Initial studies to determine the prevalence of mastitis in heifers of breeding age and in pregnant dairy heifers demonstrated that IMI were present in 97% of heifers and 75% of quarters. The most common isolates were Staphylococcus aureus, Staphylococcus hyicus, and Staphylococcus chromogenes; SCC ranged from 12.4 to 17.3 x 10(6)/ml. Approximately 29% of heifers and 15% of quarters exhibited clinical mastitis at breeding age, as evidenced by clots or flakes in mammary secretions. Histologic examination of mammary tissues demonstrated significant reductions in alveolar epithelial and luminal areas and increases in connective tissue stroma and leukocytosis, illustrating limited development and marked inflammation of infected tissues. A one-time infusion of antibiotic for nonlactating cows into infected quarters > or = 45 d prepartum reduced incidence of IMI by 59% at calving compared with the pretreatment level; the cure rate for Staph. aureus IMI was > 90%. Prophylactic treatment of uninfected quarters > or = 45 d prepartum reduced new Streptococcus sp. IMI by 93%. The mean SCC was 50% lower at calving for treated heifers, and milk yield over the first 2 mo of lactation was 10% greater than that of untreated controls. Heifers from herds using fly control had a lower prevalence of IMI than herds without fly control. Prevalences of IMI and SCC in dairy heifers were higher than previously realized, but mastitis at calving was controlled by use of therapeutic products for nonlactating cows during pregnancy.

Dairy heifers were treated 0 to 90 d, 90 to 180 d, or 180 to 270 d prepartum with one of five different antibiotic products to determine the best time and with which product they should be treated prior to calving. Two hundred thirty-three heifers were included in the study. At the initial sampling, 56.5% of quarters were infected with some type of organism and 15.4% of quarters were infected with Staphylococcus aureus. Treatments included a cephapirin dry cow product, a penicillin-novobiocin dry cow product, a penicillin-streptomycin dry cow product, an experimental dry cow product containing tilmicosin, and a cephalonium dry cow product not available in the United States. Cure rates for the five antibiotic products indicated that all were equally effective against Staph. aureus and all were significantly more effective than the spontaneous cure rate observed in untreated control quarters. No differences in efficacy were observed due to the different treatment times prepartum. However, fewer new Staph. aureus infections occurred after treatment in the group treated at 180 to 270 d prepartum, indicating that treatment in the third trimester will reduce the chances of new intramammary infections occurring after treatment and persisting to calving.

Efficacy of two therapy regimens for subclinical Staphylococcus aureus bovine mastitis was evaluated. Forty-nine lactating cows with a total of 78 subclinically infected quarters were included. Cows were divided into two treatment groups. Group 1 received intramammary infusion at each milking for six milkings with a commercial lactating cow product containing 62.5 mg of amoxicillin. Group 2 received the same intramammary infusion regimen plus one intramuscular injection daily of 9,000,000 U procaine penicillin G for 3 d. The combination of intramammary and intramuscular treatment resulted in bacteriologic cure of 51.4% of quarters and 48% of cows compared to 25% of quarters and 30.4% of cows for intramammary infusion alone. Quarters where infections were not cured supported more growth of S. aureus during treatment than glands that were cured. Mean antibiotic concentration in fore-milk samples during treatment were lower in quarters that were cured. Somatic cell counts of milk from quarters that were not cured were higher prior to initiation of treatment than those cured. Data suggest that further evaluation of new regimens for treatment of subclinical S. aureus mastitis is warranted.

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).

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.