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Ghent University, Faculty of Veterinary Medicine, Department of Physiology, Biochemistry, and Biometrics, 9820 Merelbeke, Belgium
Corresponding author: C. Burvenich; e-mail: christian.burvenich{at}UGent.be.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: primiparous dairy cow • Escherichia coli • mastitis • inoculum size • early lactation
Abbreviation key: BLC = blood leukocyte count, HR = heart rate, MP = milk production, PCV = packed cell volume, p.i. = postinfusion, PIH = postinfusion hour, QMP = quarter milk production, ROS = reactive oxygen species, RR = respiration rate, RT = rectal temperature
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Preventive measures, which are known to be efficient against contagious mastitis, such as postmilking teat disinfection (Schukken et al., 1989a, 1989b; Barkema et al., 1999) have been shown to be inefficient in the control of E. coli mastitis (Smith et al., 1985; Oliver et al., 1990; Lam et al., 1995). Several epidemiological studies have demonstrated a negative correlation between SCC and the incidence of E. coli mastitis. Therefore, SCC is thought to be a farm-related risk factor for the susceptibility to E. coli mastitis (Schukken et al., 1989a, 1989b; Barkema et al., 1999).
Various bacterial virulence factors have been studied during E. coli mastitis (Sanchez-Carlo et al., 1984a, 1984b); however, only a few have been found to substantially influence the eventual outcome of the disease. Escherichia coli is of environmental origin (Nemeth et al., 1994). Although over 100 subtypes have been recognized, no specific O-serotypes have been related to bovine E. coli mastitis (Linton and Robinson, 1984). Nevertheless, intramammary challenge with E. coli 487 caused more severe clinical signs of mastitis than did E. coli 727 (Hogan et al., 1992, 1995, 1999; Todhunter et al., 1991). Moreover, a capsulated E. coli B117 strain appeared to cause more severe clinical symptoms of mastitis because it was more difficult to opsonize than a noncapsulated E. coli P4 strain (Hill, 1981). Clinical expression after challenge with low doses of E. coli is comparable with those after inoculation of high doses (Frost et al., 1980). The adhesion factors present in enterotoxigenic E. coli, which are essential during the initial steps of adhesion in pathogenesis, do not play a role in the pathogenesis of E. coli mastitis (Nagy and Fekete, 1999). A bacterial factor, which plays an important role in the pathogenesis of E. coli mastitis, is endotoxin or LPS. Lipopolysaccharide is a potent inducer of inflammatory cytokines (Shuster et al., 1993) during growth and killing (Burvenich, 1983). Clinical signs following experimentally induced E. coli mastitis are due to mediator shock rather than to endotoxin shock, because endotoxin mainly plays a local role (Hoeben et al., 2000; Dosogne et al., 2002).
Physiological factors have a big impact on the clinical outcome of E. coli mastitis (Burvenich et al., 2003), far more than farm management and bacterial characteristics that were sited above. Several markers to predict the clinical outcome of E. coli mastitis have been studied, such as the number of circulating leukocytes, production of reactive oxygen species (ROS) by neutrophils (Heyneman et al., 1990; Vandeputte-Van Messom et al., 1993), and chemotactic activity of PMN (Kremer et al., 1993a; Kremer et al., 1993b; Dosogne et al., 1997; van Werven et al., 1997). However, predictability of clinical outcome is only significant during 1 to 2 d prechallenge and does not explain any causal relationship. These markers were mainly studied in multiparous cows, varying between second lactation (Heyneman et al., 1990; Vandeputte-Van Messom et al., 1993; Dosogne et al., 1997) to second to sixth lactation (Kremer et al., 1993a, 1993b; van Werven et al., 1997). Because BHBA alters both PMN chemotactic activity and ROS production (Heyneman et al., 1990; Kremer et al., 1993a, 1993b; Vandeputte-Van Messom et al., 1993), many E. coli challenges that were executed at Ghent University and the University of Utrecht were performed on cows with a serum BHBA concentration <1.4 mmol/L (Kremer et al., 1993b).
Early lactating cows, infected with E. coli, are much more severely affected than cows after peak lactation (Hill, 1981). This is mainly due to the impairment of early lactation leukocyte function, as observed by many research groups (Burton and Erskine, 2003). During this period a decrease in cell function of the PMN, resident in the healthy mammary gland (Dosogne et al., 2001; Mehrzad et al., 2001; Vangroenweghe et al., 2001) has been observed. The decrease in cell function was mainly related to the decrease in viability, oxidative burst, and intracellular killing by PMN (Dosogne et al., 2001; Mehrzad et al., 2001; Vangroenweghe et al., 2001).
Besides energy balance and stage of lactation, cow parity was also found to be an important physiological factor that influences severity of clinical mastitis (Gilbert et al., 1993; van Werven et al., 1997; Mehrzad et al., 2002). Blood PMN function was higher in younger animals than in cows after their fourth parturition (Gilbert et al., 1993; van Werven et al., 1997). Moreover, viability and oxidative burst have been found to be significantly different between primiparous cows and multiparous cows during the periparturient period (Mehrzad et al., 2002).
In conclusion, many studies indicate that physiological factors determine the clinical outcome of E. coli mastitis. In this study the clinical outcome of E. coli mastitis was studied in primiparous cows using the same criteria for severity as in other studies using multiparous cows (Heyneman et al., 1990; Vandeputte-Van Messom et al., 1993; Dosogne et al., 1997; van Werven et al., 1997; Hoeben et al., 2000).
The purpose of the present study was to evaluate the outcome of intramammary E. coli inoculation in primiparous cows under identical conditions as described before with multiparous cows (Heyneman et al., 1990; Vandeputte-Van Messom et al., 1993; Dosogne et al., 1997, 2002; Hoeben et al., 2000). Moreover, the relation between preinfection parameters and the outcome of infection in term of severity was assessed. Finally, 2 high-inoculum doses, with a 100-fold difference (1 x 104 and 1 x 106 cfu), were used because the amount of LPS produced is related to the number of E. coli bacteria (Burvenich, 1983; Cross et al., 1993; Monfardini et al., 1999). The primiparous cows in this study were inoculated with the same strain as described before (Vandeputte-Van Messom et al., 1993; Hoeben et al., 2000; Dosogne et al., 2002).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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For inclusion into the intramammary inoculation trial, treatment of clinical diseases (retentio secundinarum, mastitis, metritis, and so on) was not allowed within 10 d before the intramammary inoculation. Therefore, only healthy animals, free of major mastitis pathogens through 3 consecutive bacteriologically negative examinations with a foremilk SCC below 200,000 cells/mL at quarter level, were included for the intramammary E. coli challenge. Primiparous cows accepted for the intramammary challenge were inoculated between 14 and 28 d postparturition.
Machine milking was performed daily at 0800 and 1800 h using a quarter milking device (Packo & Fullwood, Zedelgem, Belgium). Daily quarter milk production (QMP), the yield of the evening and subsequent morning milking, expressed as liters per day, was measured at d -4, d -1, d 0 (postinfusion hour [PIH] 0 to 24), d1 (PIH 24 to 48), d 2 (PIH 48 to 72) and d 3 (PIH 72 to 96).
Inoculation Dose
The generation time of E. coli in mammary secretions can be as short as 20 min (Burvenich et al., 2003). Therefore, 2 high inoculum doses (1 x 104 [group A; n = 9] and 1 x 106 [group B; n = 10] cfu) were used in this experiment because we were interested in bacterial clearance rather than in bacterial growth in the affected mammary glands.
Intramammary Inoculation Procedure and Experimental Design
Inoculation was performed as described before (Hoeben et al., 2000). Briefly, E. coli P4:O32 (H37, ß-glucuronidase +, haemolysin -), maintained as a stock in lyophilization medium at -20°C, was subcultured in brain-heart infusion broth (CM225; Oxoid, Nepean, ON, Canada) at 37°C during 3 consecutive days, subsequently washed 3 times with pyrogen-free PBS, and resuspended in PBS. Just before inoculation, the suspension was diluted in pyrogen-free PBS to a final concentration of 1 x 104 cfu/mL (group A) or 1 x 106 cfu/mL (group B). On d 0, 30 min after morning milking (1.5 h after feeding), the cows were inoculated in the left front and rear quarters with a total volume of 10 mL of pyrogen-free saline solution (0.9%) per quarter.
Animals were challenged on 4 different trial days. The 1 x 104 cfu inoculum group (group A) was challenged in a group of 4 and 5 animals, and the 1 x 106 cfu inoculum group (group B) was challenged in 2 groups of 5 animals.
Sampling Procedure
Blood and milk samples were collected at d -4, d -1, d 0, d +1, d +2 and d +3 relative to the day of challenge. On the day of challenge, blood and milk samples were collected at PIH 3, 6, 9, 12, 15, 18, and 21.
Blood samples were drawn aseptically from the external jugular vein of each cow by venipuncture in evacuated tubes (lithium heparin for plasma preparation, sodium-fluoride for glucose analysis, and coagulated for serum preparation) for subsequent use in different analyses. Foremilk was aseptically collected for diagnostic bacteriology on major pathogens before inoculation and for quantification of the number of E. coli (cfu) from PIH 3 onwards, determination of SCC, and preparation of skim milk. All samples were kept on melting ice (1°C) during transport and at the laboratory until analysis was performed.
Clinical Examination
Classical clinical parameters were examined. Rectal temperature (RT), heart rate (HR), respiration rate (RR), rumen motility, skin turgor, fecal appearance, appetite, general attitude, BCS (Edmondson et al., 1989), and aspects of the mammary gland (abnormal milk, swelling, teat relaxation, and milk leakage [Massart-Leën et al., 1988]) were recorded by a veterinarian each time blood and milk samples were collected.
Severity Determination
The severity of E. coli mastitis was determined based on QMP in the noninfected quarters at d +2 postinfusion (p.i.). Animals with a QMP in the noninfected quarters at d +2 higher than 50% compared with their QMP at d -1 in the same quarters were scored as moderate responders, whereas animals with a QMP at d +2 lower than 50% were considered severe responders (Vandeputte-Van Messom et al., 1993; Dosogne et al., 1997, 1999). Moreover, severity was scored based on Wenz et al. (2001) with some slight modifications. Therefore, clinical data (RT, skin turgor, reticulorumen motility, and general attitude) obtained from PIH 9 to 48 were scored as described in Table 1
, and based on their total score, primiparous cows were classified into mild, moderate, and severe responders.
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Indicators of the Disintegration of the Blood-Milk Barrier
Milk samples for the determination of serum albumin (mg/dL), sodium (Na+), chloride (Cl-), and potassium (K+) concentration (mmol/L) were centrifuged at 1000 x g (30 min, 4°C). Fat was removed and samples of skim milk were taken and immediately frozen at -80°C until analysis. After thawing, serum albumin was quantified using a radial immunodiffusion kit (bovine low level albumin, Bethyl VET-RID; Bethyl Laboratories, Montgomery, TX). Ion concentration was analyzed using an ion-selective electrode analyzer (Ilyte; Instrumentation Laboratories, Milan, Italy).
Packed Cell Volume, Blood Leukocyte Count, and Differentiation
Blood packed cell volume (PCV) (%) was determined in hematocrit-capillaries (60 µL/75 mm; Hirschmann Laborgeräte, Eberstadt, Germany) using a micro-hematocrit centrifuge (Hawksley, London, UK). Blood leukocyte count (BLC) (log10/mL) was determined using an electronic particle counter (Coulter Counter Z2; Coulter Electronics Ltd., Luton, UK). Differential BLC was performed on blood smears. Briefly, 10 µL of homogenized whole blood was added onto a microscope slide and very thin smears were prepared, following drying and staining as described previously, differential counts were carried out (Mehrzad et al., 2001).
Colony-Forming United in the Inoculated Quarters
The number of E. coli (cfu/mL) after experimental inoculation was determined by appropriate 10-fold dilutions of each milk sample in PBS. Ten microliters of these dilutions was plated out on Columbia Sheep Blood agar (Biokar Diagnostics, Beauvois, France). All dilutions were performed in duplicate. Colonies were counted after a 24-h incubation at 37°C. The colony count was converted to cfu/mL based on the factor of dilution and finally expressed as log10/mL for statistical analysis.
Glucose and ß-Hydroxybutyrate in Serum
On arrival at the laboratory, clotted blood was incubated for 2 h at 37°C to neutralize the cryoglobulins present in bovine serum. After centrifugation (1000 x g, 20 min, 20°C), serum was aliquoted and stored at -80°C until analysis of BHBA. NaF-collected blood was centrifuged (1000 x g, 20 min, 20°C) and plasma was collected and stored at -20°C until analysis of glucose.
Glucose concentration (mmol D-glucose/L) was determined using an UV-method (D-Glucose; Roche Diagnostics, Brussels, Belgium). ß-Hydroxybutyrate (mmol/L) was determined twice in the week before challenge to check the status of the cow’s energy balance. For analysis, the method with acetone oxidation, described by Williamson and Mellanby (1970), was used.
Statistical Analysis
The 2 inoculum groups were not formally compared because inoculum dose was not randomly assigned to animals. Preinfection values of blood and milk constituents of the inoculum groups (Table 2) were compared just before intramammary E. coli inoculation using a paired 2-sided t-test, assuming unequal variances (Statistix; Analytical Software, Tallahassee, FL). The relationship between preinfection concentrations of blood and milk constituents and the milk production reduction in the noninfected quarters at d +2 (PIH 48 to 72) was assessed by linear regression with an adjustment for inoculum size.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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).
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). Rectal temperature increased from PIH 9 onwards, with peak fever reached at PIH 12 (Figure 2a). Heart rate and RR peaked at PIH 12 (Figures 2b and 2c). Rumen motility was depressed from PIH 9 onwards, resulting in a decrease in appetite. Primiparous cows returned to normal appetite and reticulorumen motility around PIH 21. Udder parameters, such as swelling and elevated quarter temperature, appeared at PIH 9 in the infected quarters. Teat relaxation, milk leakage, and diarrhea, considered as indicators for severe clinical illness, only appeared in a small number of animals. Changes to abnormal milk with clots, flakes, and a watery appearance were maximally present in all infected animals at PIH 18 (results not shown). Body condition score slightly decreased from 3.5 to 2.5 between calving and the end of the experimental challenge period.
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Milk production in the infected quarters maximally decreased (±55%) on the day of challenge. At d +1 p.i., MP gradually increased and almost totally recovered by d +3 p.i (Figure 2d
). The number of E. coli in the infected quarters peaked at PIH 6, followed by a rapid clearance until PIH 15. At PIH 72, only 5 animals still had low numbers of E. coli in the infected quarters (Figure 2e). Somatic cell count rapidly increased from PIH 6, reached a maximum at PIH 12, and remained high until the end of the experiment at PIH 72 (Figure 2f).
Blood leukocyte count decreased to nadir at PIH 12 and recovered to preinfection level at PIH 48 (Figure 2j). The presence of early immature PMN (myelocytes-metamyelocytes) in circulation increased at PIH 6 and reached peak levels at PIH 21 (Figure 2g). Similarly, maximal percentages of late immature PMN (band cells) were reached at PIH 21 (Figure 2h). Concomitantly, circulating mature PMN decreased at PIH 6 with nadir at PIH 12 (Figure 2i).
The concentration of lactose in the infected quarters decreased from PIH 9 with a minimum at PIH 21 to 24 (Figure 2k). Serum albumin in the infected quarters increased from PIH 9 and peaked at PIH 15 (Figure 2l). Sodium and chloride concentrations peaked at PIH 12 (Figures 2m and 2n), whereas potassium concentration decreased from PIH 9 and reached its greatest reduction at PIH 12 (Figure 2o). All indicators for the disintegration of the blood-milk barrier gradually recovered to preinfection levels at PIH 72, after reaching their peak values (Figures 2k to 2o).
Clinical and Laboratory Results of Primiparous Cows in Group B Inoculated with 1 x 106 cfu
Clinical reaction and changes in laboratory parameters were similar to the previously described results for group A. However, the changes during inflammation generally appeared more rapidly (approximately 3 h) in primiparous cows from group B, which were infused with the 1 x 106 cfu E. coli inoculum dose, compared with group A, which were infused with the 1 x 104 cfu E. coli inoculum dose.
All primiparous cows in this group reacted with a moderate clinical response to intramammary E. coli infusion. Milk production in the noninfected quarters was only decreased until d +1 p.i. (Figure 1). Rectal temperature increased from PIH 6, peaked at PIH 9, and gradually decreased to preinfection values at PIH 18 (Figure 2a). Peak tachycardia was reached at PIH 9 (Figure 2b), whereas maximal RR was reached at PIH 12 (Figure 2c). Reticulorumen motility and all other clinical parameters seemed to change some 3 h earlier in group B. The same trend in BCS could be observed as in group A.
Clinical scores, adapted from Wenz et al. (2001), showed an earlier reaction (approximately 3 h) in group B compared with group A. At PIH 9, most of the primiparous cows reacted moderately (n = 8), gradually decreasing at PIH 12 (n = 5). Only one animal in group B had a prolonged moderate response until PIH 24. No severe responses were present in this group at any timepoint between PIH 9 and 48.
Milk production reduction in the infected quarters was maximal (±65%) on the day of infusion and recovered rapidly to preinfection production at d +3 p.i. (Figure 2d). The number of E. coli in the infected quarters peaked at PIH 3 (Figure 2e). A rapid influx of cells into the infected quarters was observed from PIH 3, with a peak at PIH 9 (Figure 5f).
The number of circulating blood leukocytes decreased from PIH 6, peaking at PIH 9. A pronounced rebound effect was observed between PIH 15 and 48 (Figure 2j). The increase in circulating early immature cells was pronounced and peaked at PIH 15 (Figure 2g). Band cells increased at PIH 9 and peaked at PIH 18, thereafter gradually decreasing to reach prechallenge levels at PIH 72 (Figure 2h). The greatest reduction in circulating mature PMN was reached at PIH 3, gradually increasing to preinfection values at PIH 21 (Figure 2i).
The concentrations of lactose, serum albumin, sodium, chloride, and potassium followed similar kinetics as in group A, although the initial changes appeared approximately 3 h earlier (Figure 2k to o).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). In both infected and noninfected quarters, milk production seemed to recover more rapidly on d +1 p.i. in primiparous cows from group B, which received the 1 x 106 cfu E. coli inoculum dose. Animals were scored as previously described (Vandeputte-Van Messom et al., 1993) and compared with the clinical score used by Wenz et al. (2001). This resulted in a similar classification of mild and moderate responses. The only difference that could be observed between group A and group B was that the time of latency was shorter when higher inoculum doses were used.
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The number of circulating PMN, a marker to predict the clinical outcome of the disease, was similar to the levels previously observed in moderate responders (Vandeputte-Van Messom et al., 1993). Primiparous cows in this study could therefore be expected to react with a moderate clinical response. Although no PMN functionality was determined in this study, it is expected that PMN ROS production was high in these primiparous cows. Mehrzad et al. (2002) recently found that in the period from 5 wk before until 5 wk postparturition the chemiluminescence activity of PMN was higher in primiparous cows than in multiparous cows. Production of PMN ROS and chemotactic activity were inversely correlated to the clinical outcome of E. coli mastitis (Heyneman et al., 1990; Kremer et al., 1993a, 1993b). No significant relationship with the reduction in MP on d +2 p.i. was observed. This is possibly due to the limited number of animals in this study.
The onset of local clinical signs of mastitis, characterized by quarter swelling, coincided with the influx of PMN to the infected quarters. Somatic cell count increased rapidly in both groups, which is in accordance with earlier observations, where moderate responders had a rapidly occurring leucocytosis in the infected glands (Vandeputte-Van Messom et al., 1993). The extraction of mature PMN from the blood to the infected glands is known to result in early and late immature PMN recruitment from the bone marrow to restore the number of circulating PMN. In this study the recruitment of immature PMN was of short duration, when compared with Heyneman et al. (1990), who observed immature forms in circulation for at least 3 d in moderate responders and for almost 10 d in severe responders.
The rapid influx of PMN into the infected glands was associated with fast clearance of bacteria from the quarters. In this study, high inoculum doses were used for experimental induction of E. coli mastitis because we were mainly interested in bacterial clearance, rather than bacterial growth in the mammary gland. In contrast to an earlier study with the same inoculum dose in multiparous animals (Vandeputte-Van Messom et al., 1993), peak bacterial numbers were already reached around PIH 3 to 6. This peak number of bacteria was followed by a rapid clearance from the affected glands. Contrary to the induction of E. coli mastitis with low inoculum doses, where clearance is preceded by excessive bacterial growth, in this study, peak numbers were reached within 6 h p.i. and followed by a subsequent bacterial clearance. Therefore, PMN influx is thought to be fast and strong enough to rapidly clear the bacteria from the affected glands. It can be presumed that the bactericidal capacity of the PMN migrated to the infected quarters is high because more efficient PMN functionality was reported in primiparous cows recently (Mehrzad et al., 2002).
Several indicators (lactose, serum albumin, sodium, chloride, and potassium) for the disintegration of the blood-milk barrier were determined in this study. Generally, the changes could be observed approximately 3 h earlier in group B. Peak levels were almost identical for both groups, however, which coincides with all other data, indicating that the animals reacted with a mild to moderate response and that little variation in the clinical response was present in this study.
From this study, it appears that the inflammatory response in primiparous cows from group B has an earlier onset compared with group A. One possible explanation for this observation could be the 100-fold difference in the number of E. coli infused into the mammary glands because the amount of LPS produced is related to the number of E. coli bacteria (Burvenich., 1983; Cross et al., 1993; Monfardini et al., 1999). A direct effect of LPS present in the inoculum can be excluded because the bacterial cultures were washed 3 times in pyrogen-free PBS before further dilutions were made. Lipopolysaccharide, known as a potent inducer of inflammatory cytokines (Shuster et al., 1993), can be produced quite rapidly during bacterial growth following intramammary infusion. Therefore, a sufficient amount of inflammatory cytokines should be produced early during inflammation in both groups, resulting in the rapid attraction of PMN from the blood into the mammary gland, with a subsequent pronounced increase of SCC in the infected glands.
Following E. coli mastitis, treatment with a bactericidal antibiotic at PIH 10 has been shown to be inefficient to alter local and systemic symptoms already present, although the number of bacteria in the infected quarters decreased 100-fold (Monfardini et al., 1999). Therefore, it can be suggested that early inflammatory events (first 3 h) could play an important role in the further regulation of the inflammatory response to combat the invading organisms. Experimental design in this study, particularly in relation to the number and time of samplings, however, was not suitable for unraveling these elements of early acute phase response.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication July 9, 2003. Accepted for publication September 29, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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