pre-existing Staphylococcus sp. IMI on development of new IMI after experimental challenge with Staphylococcus aureus and Streptococcus agalactiae was studied. The IMI data were analyzed from five trials in which quarters were challenged with major pathogens incident to studies of teat dip efficacy. Prior to each trial, quarter IMI status was determined, and new IMI were enumerated during challenge. Percentage of new Staph. aureus IMI in uninfected quarters was 3-fold that of quarters already infected with Staphylococcus sp. Of quarters that were initially uninfected, 13.23% acquired new Staph. aureus IMI, and 4.49% of quarters infected with Staphylococcus sp. became infected. Conversely, the percentage of new Strep. agalactiae IMI in quarters infected with Staphylococcus sp. was 1.5-fold that of uninfected quarters (8.38 vs. 5.52%). The percentage of clinical Staph. aureus IMI in uninfected quarters was higher than for quarters infected with Staphylococcus sp., but percentages of clinical Strep. agalactiae IMI were similar among IMI statuses. Geometric mean SCC prior to challenge were 87 x 10(3) for uninfected quarters and 260 x 10(3)/ml for quarters infected with Staphylococcus sp. Quarters infected with Staphylococcus sp. were less susceptible to Staph. aureus IMI, but more susceptible to Strep. agalactiae IMI.