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 h 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 (R2 = 0.28), but was not associated with severity of E. coli mastitis (R2 = 0.06). In vivo bacterial growth during the early phase of infection (positive association; R2 = 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; R2 = 0.11), are major determinants for the severity of experimental E. coli mastitis (R2 = 0.56).

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 100cfu E. coli. After 6h 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.

Plasma prolactin, growth hormone and progesterone concentrations in pseudopregnant, hysterectomized and pregnant goats.

Jugular plasma prolactin (PRL), growth hormone (GH) and progesterone (P4) levels were estimated in goats under three different conditions with prolonged luteal function (P4 > or = 1 ng/ml): pseudopregnant animals (n = 4), goats hysterectomized during early pregnancy (n = 4) and does with normal pregnancy (n = 4). Mean duration (+/- S.E.M.) of luteal phases were 189 +/- 20, 171 +/- 10, and 147 +/- 2 days in the three groups, respectively. Until day 120, mean PRL levels were below 150 ng/ml in each group. After day 120 of the luteal phase, PRL concentrations were significantly higher than before, but continued to increase up to 800 ng/ml only in pregnant animals around parturition. Mean GH levels varied between 2 and 3 ng/ml in animals of each group during the luteal phase. Only after parturition, a significant elevation occurred. P4 levels in pseudopregnant animals were significantly lower than in the other two groups between days 10 and 55, and showed a gradual but continuous decline towards the end of the luteal phase. After hysterectomy of early pregnant animals, P4 concentrations decreased to levels measured in pseudopregnant animals but were significantly higher again as compared to pseudopregnant animals between days 121 and 150. It is concluded that a pseudopregnant condition, characterized by intrauterine fluid accumulation, is not related to increased plasma PRL and GH concentrations. The low and gradually decreasing plasma progesterone levels in the pseudopregnant animals probably reflect the absence of a luteotrophic stimulus by the conceptus. The progesterone profile in the animals that were hysterectomized during early pregnancy suggests that the corpora lutea of these does have been permanently changed by the presence of the conceptus during the first weeks of the luteal phase.

Induction of hydrometra in goats by means of active immunization against prostaglandin F2 alpha.

Hydrometra is a pathological condition of the uterus which is characterized by accumulation of aseptic fluid in the presence of a persistent corpus luteum. It forms a major cause of subfertility in goats. Active immunization against prostaglandin F2 alpha (PGF2 alpha), the luteolytic hormone in this species, was used to explore the possibility for artificial induction of this pseudopregnant condition. During the breeding season, 11 goats (group I) were immunized with 5 mg PGF2 alpha-ovalbumin conjugate in Freund's complete adjuvant, 4 goats were control-immunized (group CI) and 5 goats remained untreated (group C). Jugular blood samples were taken twice a week (monday and thursday) for measurements of plasma progesterone and binding of 3HPGF2 alpha. In conjunction with blood sampling, transcutaneous ultrasonographic examination of the uterus took place to detect the presence of fluid in the uterus. Before and immediately after immunization, the mean (+/-SD) duration of luteal phases (progesterone concentrations > or = 1 ng ml-1) was 16.7 +/- 1.6 (n = 39), 17.8 +/- 1.3 (n = 23) and 16.9 +/- 1.1 (n = 18) days in animals of group I, CI and C respectively. Ten goats of group I developed an antibody titre. Persistence of luteal function (mean duration +/-SD: 150.3 +/- 23.5 days) occurred in 6 of these animals and in 1 goat of group CI. Accumulation of fluid in the uterus in group I was first observed between day 31 and 38 of the prolonged luteal phase. Discharge of uterine fluid occurred as soon as the plasma progesterone concentration reached a level lower than 0.5 ng ml-1. It is concluded that immunization against PGF2 alpha is an effective method to induce pseudopregnancy in goats, providing a model for studies on luteal maintenance and uterine function in the absence of a conceptus.

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.