Bacterial growth during the early phase of infection determines the severity of experimental Escherichia coli mastitis in dairy cows

The aim of this study was to investigate the importance of bacterial growth for the severity of experimental Escherichia coli mastitis, indirectly expressed as the area under the curve of bacterial counts in milk over time. The association of pre-infusion somatic cell count and post-infusion influx of inflammatory cells in milk with severity of infection was also examined. Bacterial growth was studied through culture in milk samples (in vitro) and through monitoring of bacterial counts in milk during the early phase of infection (in vivo) in 36 cows. Individual variation in bacterial counts was more than 2 x 10(2)-fold after 6 h of in vitro incubation, and more than 8 x 10(2)-fold 6 It after intramammary infusion. In vitro growth in milk was not associated with in vivo growth during the early phase of infection, nor with severity of E. coli mastitis. Somatic cell count before experimental E. coli mastitis was negatively associated with in vivo bacterial growth during the early phase of infection (R-2 = 0.28), but was not associated with severity of E coli mastitis (R-2 = 0.06). In vivo bacterial growth during the early phase of infection (positive association; R-2 = 0.41), together with influx of inflammatory cells in milk, expressed as mean hourly increase of somatic cell count between 6 and 12 h post-infusion (negative association; R-2 = 0.11), are major determinants for the severity of experimental E. coli mastitis (R-2 = 0.56). (C) 2004 Elsevier B.V. All rights reserved

Effects of milk production capacity and metabolic status on HPA function in early postpartum dairy cows

Increasing milk yields in modern dairy cows cause concern that high yield may impair the cows' health and welfare, for example, via negative effects on metabolic status and hypothalamo-pituitary-adrenocortical (HPA) function. This study aims to investigate whether a high level of milk production, and the associated metabolic status, affects HPA function in dairy cows and changes their adaptive capacity. Additionally, it aims to establish whether possible effects of milk production level only show under challenging conditions. Holstein-Friesian cows, which produced on average 11,443 and 7727 kg of fat and protein-corrected milk (FPCM)/305 d in their previous lactation, were compared. During the dry period, the cows were fed to requirements or overfed. High milk yield and the concomitant large energy deficit were associated with 1) increased pituitary (re)activity, i.e., increased ACTH baseline concentrations and higher ACTH concentrations after corticotropin-releasing hormone (CRH) administration, and 2) decreased adrenocortical reactivity, i.e., lower cortisol responses after ACTH administration. Although significant, the effects of milk production level on HPA function were relatively small. Animals showed seemingly normal hormonal responses to CRH and ACTH administration. Also, cortisol baseline concentrations were unaffected. It seems, therefore, unlikely that the adaptive capacity of the high-producing cows was significantly impaired compared with their low-producing herdmates

In vitro growth of mastitis-inducing Escherichia coli in milk and milk fractions of dairy cows

The outcome of E. coli mastitis in cows ranges from mild to severe in individual animals. This study explored the hypothesis that milk from individual cows differs in its growth medium properties for E. coli, and whether possible variation could be related to specific milk constituents. To mimic the early phase of intramammary E. coli infection, a low inoculum size and a short incubation period were used. Cell-reduced, cell- and fat-free (skim) and cell- and fat-free and protein-reduced (whey) fractions were prepared from whole milk samples (n=18). Ten ml of whole milk, milk fractions and brain heart infusion broth (BHI) were inoculated with approximately 100 cfu E. coli. After 6 h of incubation, bacterial counts were assessed by dilution plating in triplicate. Bacterial counts in whole milk differed up to a 100-fold between cows, which was not associated with SCC. Bacterial counts were significantly higher in whey fractions than in whole milk, cell-reduced and skim fractions and variation in whey was smaller, indicating that the acid-precipitable protein fraction contains the milk constituents of major relevance for inhibition of and variation in bacterial growth. The presence of fat and cells added to bacterial growth inhibition to a lesser extent. In conclusion, in vitro growth of E. coli in milk differs substantially between individual cows within an incubation period comparable with the early phase of intramammary infection. This suggests that the growth medium properties of milk could be of importance in the pathogenesis of E. coli mastitis and subsequent outcome of disease

In vitro growth of mastitis-inducing Escherichia coli in milk and milk fractions of dairy cows

The outcome of E. coli mastitis in cows ranges from mild to severe in individual animals. This study explored the hypothesis that milk from individual cows differs in its growth medium properties for E. coli, and whether possible variation could be related to specific milk constituents. To mimic the early phase of intramammary E. coli infection, a low inoculum size and a short incubation period were used. Cell-reduced, cell- and fat-free (skim) and cell- and fat-free and protein-reduced (whey) fractions were prepared from whole milk samples (n=18). Ten ml of whole milk, milk fractions and brain heart infusion broth (BHI) were inoculated with approximately 100 cfu E. coli. After 6 h of incubation, bacterial counts were assessed by dilution plating in triplicate. Bacterial counts in whole milk differed up to a 100-fold between cows, which was not associated with SCC. Bacterial counts were significantly higher in whey fractions than in whole milk, cell-reduced and skim fractions and variation in whey was smaller, indicating that the acid-precipitable protein fraction contains the milk constituents of major relevance for inhibition of and variation in bacterial growth. The presence of fat and cells added to bacterial growth inhibition to a lesser extent. In conclusion, in vitro growth of E. coli in milk differs substantially between individual cows within an incubation period comparable with the early phase of intramammary infection. This suggests that the growth medium properties of milk could be of importance in the pathogenesis of E. coli mastitis and subsequent outcome of disease