• View in gallery
    Figure 1—

    Mean ± SEM rectal temperatures of 6 control steers administered a placebo inoculation of saline (0.9% NaCl) solution intranasally at time 0 (dashed line) and 6 steers challenged with IBRV administered intranasally at time 0 (solid line).

  • View in gallery
    Figure 2—

    Mean ± SEM serum concentrations of IFN-γ in the same groups as in Figure 1.

  • View in gallery
    Figure 3—

    Mean ± SEM serum concentrations of TNF-α in the same groups as in Figure 1.

  • View in gallery
    Figure 4—

    Mean ± SEM serum concentrations of IL-6 in the same groups as in Figure 1.

  • View in gallery
    Figure 5—

    Mean ± SEM serum concentrations of cortisol in the same groups as in Figure 1.

  • View in gallery
    Figure 6—

    Mean ± SEM serum concentrations of GH in the same groups as in Figure 1.

  • View in gallery
    Figure 7—

    Mean ± SEM serum concentrations of IGF-I in the same groups as in Figure 1.

  • 1. Engelken TJ. Preventative programs for respiratory disease in cow/calf operations. Vet Clin North Am Food Anim Pract 1997; 13:647660.

  • 2. Smith RA. Introduction. In: Bovine respiratory disease: sourcebook for the veterinary professional. Yardley, Pa: Veterinary Learning Systems Co, 1996;15.

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  • 3. Babiuk LA, van Drunen Little-van den Hurk S, Tikoo SK. Immunology of bovine herpesvirus 1 infection. Vet Microbiol 1996; 53:3142.

  • 4. Grell SN, Tjornehoj K, Larsen LE, et al. Marked induction of IL-6, haptoglobin and IFNγ following experimental BRSV infection in young calves. Vet Immunol Immunopathol 2005; 103:235245.

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  • 5. Orr CL, Hutcheson DP, Grainger RB, et al. Serum copper, zinc, calcium, and phosphorus concentrations of calves stressed by bovine respiratory disease and infectious bovine rhinotracheitis. J Anim Sci 1990; 68:28932900.

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  • 15. Gardner BA, Dolezal HG, Bryant LK, et al. Health of finishing steers: effects on performance, carcass traits, and meat tenderness. J Anim Sci 1999; 77:31683175.

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  • 16. Daniel JA, Whitlock BK, Wagner CG, et al. Regulation of the GH and LH response to endotoxin in sheep. Domest Anim Endocrinol 2002; 23:361370.

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  • 17. Reuter RR, Carroll JA, Hulbert LE, et al. Technical note: development of a self-contained, indwelling rectal temperature probe for cattle research. J Anim Sci 2010; 88:32913295.

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  • 18. Elsasser TH, Rumsey TS, Hammond AC. Influence of diet on basal and growth hormone-stimulated plasma concentrations of IGF-I in beef cattle. J Anim Sci 1989; 67:128141.

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  • 19. Tizzard IR. How inflammation is triggered. In: Tizzard IR, ed. Veterinary immunology: an introduction. 7th ed. St Louis: Saunders-Elsevier, 2004;1127.

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  • 20. Ackermann MR, Derscheid R, Roth JA. Innate immunology of bovine respiratory disease. Vet Clin North Am Food Anim Pract 2010; 26:215228.

  • 21. Peisen JN, McDonnell KJ, Mulroney SE, et al. Endotoxininduced suppression of the somatotropic axis is mediated by interleukin-1b and corticotropin-releasing factor in the juvenile rat. Endocrinology 1995; 136:33783390.

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  • 22. Elsasser TH, Rumsey TS, Hammond AC, et al. Influence of parasitism on plasma concentrations of growth hormone, somatomedin-C, and somatomedin binding proteins in calves. J Endocrinol 1988; 116:191200.

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  • 23. Fan J, Molina PE, Gelato MC, et al. Differential tissue regulation of insulin-like growth factor-I content and binding proteins after endotoxin. Endocrinology 1994; 134:16851692.

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Evaluation of endocrine and immune responses of steers challenged with infectious bovine rhinotracheitis virus

Shollie M. FalkenbergUSDA Agriculture Research Services, National Animal Disease Center, Ames, IA 50010.

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Jeffery A. CarrollLivestock Issues Research Unit, USDA Agriculture Research Services, Lubbock, TX 79424.

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Ted ElsasserUSDA Agriculture Research Services, Bovine Genomics, Beltsville, MD 20705.

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Tim BestDepartment of Animal and Dairy Sciences, Mississippi State University, Starkville, 39762.

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James SartinDivision of Animal Science and College of Veterinary Medicine University of Auburn, Auburn, AL 36849.

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Joe O. BuntynDepartment of Animal Science, University of Nebraska – Lincoln, Lincoln, 68583.

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Ty B. SchmidtDepartment of Animal Science, University of Nebraska – Lincoln, Lincoln, 68583.

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Abstract

Objective—To evaluate the endocrine and immune responses of steers challenged with infectious bovine rhinotracheitis virus (IBRV).

Animals—12 crossbred beef steers.

Procedures—Steers were randomly assigned to IBRV– (control) or IBRV+ treatment groups. Experimentally challenged steers (IBRV+) received a dose of IBRV intranasally (8.0 50% tissue culture infective doses), IBRV– steers received a saline (0.9% NaCl) solution placebo intranasally, and each group was placed in an isolated paddock. At 72 hours after challenge, all steers were fitted with indwelling jugular catheters and placed into individual stanchions. Blood samples were collected on days 4 through 8. Serum was analyzed for concentrations of cortisol, interleukin-6, interferon-γ, tumor necrosis factor-α, growth hormone, and insulin-like growth factor I.

Results—From 72 to 144 hours after challenge inoculation, the IBRV+ group had significantly greater mean rectal temperature, compared with the IBRV– group; the greatest temperatures in both groups were observed at 72 hours. Serum cortisol concentrations were increased in both groups from hours 72 to 136 and serum interferon-γ concentrations were greater in the IBRV+ from 94 to 112 hours after inoculation. Growth hormone concentration was greater in the IBRV+ group at various time points, but no difference in insulin-like growth factor- I concentration was observed.

Conclusions and Clinical Relevance—Results indicated that IBVR challenge altered growth hormone concentration at some time points but was not associated with large increases in circulating proinflammatory cytokines.

Abstract

Objective—To evaluate the endocrine and immune responses of steers challenged with infectious bovine rhinotracheitis virus (IBRV).

Animals—12 crossbred beef steers.

Procedures—Steers were randomly assigned to IBRV– (control) or IBRV+ treatment groups. Experimentally challenged steers (IBRV+) received a dose of IBRV intranasally (8.0 50% tissue culture infective doses), IBRV– steers received a saline (0.9% NaCl) solution placebo intranasally, and each group was placed in an isolated paddock. At 72 hours after challenge, all steers were fitted with indwelling jugular catheters and placed into individual stanchions. Blood samples were collected on days 4 through 8. Serum was analyzed for concentrations of cortisol, interleukin-6, interferon-γ, tumor necrosis factor-α, growth hormone, and insulin-like growth factor I.

Results—From 72 to 144 hours after challenge inoculation, the IBRV+ group had significantly greater mean rectal temperature, compared with the IBRV– group; the greatest temperatures in both groups were observed at 72 hours. Serum cortisol concentrations were increased in both groups from hours 72 to 136 and serum interferon-γ concentrations were greater in the IBRV+ from 94 to 112 hours after inoculation. Growth hormone concentration was greater in the IBRV+ group at various time points, but no difference in insulin-like growth factor- I concentration was observed.

Conclusions and Clinical Relevance—Results indicated that IBVR challenge altered growth hormone concentration at some time points but was not associated with large increases in circulating proinflammatory cytokines.

One of the most economically devastating diseases the beef industry faces is BRD. The economic cost of BRD from death and morbidity is estimated at $480 million1 to $624 million annually.2 The BRD complex is generally thought to be the consequence of the clinical manifestation of a variety of factors that culminate in acute or chronic bronchopneumonia. Factors and agents associated with the BRD complex include infectious agents (viral and bacterial), stressors, and host susceptibility, making it a difficult disease to diagnose and treat. Each of these factors or agents can potentiate the effects of others, and this can lead to an overwhelming of the body's immune defense mechanisms.

There are a variety of viral pathogens associated with the BRD complex that have been implicated as leading to immunosuppression and allowing opportunistic bacteria the ability to overwhelm the host defenses. Researchers have reported that clinical signs of illness and alterations in immune system typically begin approximately 5 to 7 days after cattle are inoculated and persist for 2 to 3 days.3–7 However, there is little information available regarding the effect of the immune response to IBRV on endocrine factors such as GH and IGF-I. Previously, plasma IGF-I concentrations were reduced by infection with Sarcocystis cruzi or Eimeria bovis even after return of feed intake and a decrease in acute phase proteins, but no differences in GH concentrations were detected.8

Generally at the onset of the proinflammatory response, several of the first-responder cytokines (IL-1, 4, and 6; TNF-α; and INF-γ9) have a purely catabolic function, promoting a physiologic state of readiness to retrieve needed energy substrates from storage depots such as fat and in some instances muscle.10 The initial responses are generally catabolic for 2 main reasons10: when fever is present, the caloric demand can increase as much as 30% for each 1°C increase in core body temperature11; and if the increase in temperature is accompanied by a decrease in feed intake that occurs during proinflammatory stress, the needed calories are channeled away from other resources and processes in a prioritized manner that is proportional to the severity of the response resulting from the immune stimulus. Prioritization of nutrients to different tissues has been postulated to occur to ensure the survival of the animal12 and implies that surviving an immune challenge takes priority over other biological needs, especially growth and tissue accretion in a young animal.10

The somatotrophic axis includes GH, IGF-I, IGF-II, and a signaling and receptor system that influences the control of metabolism and physiologic processes. Results of previous studies10,13,14 suggest that immune challenge methodologies can cause alterations in the somatotrophic axis and provide support for a bidirectional communication pathway between the endocrine and immune systems. On the basis of the decrease in measures of growth (average daily gain, final body weight, and carcass weight) in cattle with clinical signs of the BRD complex,15 pathogens associated with the BRD complex could potentially affect the somatotrophic axis. General response mechanisms that occur when the host is challenged with a pathogen include activation of immune components, endocrine responses that favor catabolic reactions rather than anabolic reactions, repartitioning of nutrients to meet the increase in energy demands, and suspension of less essential biological functions.16 Therefore, the objective of the study reported here was to evaluate the endocrine and proinflammatory immune responses of vaccinated steers challenged with IBRV.

Materials and Methods

The experiment was conducted at the Leveck Animal Research Facility at Mississippi State University. The Animal Care and Use Committee at Mississippi State University approved the experimental protocol for this study.

Animals—Thirty days prior to weaning, 12 crossbred steers (mean ± SD body weight, 207.89 ± 15.45 kg) were identified at the Prairie Research Facility in Prairie, Miss. After selection, steers were vaccinated against Mannheimia haemolytica, parainfluenza virus, bovine respiratory syncytial virus, and bovine viral diarrhea virus. At weaning, a blood sample was obtained to ensure all 12 steers were seronegative for IBRV, and then the steers were transported to the research facility. Steers had an adjustment period of 30 days after arrival, were allowed free access to hay and water, and were fed a diet containing 35% cotton gin mote, 35% hay, and 30% corn gluten supplementation diet to obtain 2.2 kg of weight gain/d. During the adjustment period, the steers were intensively handled to ensure they were accustomed to being haltered and restrained in metabolism crates used during the trial period. Twenty-four hours prior to the viral challenge, steers were fitted with an indwelling rectal temperature-monitoring device17 that was programmed to record temperature at 1-minute intervals. Body weight was recorded, and a blood sample was also obtained via a single venipuncture of the jugular vein into a plain evacuated tube. By use of a random number generator programmed to generate numbers from 1 through 12 (corresponding to ear tag numbers of steers), steers were randomly assigned to treatment groups (IBRV– and IBRV+). The first 6 random numbers generated were designated as IBRV– and the next 6 numbers were designated as IBRV+. A 3-person panel from the Mississippi State Beef Unit (not associated with the trial; panel members were unaware of group assignments) was used to monitor clinical illness scores7 with a removal-from-trial criterion of any steer receiving a score ≥ 4. Although the panel was unaware of treatment group assignments, when cattle were moved into the metabolism stanchions, the panel always evaluated the IBRV– group first (to prevent cross contamination); thus panel members may have assumed the identity of the treatment groups. For the duration of the trial, no difference in clinical illness scores between IBRV– and IBRV+ groups was observed. After sorting, IBRV– and IBRV+ steers were moved to different isolated paddocks where the diet was provided and steers had free access to water.

The IBRV– and IBRV+ steers were housed in different paddocks prior to entering the experimental facility and were maintained in different rooms in the facility to ensure there was no cross contamination. To prevent cross contamination, all personnel followed Animal Biosafety Level 2 protocol, which included use of clean personal protective clothing and gloves each time personnel entered a room, and use of footbaths prior to and following entry into any room where cattle were housed. Each room was fully equipped and no equipment was shared between rooms.

Experimental design—On day 0 (0 hours), prior to the steers receiving the respective treatments, day 1, (24 hours after inoculation), and day 2 (48 hours after inoculation), a blood sample was collected via a single venipuncture of the jugular vein. Each day, the IBRV– group was brought to the facility first to prevent cross contamination between the IBRV+ and IBRV– groups. After the blood samples were obtained, steers were returned to the isolated paddocks. Blood samples were allowed to clot for 30 minutes at 21°C and then centrifuged at 3,000 × g for 30 minutes at 4°C. Serum was collected and transferred into 15-mL microcentrifuge tubes for storage and frozen (−80°C) for later analysis.

On day 0, after the blood sample had been obtained, the IBRV+ group received 4 mL of IBRV inoculum (2 mL/nostril [8.0 50% tissue culture infective dose]), and the IBRV– group received a 4-mL placebo inoculum of saline (0.9% NaCl) solution (2 mL/nostril). The viral agent used in this study was the IBR challenge virus (lot number 05–08) isolated from the Cooper strain.a Each virus ampule was thawed and contents diluted with PBS solution to a total of 4 mL, and then ampules were pooled. Inoculum (4 mL) for each steer was placed in a prelabeled 15-mL conical tube that was capped and maintained at 26°C prior to administration (< 30 minutes later). Viral and placebo doses were administered with a glass and metal atomizer attached to a compressor-nebulizer.7 The atomizing device was sterilized between administration of viral and placebo doses by flushing the atomizer with 70% isopropyl alcohol and allowing it to dry for 15 minutes.

On day 3 (72 hours after inoculation), all steers were fitted with an indwelling jugular vein catheter for serial blood collection. Catheters consisted of approximately 150 mm of polytetrafluorethylene tubingb that was inserted into the jugular vein by use of a 14-gauge, 5.1-cm thin-walled stainless steel biomedical needle (outer diameter, 2.11 mm). Catheters and needles were sterilizedc before use. The catheter was maintained in place with tag cement and a 5.1-cm-wide porous surgical tape and fitted with extensions made of sterile plastic tubingd for collection of blood samples without disturbing the steers. After catheters were inserted and functioning, steers were moved to individual metabolism stanchions. While in the metabolism stanchions, steers received the same diet and had ad libitum access to water. Beginning at 74 hours after IBRV inoculation and continuing until 96 hours after inoculation (after all catheters were inserted and steers were in the stanchions), blood was collected every 4 hours. Starting at 96 hours after IBRV inoculation, blood samples were collected every hour for 64 hours (from hours 96 to 160), after which samples were collected every 4 hours (from hours 164 to 208), every 12 hours (from hours 220 to 364), and then every 24 hours until the completion of the trial (676 hours). When a blood sample was obtained, approximately 5 mL of waste sample was withdrawn from the catheter and discarded. Then the collection tubee was connected to the catheter and a 14-mL sample was obtained from each steer. The catheter was flushed with 5 mL of saline solution followed by 3 mL of heparinized saline solution to maintain patency of the catheter. After the first 12-hour blood sample was obtained (at hour 220) the catheters were removed and steers were removed from the stanchions and returned to isolation paddocks and offered the same diet and free access to water. Blood samples collected in 12- and 24-hour increments were collected via a single venipuncture of the jugular vein. All steers were monitored daily for any clinical signs of illness to ensure recovery from the sampling procedures. Study protocol indicated that any steers that had clinical signs of illness would be immediately removed from the trial and evaluated by the attending veterinarian.

Endocrine and cytokine assays—Serum concentrations of the proinflammatory cytokines TNF-α, IL-6, and IFN-γ were assayed per the manufacturer's protocol by use of a customized multiplex ELISAf validated for bovine cytokines. For all cytokines, a pool of serum samples from a previous lipopolysaccharide challenge in beef cattle was used as a positive control; the intra-assay coefficient of variation and inter-assay coefficient of variation were < 10%.

Serum samples were also analyzed for GH and IGF-I as described.18 For the GH assay, rabbit anti-bGH (R1–1–4) antibody was used at a final dilution of 1:60,000. At this dilution, the antibody bound 23% of the tracer. Minimal sensitivity of the assay was 150 pg of bGH/assay tube with 50% binding of tracer achieved at 1,800 pg/tube. Increasing volumes of plasma resulted in displacement of tracer counts in a manner similar to that of the standard curve. Percentage recovery of non-labeled bGH was a mean of 97% for 300, 600, and 1,200 pg added to 200 μL of serum. Intra- and inter-assay coefficients of variation were ≤ 10%.

Concentrations of IGF-I were determined following acidification for 36 hours with glycyl-glycine buffer to achieve a final pH of 3.6. Each sample was then diluted 1:80 with assay buffer. Anti-human-bovine IGF-I primary serumg was used at a final dilution of 1:10,000. Dilutions of plasma resulted in displacement of radioactive tracer in a manner similar to the displacement generated in the standard curve. The minimal detectable concentration of IGF-I was 32 pg/tube; recovery of nonlabeled IGF-I added to plasma before acidification was a mean of 95% with intra- and interassay coefficients of variation ≤ 10%.

Statistical analysis—Summary statistics were calculated for each variable, and mean ± SEM values were determined for each treatment. Change in variables over time was analyzed by use of ANOVA for repeated measures as a completely random design; the model included sampling time, treatment, and treatment × sampling time. Treatment × sampling time was used as the error term to test whole-plot effects. Rectal temperature was recorded at 1-minute intervals; mean values over 24-hour intervals were calculated from these 1-minute data points and used to facilitate comparisons with other variables. When results of the F test were significant (P < 0.05), group means at various sampling times were compared by use of the least significant difference test by sample and adjusted using the Tukey test.

Area under the curve was calculated for INF-γ, TNF-α, IL-6, cortisol, GH, and IGF-I by use of the trapezoidal method to determine the time-concentration value (ng•h/mL or pg•h/mL) of each variable over a given period. Only when values for the variables were significant (F test with P < 0.05) was the area under the curve calculated. Results from these calculations were analyzed by use of ANOVA. The model included treatment as a fixed effect for each of the cytokines and hormones. For all comparisons, values of P < 0.05 were considered significant.

Results

A significant (P < 0.001) interaction of the 2 main effects (treatment × sampling time) was detected for rectal temperature (Figure 1). No significant differences in mean rectal temperature between groups were observed from 24 hours before to 48 hours after inoculation; mean rectal temperature was significantly greater in the IBRSV+ group from 72 to 120 hours. Mean rectal temperature in both groups was greatest at 72 hours. At 144 hours, mean rectal temperature in the IBRV+ group appeared to return to near baseline.

Figure 1—
Figure 1—

Mean ± SEM rectal temperatures of 6 control steers administered a placebo inoculation of saline (0.9% NaCl) solution intranasally at time 0 (dashed line) and 6 steers challenged with IBRV administered intranasally at time 0 (solid line).

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

A significant (P < 0.001) interaction of the 2 main effects (treatment × sampling time) was detected for IFN-γ concentration. The IBRV+ group had significantly increased mean serum concentration of IFN-γ, compared with the IBRV– steers, from 94 through 112 hours, with peak mean concentration at 96 hours (Figure 2). Mean serum IFN-γ concentration in the IBRV– group was significantly greater than in the IBRV+ group at 136 hours. At approximately 152 hours, IFN-γ concentration in both treatment groups appeared to have returned to near baseline. For serum concentration of IFN-γ, the area under the curve measured during hours 96 to 120, which was considered the critical immune response period, was significantly greater in the IBRV+ group.

Figure 2—
Figure 2—

Mean ± SEM serum concentrations of IFN-γ in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

No significant (P > 0.10) interaction of the 2 main effects (treatment × sampling time) was detected for TNF-α concentration; however, evaluation of pairwise comparisons between the treatment groups at specific time points detected notable differences (Figure 3). Mean serum concentration of TNF-α at 136 hours was significantly greater in the IBRV+ group, compared with the IBRV– group. Starting at 144 hours and continuing to approximately 160 hours after IBRV inoculation, both groups had concentrations of TNF-α that were greater than baseline (−24 hours), but no differences were observed between the groups. At 184 hours, mean serum concentrations of TNF-α for both groups were not significantly different from baseline concentrations. For serum concentration of TNF-α, the area under the curve measured during hours 96 to 120 was not significantly different between groups.

Figure 3—
Figure 3—

Mean ± SEM serum concentrations of TNF-α in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

No significant (P > 0.10) interaction of the 2 main effects (treatment × sampling time) was detected for IL-6 concentration; evaluation of pairwise comparisons between the treatment groups at specific time points detected notable differences. Mean serum concentrations of IL-6 in both groups were generally low throughout the study, but an increase was observed in the IBRV+ group at 136 hours when concentration in the IBRV+ group was significantly greater than in the IBRV– group (Figure 4). By 152 hours, concentrations in both groups were not significantly different from baseline. There was no significant difference between groups for area under the curve for serum IL-6 concentration measured during hours 96 to 120.

Figure 4—
Figure 4—

Mean ± SEM serum concentrations of IL-6 in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

A significant (P < 0.05) interaction of the 2 main effects (treatment × sampling time) was detected for serum cortisol concentration (Figure 5). From hours 72 to 86, the IBRV– group had significantly increased concentrations, compared with the IBRV+ group. At 140 hours, concentrations in both groups appeared to be similar to baseline. At 94, 96, and 120 hours, mean cortisol concentrations in the IBRV– group were significantly greater than in the IBRV+ group. The greatest mean cortisol concentration for the IBRV– group was observed at approximately 96 hours after inoculation (24 hours after being moved into the metabolic stanchions). The area under the curve for cortisol concentration measured from 96 to 120 hours was not significantly different between groups.

Figure 5—
Figure 5—

Mean ± SEM serum concentrations of cortisol in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

A significant (P = 0.03) interaction of the 2 main effects (treatment × sampling time) was detected for serum GH concentration. Mean serum GH concentrations were significantly greater in the IBRV– group, compared with the IBRV+ group, at 124 hours after inoculation and at various further time points until 196 hours (Figure 6). The area under the curve for mean serum GH concentrations measured from 96 to 120 hours after inoculation was not significantly different between groups (Table 1). The area under the curve for mean serum GH concentrations measured from 120 to 144 hours and for 144 to 192 hours was significantly greater in the IBRV– group, compared with the IBRV+ group.

Figure 6—
Figure 6—

Mean ± SEM serum concentrations of GH in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

Table 1—

Mean area under the concentration-versus-time curve for IFN-γ, cortisol, and GH in 6 contro steers administered a placebo inoculation of saline (0.9% NaCl) solution intranasally at time 0 (IBRV-) and 6 steers challenged with IBRV (IBRV+) administered intranasally at time 0.

  Treatment group  
VariableTime after inoculation (h)IBRVIBRV+SEM*P value
INF-γ (pg•h/mL)96–120610.902,790.55873.380.05
Cortisol (ng•h/mL)96–1202,826.001,810.83624.360.18
GH (ng•h/mL)96–120197.36165.5123.710.31
GH (ng•h/mL)120–1441,335.56755.64131.060.006
GH (ng•h/mL)144–1923,883.121,977.12632.450.04

SEM for both groups.

No significant interaction of the 2 main effects (treatment × sampling time) was detected for serum IGF-I concentrations. The IBRV– group had significantly greater mean serum IGF-I concentration at 24 hours after inoculation, compared with the IBRV+ group (Figure 7); this was the only time point at which a difference was detected. Serum concentrations decreased rapidly at 96 hours for both groups and generally remained low for the remainder of the trial.

Figure 7—
Figure 7—

Mean ± SEM serum concentrations of IGF-I in the same groups as in Figure 1.

Citation: American Journal of Veterinary Research 74, 12; 10.2460/ajvr.74.12.1522

Discussion

Prior to the study, all steers were seronegative for IBRV. At the conclusion of the study, all IBRV– steers were still seronegative, whereas all IBRV+ steers had seroconverted and had a mean titer of 1:8. The increases in rectal temperature and INF-γ concentration (at 136 hours) observed in the IBRV– group may have been the result of the stress of confinement in the IBRV– group, because seroconversion against IBRV was not detected.

The IBRV+ group had a mean rectal temperature that was significantly greater than that of the IBRV– group from 72 to 120 hours after IBRV inoculation as well as greater mean concentrations of IFN-γ from 94 to 112 hours. This combination of factors may have contributed to the decreased GH concentration in the IBRV+ group, compared with the IBRV– group, from hours 124 to 196. The IFN-γ concentrations in the IBRV+ group increased over time, whereas TNF-γ and IL-6 concentrations did not, which was indicative of a virus-induced immune response. Bovine respiratory viruses such as IBRV can infect the lung epithelial cells and cause activation of toll-like receptors 3, 7, 8, and 9, initiating a signaling cascade geared toward activation of immune responses.20

Growth hormone mediates proliferation of a number of cell types and has a plethora of effects on the cells and organs associated with immune function. Many of the biological effects attributed to GH are mediated either directly or indirectly by IGF-I.14 Therefore, the role of GH as a key anabolic hormone that is part of the somatotrophic axis has been evaluated as a potential endocrine mediator of hypertrophy and growth during an immune stimulus. The relationship between GH and IGF-I during an immune stimulus has also been assessed in an effort to understand alterations in the somatotrophic axis. Growth hormone concentrations were reduced during an endotoxin challenge in rats21 and during a parasitic infection in cattle,22 although there are differences in the immune responses initiated by parasitic and viral infections. The anabolic action of GH is accomplished in part by the activation of cells in the liver to produce IGF-I,14 and parasite-infected cattle have decreased IGF-I concentrations and reduced growth rates.22 It is also suggested that the reduction in IGF-I concentration during an immune challenge likely reflects decreased IGF-I synthesis in multiple tissues.10 Uncoupling of the somatotrophic axis occurs during immunologic challenge, in which decreased concentrations of GH precede the decrease in IGF-I concentrations, and then GH concentration returns to reference range; IGF-I concentration is still decreased 24 hours after challenge.23 Although no significant changes in IGF-I concentration were detected in the present study, there was a decrease at 96 hours after inoculation that remained throughout most of the duration of the trial.

The reduction in total GH concentration (area under the curve) during hours 124 to 196 for the IBRV+ group suggested alterations similar to those observed during endotoxin challenges. Generally, changes in GH concentration during endotoxin challenges occur during the challenge period, but it would appear there was a delayed suppression of GH associated with the viral inoculation in the present study; this delayed suppression could be attributed to the type of immune stimulus used. Endotoxin challenges initiate a rapid innate immune response and are generally recognized and resolved quickly by the host's immune system, whereas the viral challenge in the present study elicited a sustained immune response and no immunity-related variables associated with febrile responses were detected until the third day after inoculation. Another factor that could be implicated in the suppression of GH and IGF-I concentrations was the increase in cortisol concentration associated with the steers being moved to metabolic stanchions at 72 hours after inoculation. Cortisol concentrations were increased for both groups from 72 to 136 hours after inoculation, and the increase in cortisol concentration was concurrent with the increase in IFN-γ concentration in the IBRV+ group. The dual activation of the immune response and the hypothalamus–pituitary–adrenal axis could have contributed to the reduction in GH observed in the IBRV+ group. The transient decrease in IGF-I concentration was concurrent with the steers being moved into the metabolic stanchions and the increase in cortisol concentration, but concentrations of GH did not return to initial baseline values in either group for the remainder of the trial.

Results of this study suggested that IBRV can induce minor alterations in the endocrine variables (GH and IGF-I) associated with the somatotrophic axis. These minor alterations may have been potentiated by the increase in cortisol concentration, the combined increase in cortisol and IFN-γ concentrations, or solely by the increase in IFN-γ concentration. However, with the limited clinical signs of illness and limited changes in cytokine concentrations, the results could indicate that the IBRV challenge, although sufficient to elicit a febrile response, was only able to induce a mild proinflammatory response. Therefore, a better understanding of multiple interactions associated with stressors and immune challenges needs to be elucidated to clarify the effects of viral inoculation on the response of the somatotrophic axis.

ABBREVIATIONS

bGH

Bovine growth hormone

BRD

Bovine respiratory disease

GH

Growth hormone

IBRV

Infectious bovine rhinotracheitis virus

IFN-γ

Interferon γ

IGF

Insulin-like growth factor

IL-6

Interleukin 6

TNF-α

Tumor necrosis factor α

a.

USDA, Center for Veterinary Biologics, Ames, Iowa.

b.

PTFE 6417–41 18TW (od = 1.66 mm), Cole-Palmer, Vernon Hills, Ill.

c.

Nolvasan, Pfizer, New York, NY.

d.

Tygon S-50 HL (id = 1.59 mm; od = 3.18 mm), VWR Scientific, Randor, Pa.

e.

Monovet, Sarstedt Inc, Newton, NC.

f.

Searchlight Multiplex Immunoassay Kit, Billerica, Mass.

g.

Anti human/bovine IGF-I primary serum, GroPep, Adelaide, Australia.

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Contributor Notes

Address correspondence to Dr. Schmidt (tschmidt4@unl.edu).

This manuscript represents a portion of a dissertation submitted by Dr. Falkenberg to the Mississippi State University Department of Animal and Dairy Sciences as partial fulfillment of the requirements for a PhD.

Presented as an abstract at the Annual Meeting of the American Society of Animal Science, Denver, July 2010.

Mention of trade names or commercial products in this article is solely for the purpose of providing scientific information and does not imply recommendation or endorsement by the USDA.