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Journal of Animal Science - Animal Production

Effect of timing of Mannheimia haemolytica challenge following short-term natural exposure to bovine viral diarrhea virus type 1b on animal performance and immune response in beef steers1

 

This article in JAS

  1. Vol. 94 No. 11, p. 4799-4808
     
    Received: June 10, 2016
    Accepted: Aug 20, 2016
    Published: October 13, 2016


    3 Corresponding author(s): the.blake.wilson@okstate.edu
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doi:10.2527/jas.2016-0712
  1. L. Carlos-Valdez*22,
  2. B. K. Wilson 3*,
  3. L. O. Burciaga-Robles*44,
  4. D. L. Step,
  5. B. P. Holland*55,
  6. C. J. Richards*,
  7. M. A. Montelongo,
  8. A. W. Confer,
  9. R. W. Fulton and
  10. C. R. Krehbiel*
  1. * Department of Animal Science, Oklahoma State University, Stillwater 74078
     Center for Veterinary Health Sciences, Oklahoma State University, Stillwater 74078

Abstract

Bovine respiratory disease (BRD) is the most common and economically detrimental disease of beef cattle during the postweaning period, causing the majority of morbidity and mortality in feedlots. The pathogenesis of this disease often includes an initial viral infection, which can predispose cattle to a secondary bacterial infection. The objective of this experiment was to determine the effects of timing of an intratracheal Mannheimia haemolytica (MH) challenge relative to 72 h of natural exposure to bovine viral diarrhea virus (BVDV) type 1b persistently infected (PI) calves on performance, serum antibody production, total and differential white blood cell (WBC) count, rectal temperature, clinical severity score (CS), and haptoglobin (Hp). Steers (n = 24; 276 ± 31 kg initial BW) were randomly allocated to 1 of 3 treatments (8 steers/treatment) in a randomized complete block design. Treatments were steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON), steers intratracheally challenged with MH 84 h after being exposed to calves PI with BVDV 1b for 72 h (LateCh), and steers intratracheally challenged with MH 12 h after being exposed to calves PI with BVDV 1b for 72 h (EarlyCh). Performance (ADG, DMI, and G:F) was decreased (P < 0.001) for both EarlyCh and LateCh from d 0 to 4. From d 5 to 17, LateCh appeared to compensate for this lost performance and demonstrated increased ADG (P = 0.01) and G:F (P = 0.01) compared with EarlyCh. Both EarlyCh and LateCh had decreased platelet counts (P < 0.001) compared with CON. Antibody concentrations of BVDV and MH were higher (P < 0.05) for both EarlyCh and LateCh compared with CON. Rectal temperature, CS, and Hp increased (P < 0.001) across time from h 4 to 48, h 4 to 36, and h 8 to 168, respectively. Within 24 h of MH challenge, WBC and neutrophil concentrations within the blood increased whereas lymphocyte concentrations decreased. The timing of BVDV exposure relative to a MH challenge appears to influence the CS and acute phase response associated with BRD. As typical beef cattle marketing channels allow for variation in the timing of respiratory pathogen exposure, understanding the physiological changes in morbid cattle will lead to improved management of BRD.



INTRODUCTION

Bovine respiratory disease (BRD) causes approximately 75% of morbidity and over 50% of mortality in feedlots (Smith, 1998). Although mortality due to BRD is a concern, morbidity most likely costs the industry more, considering the expenses associated with medications, labor, facility use, chronic illness, and decreased performance and carcass quality (Smith, 1998; Gardner et al., 1999; Gagea et al., 2006). Bovine respiratory disease is a multifactorial disease that involves environmental conditions and other stressors, and active infection with a number of respiratory viruses can predispose cattle to pneumonia from several bacterial pathogens (Czuprynski et al., 2004). A predominant viral/bacterial synergism that exists between respiratory pathogens isolated from feedlot cattle includes bovine viral diarrhea virus (BVDV), which can predispose cattle to a secondary bacterial infection with Mannheimia haemolytica (MH).

Burciaga-Robles et al. (2010) evaluated the effects of an intratracheal challenge with MH with or without previous exposure (72 h) to steers persistently infected (PI) with BVDV 1b. Changes in serum antibody production, white blood cell (WBC) counts, and cytokine concentrations consistent with an immune challenge were observed. These results suggest that exposure of naïve calves to calves PI with BVDV increases the potential for secondary infections and negative impacts to animal health and performance. Due to the potential for immunosuppression by BVDV 1b, it was hypothesized that steers exposed to BVDV 1b for 72 h and challenged with MH 84 h later would have a reduced immune response compared with steers receiving the MH challenge shortly (12 h) after exposure to BVDV 1b. Our objective was to determine the effects of timing of a MH challenge relative to BVDV exposure on animal performance and immune response in growing beef cattle.


MATERIALS AND METHODS

All procedures were approved by the Oklahoma State University Institutional Animal Care and Use Committee (Animal Care and Use Protocol AG-06-16).

Cattle Description

A total of 24 Angus crossbred steers (276 ± 31 kg initial BW) were housed at the Nutrition Physiology Research Center (NPRC) at Oklahoma State University, Stillwater, OK, to determine the effects of timing of bacterial challenge relative to viral exposure on performance and immune responses in growing cattle. All animals had been weaned, were deemed clinically healthy by the attending veterinarian, and were considered seronegative to all pathogens involved in the study as determined with paired serum samples collected 14 d apart prior to the start of the experiment.

Experimental Treatments

The 24 steers were randomly allocated to 1 of 3 treatments (8 steers/treatment) arranged as a randomized complete block design. Treatments were control steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON), steers intratracheally challenged with MH 84 h after being exposed to calves PI with BVDV 1b for 72 h (LateCh), and steers intratracheally challenged with MH 12 h after being exposed to calves PI with BVDV 1b for 72 h (EarlyCh). This experiment was conducted in 2 periods with a 2-wk interval between the periods to facilitate sample collection. Steers were blocked by BW into 2 groups of 12 and the challenge procedures and sample collections were staggered with a 2-wk interval between the 2 groups. Steers exposed to the calves PI with BVDV were transported approximately 3.2 km to the Willard Sparks Beef Research Center at Oklahoma State University, Stillwater, OK, where they were commingled in a 6 by 10.8 m pen with 2 steers previously confirmed as being PI with BVDV 1b via immuohistochemistry and genotyping by sequencing the 5′-untranslated region of the viral genome and antigenic analyses with panels of monoclonal antibodies to BVDV 1b (Ridpath and Bolin, 1998; Fulton et al., 2006a,b). During this period, all calves in the experimental group shared a common feed bunk and water tank to ensure exposure to the animals PI with BVDV. For both the EarlyCh and LateCh groups, the length of exposure to the calves PI with BVDV 1b was 72 h. The LateCh were exposed to the calves PI with BVDV 1b 72 h (84 h prior to MH challenge) prior to EarlyCh (12 h prior to MH challenge). After the time of BVDV exposure, calves were returned to the NPRC, where they remained for the remainder of the experiment. Steers not exposed to calves PI with BVDV 1b were not transported the short distance and remained at the NPRC to minimize risk of exposure to BVDV or other potential respiratory pathogens. Steers challenged with MH received 10 mL of a solution containing 6 × 109 cfu of MH serotype 1 that was reconstituted and grown prior to the challenge as previously described (Mosier et al., 1998). Steers not challenged with MH were intratracheally dosed with 10 mL of a PBS solution (Sigma-Aldrich, St. Louis, MO.). Inoculation with the MH culture or PBS was performed as described by Dowling et al. (2002) with modifications (Burciaga-Robles et al., 2010). Challenge with MH or PBS occurred on the same day at the same time (d 0) for all treatment groups beginning at 0800 h, either 12 or 84 h after the 72 h exposure to calves PI with BVDV 1b for EarlyCh and LateCh, respectively.

The experiment consisted of 24 d during which the animals were kept in individual pens (3.7 by 3.7 m) with the exception of d 0 (day of MH challenge) to 4. During those days, animals were placed in individual stanchions (1.1 by 2.3 m) with headlocks to allow for the challenge procedures and collection of blood samples (Burciaga-Robles et al., 2010). During the experiment, steers were offered feed at 3% of BW on a DM basis; feed was delivered twice daily. The dry-rolled corn–based diet contained 40% ground alfalfa hay and was formulated to meet or exceed nutrient requirements providing 148 g CP, 1.79 Mcal NEm, and 1.12 Mcal NEg per kilogram of DM (NRC, 1996). Animals were weighed on d −7, 0, 4, and 17. Average daily gain was calculated using BW and days on feed and G:F was calculated using DMI for the corresponding periods.

Rectal Temperatures and Subjective Clinical Severity Scores

Rectal temperatures (TEMP) were recorded using a digital veterinary thermometer (GLA M-500; GLA Agricultural Electronics, San Luis Obispo, CA.). In addition, all steers were monitored by trained personnel throughout the length of the experiment for clinical signs of morbidity. The clinical evaluation used has been described by Step et al. (2008). Briefly, the subjective criteria included depression (e.g., hanging head, sunken eyes, arched back, and difficulty getting up from lying down), abnormal appetite, and respiratory signs (e.g., labored breathing). Based on the severity of signs, the evaluator assigned a numeric clinical severity score (CS) ranging from 0 to 4, where 0 was assigned for clinically healthy, 1 was assigned for mild, 2 for moderate, 3 for severe, and 4 for moribund (steer would not rise from recumbency or assistance was needed). Rectal temperature and CS were recorded prior to BVDV exposure and at −96, −2, 2, 4, 6, 8, 12, 18, 24, 36, 48, 72, and 96 h relative to MH challenge.

Serum Haptoglobin

Blood samples (Clott activator; Becton Dickinson Vacutainer Systems, Franklin Lakes, NJ) were collected via jugular venipuncture with an 18-gauge needle (−96 −2, 2, 6, 12, 18, 24, 36, 48, 72, 96, and 168 h following the 0 h MH challenge). Samples were allowed to clot for 24 h at 4°C. After the clotting time, chilled blood samples were centrifuged at 3,000 × g at 4°C for 20 min. Serum was harvested in 2-mL centrifuge tubes and stored at −20°C until further analyses were performed. Once all the serum samples were collected, a Bovine Haptoglobin ELISA Test (Immunology Consultants Lab, Portland, OR) was used to determine the haptoglobin (Hp) concentration of each serum sample. Prior to the analysis, serum samples were diluted 1:10,000 in Tris-buffered saline with Tween 20 (pH 4.0; Sigma-Aldrich). The intra- and interassay CV were below 5%.

Hemogram

Anticoagulated blood samples (EDTA; Becton Dickinson Vacutainer Systems) were collected via jugular venipuncture with an 18-gauge needle at −168, −96, −12, −2, 6, 18, 72, 96, 168, and 336 h following MH challenge. Samples were immediately submitted to a commercial lab (Antech Diagnostic Lab, Stillwater, OK) for total and differential WBC count determination, total red blood cells, total platelets, hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH) concentration.

Mannheimia haemolytica and Bovine Viral Diarrhea Virus Antibodies

Blood samples were collected via the same procedure described for Hp and harvested serum samples from d −7, 0, 4, 7, and 14 were used to determine MH whole cell (WC) and leukotoxin (LKT) antibodies by a formalin-killed MH serotype 1 by an ELISA test as described by Confer et al. (1995, 1996). Antibody responses were expressed as nanograms of immunoglobulin binding based on a set of IgG standards on each plate. The intra- and interassay CV were below 5%. Blood samples were collected via the same procedure described for Hp, and harvested serum samples from d −7, 0, 4, 7, 14, 28, and 42 were submitted to a diagnostic laboratory (Oklahoma Animal Disease and Diagnostic Lab, Stillwater, OK) where a virus neutralization test in Madin–Darby bovine kidney cell monolayers in 96-well microtiter plates was used to quantify virus-neutralizing antibodies to BVDV. The viruses used were cytopathic BVDV 1a (Singer strain), cytopathic BVDV 1b (strain TGAC 8HB), and cytopathic BVDV 2a (strain 125-C). A 1:4 dilution was the lowest tested, and titers of less than 1:4 were considered negative. The highest dilution tested was 1:2,048.

Statistical Analyses

The experiment was designed as a randomized complete block with animal serving as the experimental unit and period serving as the block. Period was included in the random statement. Data for BVDV antibody titers, MH WC and LKT antibody titers, TEMP, CS, and Hp concentrations were analyzed using repeated measures analysis of the MIXED procedure of SAS with a nonstructured covariance structure and slice output option (SAS version 9.1; SAS Inst. Inc., Cary, NC.). The repeated term was time (hour or day), and the model included the main effects of treatment (CON, EarlyCh, and LateCh), time, their interactions, and period. When an EarlyCh and/or LateCh × time interaction was significant (P ≤ 0.05), the slice output option was used to determine the time points at which time the effect was different across treatments. Least squares means were separated using the pdiff statement of SAS.


RESULTS

Body weight was not affected (P ≥ 0.67) by timing of MH challenge in relation to BVDV exposure (Table 1). From d 0 to 4, both EarlyCh and LateCh had lower (P < 0.001) ADG than CON. From d 5 to 17, LateCh appeared to compensate and had greater (P = 0.01) ADG than both CON and EarlyCh. Across the entire period, EarlyCh had lower (P ≤ 0.03) ADG than CON and LateCh. From d 0 to 4, DMI (kg/d) was greatest (P < 0.001) for CON, intermediate for LateCh, and lowest for EarlyCh. In addition, exposure to steers PI with BVDV 1b for 72 h decreased (P ≤ 0.02) DMI as a percent of BW from d 0 to 4, regardless of timing of MH challenge. From d 0 to 4, G:F was lower (P ≤ 0.001) for steers exposed to steers PI with BVDV compared with CON. Across the entire experiment, G:F was greatest (P ≤ 0.03) for CON, intermediate for LateCh, and lowest for EarlyCh.


View Full Table | Close Full ViewTable 1.

Effects of bovine viral diarrhea virus (BVDV) type 1b exposure and subsequent Mannheimia haemolytica (MH) challenge on steer performance

 
Experimental treatment1
Item CON EarlyCh LateCh SEM P-value
BW, kg
    Initial 270 275 283 17. 7 0.67
    d 0 282 294 290 16.5 0.77
    d 4 291 287 280 16.6 0.77
    d 17 305 299 309 12.2 0.84
ADG, kg
    d 0 to 4 2.21a −1.51b −2.62b 0.74 <0.001
    d 5 to 17 1.14a 0.88b 2.26c 0.31 0.01
    d 0 to 17 1.39a 0.32b 1.12a 0.27 0.03
DMI, kg/d
    d 0 to 4 6.58a 5.13b 5.80c 0.24 <0.001
    d 5 to 17 6.38 6.06 6.72 0.31 0.33
    d 0 to 17 6.42 5.84 6.36 0.22 0.15
DMI, % BW
    d 0 to 4 2.31a 1.81b 1.85b 0.13 0.02
    d 5 to 17 2.51 2.08 2.26 0.17 0.49
    d 0 to 17 2.19 2.02 2.16 0.14 0.39
G:F, kg/kg
    d 0 to 4 0.32a −0.31b −0.50b 0.14 0.001
    d 5 to 17 0.17a 0.12b 0.33c 0.05 0.01
    d 0 to 17 0.22a 0.04b 0.17c 0.05 0.03
a–cWithin a row, means with different superscripts are different (P < 0.05).
1CON = steers not exposed to calves PI with BVDV 1b and not challenged with MH; LateCh = steers intratracheally challenged with MH 84 h after being exposed to calves PI with BVDV 1b for 72 h; EarlyCh = steers intratracheally challenged with MH 12 h after being exposed to calves PI with BVDV 1b for 72 h.

There was an overall treatment × time interaction (P < 0.001) for BVDV 1b antibody concentrations (Fig. 1). Antibody concentrations also increased on d 28 and 42 (P ≤ 0.006) for EarlyCh and LateCh compared with CON. In challenged steers, BVDV antibody concentrations were greater (P < 0.05) for LateCh compared with EarlyCh on d 28 and 42. There was a treatment × time interaction (P < 0.001) for MH WC (Fig. 2a) and LKT (Fig. 2b) antibody concentrations. Antibody concentrations for MH WC and LKT increased over time for EarlyCh and LateCh and were greater (P ≤ 0.05) compared with CON on d 7 and 14.

Figure 1.
Figure 1.

Serum bovine viral diarrhea virus (BVDV) 1b neutralization antibody concentrations in calves exposed to steers persistently infected (PI) with BVDV for 72 h followed by an intratracheal challenge with 6 × 109 CFU of Mannheimia haemolytica (MH) serotype 1 on d 0 , occurring 12 and 84 h after the 72 h BVDV exposure for EarlyCh and LateCh, respectively compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There was a treatment × time interaction (P < 0.001; SEM = 8.60). Values plotted represent least squares means ± SE of the mean, calculated for 8 animals per experimental group. a–cWithin day, least squares means with different superscripts are different (P < 0.05).

 
Figure 2.
Figure 2.

Serum concentration of Mannheimia haemolytica (MH) whole cell (WC) antibodies (panel a) and MH leukotoxin (LKT) antibodies (panel b) in calves exposed to steers persistently infected (PI) with bovine viral diarrhea virus (BVDV) for 72 h followed by an intratracheal challenge with 6 × 109 CFU of MH serotype 1 on d 0 , occurring 12 and 84 h after the 72 h BVDV exposure for EarlyCh and LateCh, respectively compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There was a treatment × time interaction (P < 0.009, SEM = 0.293; panel A) for MH WC antibodies and a treatment × time interaction (P < 0.02, SEM = 0.135; panel B) for MH LKT antibodies. Values plotted represent least squares means ± SE of the mean, calculated for 8 animals per experimental group for both MH WC antibodies and MH LKT antibodies. a,bWithin day, least squares means with different superscripts are different (P < 0.05).

 

Rectal temperatures were greater for EarlyCh and LateCh from 4 to 72 h following MH challenge compared with CON (treatment × time interaction, P < 0.001; Fig. 3). In addition, TEMP was greater (P ≤ 0.05) for LateCh compared with EarlyCh from 12 to 18 h after MH challenge. Subjective CS responded with a treatment × time interaction (P < 0.001; Fig. 4). The EarlyCh and LateCh had a greater (P ≤ 0.05) CS than CON beginning at 4 (LateCh) or 12 h (EarlyCh) after MH challenge and continuing through 96 h. In addition, CS was greater (P ≤ 0.05) for LateCh compared with EarlyCh from 8 to 96 h after MH challenge. Similarly, Hp concentrations responded with a treatment × time interaction (P < 0.001; Fig. 5). Haptoglobin concentrations were greater (P ≤ 0.05) for EarlyCh and LateCh compared with CON from 18 to 168 h after MH challenge. In addition, Hp concentrations were greater (P < 0.05) for LateCh compared with EarlyCh from 48 to 96 h after MH challenge.

Figure 3.
Figure 3.

Rectal temperature of calves exposed to steers persistently infected (PI) with bovine viral diarrhea virus (BVDV) for 72 h followed by an intratracheal challenge on d 0 with 6 × 109 cfu of Mannheimia haemolytica (MH) serotype 1 at 12 and 84 h after the 72 h BVDV exposure for EarlyCh and LateCh, respectively, after BVDV exposure compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There was a treatment × time interaction (P < 0.001, SEM = 0.06). Values plotted represent least squares means ± SE of the mean, calculated for 8 animals per experimental group. a–cWithin a day, least squares means with different superscripts are different (P < 0.05).

 
Figure 4.
Figure 4.

Subjective clinical severity scores (CS) of calves exposed to steers persistently infected (PI) with bovine viral diarrhea virus (BVDV) for 72 h followed by an intratracheal challenge on d 0 with 6 × 109 cfu of Mannheimia haemolytica (MH) serotype 1 at 12 and 84 h after the 72 h BVDV exposure for EarlyCh and LateCh, respectively, after BVDV exposure compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There was a treatment × time interaction (P < 0.0001, SEM = 0.08). Values plotted represent least squares means ± SE of the mean, calculated for 8 animals per experimental group. a–cWithin a day, least squares means with different superscripts are different (P < 0.05).

 
Figure 5.
Figure 5.

Serum haptoglobin concentrations in calves exposed to steers persistently infected (PI) with bovine viral diarrhea virus (BVDV) for 72 h followed by an intratracheal challenge on d 0 with 6 × 109 cfu of Mannheimia haemolytica (MH) serotype 1, at 12 and 84 h fter the 72 h BVDV exposure for EarlyCh and LateCh, respectively, after BVDV exposure compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There was a treatment × time interaction (P < 0.001, SEM = 95.76). Values plotted represent least squares means ± SE of the mean, calculated for 8 animals per experimental group. a–cWithin a day, least squares means with different superscripts are different (P < 0.05).

 

There was a treatment × time interaction (P = 0.007) for total WBC counts (Fig. 6a). Steers in the LateCh group had lower (P < 0.05) WBC counts at −24 and −2 h of MH challenge, whereas steers in the EarlyCh group had lower (P < 0.05) WBC counts at 72 and 96 h after MH challenge. Both EarlyCh and LateCh had greater (P < 0.05) total WBC counts than CON 18 h after MH challenge.

Figure 6.
Figure 6.

Total white blood cell (panel a), neutrophil (panel b), and lymphocyte (panel c) counts in calves exposed to steers persistently infected (PI) with bovine viral diarrhea virus (BVDV) for 72 h followed by an intratracheal challenge on d 0 with 6 × 109 cfu of Mannheimia haemolytica (MH) serotype 1, at 12 and 84 h after the 72 h BVDV exposure for EarlyCh and LateCh, respectively, after BVDV exposure compared with steers not exposed to calves PI with BVDV 1b and not challenged with MH (CON). There were treatment × time interactions (P < 0.007, SEM = 0.66 [panel A]; P < 0.0001, SEM = 312.24 [panel B]; and P < 0.03; SEM = 350.54 [panel C]). Values plotted represent least squares means ± SE, calculated for 8 animals per experimental group. a,bWithin a day, least squares means with different superscripts are different (P < 0.05).

 

There was a BVDV × MH interaction (P = 0.02) for neutrophil concentrations (Fig. 6b). Neutrophil concentrations were lower (P < 0.05) for CON and LateCh compared with EarlyCh at −12 h and remained lower for LateCh until −2 h. However, at 18 h, both EarlyCh and LateCh had greater neutrophil concentrations (P < 0.05) compared with CON. At 168 h after MH challenge, LateCh had greater (P < 0.05) greater neutrophil concentrations than CON. Lymphocyte concentrations were lower (P < 0.05) for LateCh beginning at −12 h and for EarlyCh at −2 h compared with CON and remained lower until 18 h (Fig. 6c). At 96 h, EarlyCh had lower (P < 0.05) lymphocyte concentrations compared with LateCh and CON.

A treatment × time interaction (P = 0.02) was observed for basophil concentrations and red blood cells. However, there was not a statistical difference among experimental treatments (P ≥ 0.05) within each time point, so overall means for these variables are presented in Table 2. Monocyte, eosinophil, and basophil concentrations were not affected (P ≥ 0.20) by treatment (Table 2). Similarly, hematocrit, hemoglobin, MCV, and MCH were not affected (P ≥ 0.22). The concentration of platelets was greatest (P = 0.001) for CON, intermediate for EarlyCh, and lowest for LateCh.


View Full Table | Close Full ViewTable 2.

Effects of bovine viral diarrhea virus (BVDV) type 1b exposure and subsequent Mannheimia haemolytica (MH) challenge on steer hemogram

 
Treatments1
P-value
Item CON EarlyCh LateCh SEM Treatment Treatment × time
Monocytes/μL 344.2 378.6 355.6 46.5 0.77 0.87
Eosinophils/μL 238.5 304.9 207.2 48.8 0.30 0.73
Basophils/μL 124.1 102.4 100.1 10.3 0.20 0.02
Hematocrit, % 33.66 33.11 33.45 1.01 0.73 0.99
Hemoglobin, g/100 mL 12.26 11.89 12.07 0.29 0.45 0.79
Mean corpuscular volume, fL 37.71 37.28 38.6 0.79 0.22 0.84
Mean corpuscular hemoglobin, % 13.83 13.52 13.93 0.20 0.35 0.99
Platelets, 103/μL 649a 566b 478c 31.7 0.001 0.62
Red blood cells, 106/μL 8.80 8.83 8.43 0.22 0.08 0.01
a–cWithin a row, means with different superscripts are different (P < 0.05).
1CON = steers not exposed to calves PI with BVDV 1b and not challenged with MH; LateCh = steers intratracheally challenged with MH 84 h after being exposed to calves PI with BVDV 1b for 72 h; EarlyCh = steers intratracheally challenged with MH 12 h after being exposed to calves PI with BVDV 1b for 72 h.

DISCUSSION

Experiments have evaluated the effects of calves PI with BVDV in feed yards as a main source of BVDV infection among seronegative cattle within pens. Fulton et al. (2005) used vaccinated and nonvaccinated calves exposed to calves PI with BVDV 1b to determine infection with BVDV in the feed yard. When a PI animal was included in a pen, seroconversion occurred in 70 to 100% of nonvaccinated penmates (Fulton et al., 2005). Our results confirm those of earlier studies demonstrating that PI calves can serve as a natural method of challenge to seronegative BVDV steers for immunological studies. In addition, we have confirmed that calves PI with BVDV 1b can serve as an effective source of infection to healthy animals as an experimental model (Burciaga-Robles et al., 2010; Wilson et al., 2016). Seroconversion to BVDV 1b was detected through increased BVDV antibody concentrations in all EarlyCh and LateCh. All steers exposed to calves PI with BVDV in the current experiment contracted the infection and demonstrated an independent immune response. Similar results have been observed using intranasal inoculation of BVDV type 2 in young calves (Archambault et al., 2000) and with calves infected with BVDV type 1 (Kelling et al., 2007).

In the current experiment, a greater BVDV antibody response was observed for LateCh compared with EarlyCh on d 28 and 42. We believe this increase in BVDV antibody concentration is the result of the additional 72-h delay (84 h prior compared with 12 h prior) after BVDV exposure prior to the MH challenge and sample collection in the LateCh group. This additional time would allow for increased replication of the virus in the host prior to the MH challenge on d 0. It is also possible that this difference in BVDV antibody concentrations is the result of a sort of immunological interference from MH due to the timing of the MH challenge in relation to BVDV exposure.

Multiple products and components of MH serotype 1 have been proposed as virulence factors. However, LKT and lipopolysaccaride are considered the most important factors in the virulence of these bacteria (Rice et al., 2007). In the present experiment, serum concentrations of MH WC antibodies and MH LKT antibodies were increased across time for both EarlyCh and LateCh groups. However, there was no effect of timing of MH challenge in relation to BVDV exposure between EarlyCh and LateCh treatments. Using a similar challenge model, Burciaga-Robles et al. (2010) reported that MH LKT antibodies increased over time and, on d 7, 14, and 28, were greatest for steers challenged with MH, lowest for steers not challenged with MH, and intermediate for steers exposed to BVDV 1b followed by an intratracheal challenge with MH (Burciaga-Robles et al., 2010).

Morbidity resulting from BRD decreased feedlot performance, potentially due to the febrile response that is known to accelerate protein and energy metabolism (Jim et al., 1993) and, also, the decrease in DMI could decrease growth rate in cattle suffering from BRD (Thompson et al., 2006). Sowell et al. (1998) referenced a 30% reduction in time spent at the bunk in morbid calves compared with healthy calves with much of the difference occurring in the first 4 d. Schneider et al. (2009) reported that BRD decreased ADG during both the acclimation period (4 to 6 wk; 0.37 ± 0.03 kg) and for the overall test period (0.07 ± 0.01 kg). The first 28 d of a receiving period, ADG was 0.23 kg lower for calves that became sick compared with healthy calves (Smith, 1998). In addition, calves treated for BRD had 0.14 kg/d lower ADG than cattle that were not treated (Bateman et al., 1990).

In the present experiment, ADG was decreased for EarlyCh whereas LateCh appeared to compensate from d 5 through 17. Reasons for this difference are unclear but may suggest that a decrease in ADG is more severe when a secondary bacterial infection occurs within a short time (12 h rather than 84 h) of BVDV exposure. Dry matter intake, expressed both as kilograms per day and as a percentage of BW, was decreased from d 0 to 4 for both EarlyCh and LateCh groups following MH challenge. It appears that the depression in DMI from d 0 to 4 is most likely affecting the performance results observed in the present experiment and that these differences are not differences associated with observed CS or immune response. The decreased DMI early in the experiment likely caused a reduction in gut fill that resulted in decreased performance by the EarlyCh and LateCh groups. Interestingly, the greatest depression in DMI (kg/d) occurred in the EarlyCh group, whereas TEMP, CS, and Hp concentrations were greater for the LateCh group compared with the EarlyCh group. In support of this possible explanation, gain efficiency was decreased from d 0 to 17 for both EarlyCh and LateCh compared with CON but was lowest for the EarlyCh. This was further evidence of the performance differences in the current experiment not being linked to CS or immune response. It should be noted that the performance data in the present experiment was measured over short time intervals on individual animals.

Increased TEMP (or fever) and depression are part of the clinical signs of acute BRD. In a recent study, no effect of exposure to steers PI with BVDV on TEMP was identified during the first 24 h following an intratracheal MH challenge (Burciaga-Robles et al., 2010). However, from 36 to 72 h, animals exposed to steers PI with BVDV had a higher TEMP compared with animals that received only MH and controls (Burciaga-Robles et al., 2010). In the present experiment, TEMP were elevated from 4 to 72 h after MH challenge. In a similar study, it was observed that all calves infected with BVDV had a mildly affected general appearance and an elevated TEMP (>39.5°C) 7 d after infection (Gånheim et al., 2005). In their experiment, only 2 of the calves in the MH group were mildly depressed and 3 had a rise in TEMP (>39.5°C) the day after inoculation. However, the most severe CS were observed in calves challenged with both BVDV and MH. All calves had fever and depression, which started 1 to 3 d after MH or 6 to 8 d after BVDV inoculation and lasted for 3 to 11 d (Gånheim et al., 2005). These results support a viral–bacterial synergism on alteration of TEMP and CS.

We did not detect significant alterations in TEMP or CS in steers exposed to calves PI with BVDV until after MH challenge. Rectal temperatures exceeded 40°C at 4 h after MH challenge and peaked at 6 and 8 h; peak TEMP were greater than 40.7°C and greater than 41.3°C for EarlyCh and LateCh, respectively. The CS was also increased soon (4 h) after MH challenge, which closely corresponded to TEMP. The greater and more extended increase in TEMP and CS for LateCh compared with EarlyCh suggests that timing of immunosuppression due to BVDV in relation to bacterial challenge may alter the severity of response to the disease. This may contribute to the challenge of observing cattle early in the disease process.

Haptoglobin is one of several proteins produced by the liver of cattle during the acute-phase response and is not detectable in the serum of healthy animals (Wittum et al., 1996). We observed significant changes in serum Hp concentrations for both LateCh and EarlyCh after challenge with MH, with a more pronounced effect for the LateCh from 48 to 96 h of the experiment. Similarly, Gånheim et al. (2003) reported that calves exhibited differences in CS and Hp concentrations after challenge with both BVDV and MH compared with calves inoculated with either BVDV or MH, emphasizing the importance of coinfections. Heegaard et al. (2000) showed serum Hp concentrations peaking at 7 to 8 d after inoculation using 1- to 2-wk-old calves intranasally infected with bovine respiratory syncytial virus. The current experiment demonstrates that timing of MH challenge relative to BVDV exposure has an important impact on the production of Hp. The differences in timing of peak Hp concentrations between the current experiment and the observations of Heegaard et al. (2000) may be due to the virus (bovine respiratory syncytial virus) involved and challenge doses.

Researchers have reported that BVDV inoculation induced leukopenia, mainly due to a decrease in lymphocytes but also in neutrophils and monocytes, although the decrease in monocyte numbers was not significant (Archambault et al., 2000). In the present experiment, total WBC were decreased at −12 and −2 h relative to MH challenge for the LateCh and tended to decrease for the EarlyCh at 96 h after MH challenge. Similar to the above study, the decrease in WBC was mainly due to the reduction of lymphocytes as a result of exposure to BVDV (Archambault et al., 2000). It appears from the present experiment that the decrease in total WBC and lymphocytes was related to the timing of BVDV exposure, although timing of BVDV exposure did not influence the subsequent WBC response to challenge with MH. Our findings are similar to Burciaga-Robles et al. (2010) in that neutrophils increased in response to MH challenge and lymphocytes decreased in response to exposure to BVDV, but there was not a BVDV exposure × MH interaction. In contrast, others reported that lymphopenia was more severe in calves inoculated with BVDV and MH, which was associated with more severe CS (Gånheim et al., 2005). Differences in challenge procedures and age and weight of the calves may explain differences among various experiments.

Although treatment × time interactions were observed for basophils and red blood cells in the current experiment, no statistical differences existed among experimental treatments for any blood variables other than platelets. These findings are in contrast to Burciaga-Robles et al. (2010) in that eosinophils, basophils, hematocrit, hemoglobin, MCV, MCH, and red blood cells were all influenced by exposure to BVDV, MH, or the combination whereas platelets were not different among treatments. Burciaga-Robles et al. (2010) reported that PI BVDV exposure resulted in decreased eosinophil concentrations whereas the MH challenge resulted in greater eosinophil concentrations. Similarly, basophil concentrations were decreased when steers were exposed to BVDV but increased in steers challenged with MH. Hematocrits tended to be decreased in steers challenged with MH, whereas hemoglobin was decreased. A BVDV × MH interaction was observed due to decreased hemoglobin concentrations in steers not exposed to BVDV but challenged with MH compared with steers that were exposed to BVDV and challenged with MH. Similar to hemoglobin concentrations, a BVDV × MH interaction was observed for MCH due to decreased MCH when steers not exposed to animals PI with BVDV but challenged with MH compared with steers exposed to BVDV and challenged with MH. Both hemoglobin concentration and MCH were decreased in steers challenged with MH. Red blood cell concentrations were also decreased in steers challenged with MH (Burciaga-Robles et al., 2010). Although more blood variables were impacted by BVDV exposure and MH challenge in the experiment by Burciaga-Robles et al. (2010), the clinical relevance of many of these responses is unknown.

In conclusion, total WBC and neutrophil concentrations were increased and lymphocyte concentrations were decreased by BVDV exposure and subsequent MH challenge. Delaying the MH challenge for 84 h after a 72 h exposure to calves PI with BVDV increased BVDV antibody concentrations, TEMP, CS, and serum Hp concentrations compared with delaying the MH challenge for only 12 h after BVDV exposure. However, the increased clinical and acute phase protein response did not affect short-term performance, which appeared to be mostly driven by the greater decrease in DMI for EarlyCh from d 0 to 4. It should also be noted that differences observed early in the feeding period may not reflect the long-term consequences of BVDV. Delaying MH inoculation for an additional 72 h after BVDV exposure results in a more severe immune response during a combination BVDV and MH challenge. More research examining the timing of pathogen exposure is needed, but these results indicate that the timing of initial pathogen exposure relative to a secondary stressor or pathogen exposure may be more significant than previously thought. As a result, delayed exposure to a secondary pathogen after arrival at a feedlot resulting from resorting and additional commingling could be more detrimental than exposure to the same secondary pathogen at an auction market or order buying facility.

 

References

Footnotes


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