Calves are born immunologically naïve and agammaglobulinemic because the bovine placenta does not permit the transfer of maternal antibodies to the fetus prior to parturition. Therefore, it is essential that a newborn calf receive maternal antibodies via colostrum as soon as possible after birth to protect it against disease until its own immune system can mature.
Maternal antibodies help protect neonatal calves against clinical disease caused by BRSV,1–3 BVDV,1,4,5 BHV1,6 PI3,7 BCV4, and Mannheimia haemolytica.8 Although results of multiple studies5,6,9,10 indicate that high concentrations of circulating maternal antibodies may interfere with a calf's ability to seroconvert (develop a ≥ 4-fold increase in antibody titer against a specific pathogen) following exposure to an antigen, results of other studies11,12 suggest that the rate of antibody decay for calves with circulating maternal antibodies that are vaccinated may be slower than that for similar calves that are not vaccinated. Calves that are vaccinated while they still have circulating maternal antibodies can develop an anamnestic response to subsequent vaccination, even when a humoral response to the initial vaccination was not detected.13–15 Furthermore, vaccination of calves with circulating maternal antibodies can elicit a cell-mediated immune response despite an apparent lack of a humoral response.6,14 Bovine respiratory disease is associated with multiple pathogens including BRSV, BHV1, BVDV types 1 (BVDV1) and 2 (BVDV2), PI3, BCV, and M haemolytica16 and is a common problem on many dairy operations. However, studies conducted to assess the prevalence of these pathogens or their association with the incidence of BRD in calves on commercial dairy farms in North America are lacking. Knowledge of which pathogens are most likely associated with disease in a given population of animals is important for the development of appropriate control and prevention strategies, particularly vaccination programs, for those animals.
The objective of the study reported here was to investigate the effect of clinical BRD and vaccination against BRD on the rate of decay of antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica in dairy heifer calves ≤ 3 months old. Our hypotheses were that calves with circulating maternal antibodies would not develop detectable antibody responses following clinical BRD or vaccination against BRD and that the rate of antibody decay would vary significantly between calves with and without BRD and between calves that were and were not vaccinated against BRD.
Material and Methods
ANIMALS
All study protocols were approved by the Animal Care Committee at the University of Guelph and the Institutional Animal Care and Use Committee at the University of Minnesota. A convenience sample of commercial Holstein dairy herds located within a 2-hour radius of the University of Guelph or the University of Minnesota was selected to participate in the study with the informed consent of the herd owners. All calves were home-raised (ie, calves were not sent to a heifer raiser prior to 3 months of age) in accordance with routine farm management practices. For all participating herds, a case of BRD was defined as a calf with an increase in resting respiratory rate, sound, or effort and a fever (> 39.5°C) with at least 1 additional clinical sign such as a cough, nasal discharge, depression, hyporexia, or rough hair coat. Disease events and treatments for individual calves from birth to 3 months of age were recorded and maintained by farm personnel on standardized forms.
STUDY DESIGN
To determine the effect of clinical BRD on serum antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica, a retrospective case-control study was performed. Antibody titers against each pathogen were compared between calves that were and were not treated for BRD. A sample size of 38 calves/group was required to detect a difference of 0.8 in log2 reciprocal antibody titer between calves that were and were not treated for BRD with 95% confidence and 80% power. Thus, 76 unvaccinated calves from 5 herds were enrolled in the study. From each herd, 7 to 10 calves that had been treated for BRD before 3 months of age (cases) were matched with the same number of calves that had not been treated for BRD (controls). Each case calf was matched to a control calf on the basis of date of birth (matched calves were born within 7 days of each other). When possible, case calves that had been treated for BRD between 14 and 46 days of age (the 25th and 75th percentiles for age at first BRD treatment for all calves in the 5 participating herds) were selected for the study, and within a herd, all cases and controls were enrolled in the study in the shortest time period possible.
To determine the effect of vaccination against BRD on antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica, calves that were part of a larger randomized clinical trial17 were retrospectively selected for evaluation. In that trial,17 a block randomization procedure was used to allocate calves to 1 of 4 treatment groups; calves were administered a multivalent MLV vaccinea against BRSV, BVDV1, BVDV2, BHV1, and PI3 (2 mL, IM) at 2 weeks of age (between 15 and 21 days of age; 2-week group), 5 weeks of age (between 35 and 42 days of age; 5-week group), or both 2 and 5 weeks of age (2-dose group) or were administered sterile saline (0.9% NaCl) solution (2 mL, IM) at 2 and 5 weeks of age (unvaccinated control group). All calves were born and raised on 1 farm (herd size, 750 cows) with a high incidence of BRD in calves (44% of calves were treated for BRD prior to 3 months of age during the observation period).17 For the present study, a sample size of 6 calves/group was required to detect a difference of 2 in log2 reciprocal antibody titer among the treatment groups with 95% confidence and 80% power. Thus, 24 calves were evaluated (6 calves in each treatment group [2 week, 5 week, 2 dose, and unvaccinated control]).
SAMPLE COLLECTION
From all calves, jugular venipuncture was used to collect a blood sample (10 mL) into a sterile blood collection tube without an anticoagulantb at approximately 1 to 7 days of age (enrollment), 15 to 21 days of age (2 weeks), 35 to 42 days of age (5 weeks), and 90 to 120 days of age (12 weeks). Samples were kept chilled until centrifuged at 970 × g for 10 minutes at approximately 20°C. The serum was then harvested from each sample and stored frozen at −20°C until analysis.
SERUM NEUTRALIZATION
Serum samples were submitted to the Animal Health Laboratory at the University of Guelph for determination of antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica. Antibody titers against BRSVc, BVDV1d, BVDV2e, BHV1f, PI3,g and BCVh were determined by use of conventional microtiter virus neutralization assays. Briefly, serum samples were heated at 56°C for 30 minutes. Two-fold serial dilutions of each sample, starting at 1:2 for BRSV, BVDV1, BVDV2, BHV1, and PI3 or 1:4 for BCV, were prepared in duplicate in wells of a microtiter plate. Cell culture fluid that contained 100 cell-culture infectious-dose units of the appropriate virus was added to each well in a volume equal to the diluted serum sample. The microtiter plates were then incubated for 1 hour at 37°C (BRSV, BVDV1, BVDV2, BHV1, and PI3) or 4°C (BCV), followed by the addition of Georgia bovine kidney cells (BRSV), Madin-Darby bovine kidney cells (BVDV1, BVDV2, BHV1, and BCV), or bovine turbinate cells (PI3) that were suspended in growth medium and supplemented with equine fetal serum (BVDV1, BVDV2, and BHV1) or bovine fetal serum (BRSV, PI3, and BCV), both of which tested negative for BVDV. The plates were incubated at 37°C in a humidified incubator for 3 to 4 days, and then each cell monolayer was visually inspected for characteristic viral cytopathic effects with an inverted microscope. The antibody titer was determined as the 50% endpoint, or the serum dilution at which half of the wells had evidence of cytopathic effects. Known positive and negative serum controls were included for each assay. All samples from an individual calf were analyzed for a given pathogen at the same time to minimize test-to-test variation.
To determine antibody titers against M haemolytica, 96-well platesi were coated with a combination of monoclonal antibodiesj specific for the M haemolytica leukotoxin. Ascites fluid was diluted 1:6,000 in PBS solution (pH, 7.4), and 50 μL of the diluted fluid was added to each well. The plates were incubated at 37°C for 2 hours and washed 4 times (180 μL/well) with a wash buffer that consisted of PBS solution with 0.5% fish skin gelatink and 0.05% Tween 20.k The plates were blocked with 60 μL of 5% normal horse serum in PBS solution/well and incubated for 1 hour at 37°C, then washed 4 times as previously described. Lyophilized logarithmicphase culture supernatant of M haemolytica serotype A1 (American Type Culture Collection 43270) grown in RPMI 1640 medium was reconstituted to 4 times the original concentration in wash buffer, and 50 μL was added to each well. The plates were incubated at 37°C for 30 minutes, then washed as previously described. The serum samples were diluted 1:80 in a sample diluent that consisted of PBS solution with 1.5% Tween 20, 0.5% fish skin gelatin, and 0.3M NaCl; 50 μL of each diluted sample was added to each well, and a 2-fold dilution series of a positive-control serum sample (titer, 1:327,680) obtained from a colostrum-deprived calf that was vaccinated with culture supernatant of M haemolytica serotype A1 was used as a standard on all plates. The plates were incubated at 37°C for 1 hour and washed 4 times as described. A mouse monoclonal antibody against bovine IgG that was conjugated to alkaline phosphatasel was added to each well to detect bound antibodies against M haemolytica. The plates were incubated at 37°C for 30 minutes and washed 4 times as described, and then p-nitrophenol phosphatem dissolved in diethanolamine buffer was added to each well as a substrate for color development. Optical densities were measured with a plate readern at 405 and 630 nm. Samples with optical densities that did not fall into the linear range determined by the positive-control sample were assayed again at a dilution of 1:20 or 1:320 as required.
STATISTICAL ANALYSIS
The reciprocals of the virus neutralization antibody titers were log2 transformed for analyses, and the M haemolytica antibody titers were likewise calculated and expressed on a log2 scale.18 For each calf, the change in antibody titer for each pathogen between sampling intervals was investigated for evidence of seroconversion (≥ 4-fold increase in titer). Descriptive statistics were calculated, and the median titer at the median age of sampling was plotted for each pathogen. A summary statistic approacho was used to create a calf-level regression of the slope of antibody titers across the 4 sampling points, by use of the actual age of each calf at the time of sample acquisition.11,19 For each pathogen (BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica), the slope of the regression represented the rate of antibody decay.
The primary variable of interest for the linear regression model was the BRD case or control status for the retrospective case-control study and treatment group (2 week, 5 week, 2 dose, or unvaccinated control) for the trial to determine the effect of vaccination against BRD on antibody titers (vaccination trial). A random effect for herd was included to account for the clustering of calves within herds. The association between the quadratic of the slope for each pathogen and the primary variable of interest was evaluated to assess for curvature of the regression and was not found to be significant, which indicated the rate of decay of the log2 transformed reciprocal antibody titers was linear for all pathogens. Covariates included in the linear regression model for the retrospective case-control study included the initial antibody titer determined at enrollment (1 to 7 days old) and the interaction between initial antibody titer and the case or control status of the calf. Covariates included in the regression model for the vaccination trial included initial antibody titer, whether the calf was treated for BRD prior to 3 months of age (yes or no), and the respective interactions between those covariates and treatment group. Residuals were assessed to determine whether the model met the assumptions for ANOVA and to evaluate how well the model fit the data.20 The assumption of normality was also tested. During the regression analysis, covariates with values of P > 0.05 were removed by backward elimination in an iterative process, and outliers were included and removed from the model to evaluate their effects on the model outcomes.
For the retrospective case-control study, linear regression was used to assess the associations between the respective initial log2 reciprocal antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica and the calf's case or control status while controlling for actual age at sample acquisition and the inclusion of a random effect for herd. A multivariable logistic regression model was also created to assess the associations between case or control status and the respective initial antibody titers against each pathogen and whether the calf seroconverted to the pathogen in question (yes or no), while controlling for the actual age at sample acquisition and the inclusion of a random effect for herd. Covariates with values of P > 0.05 were removed by backward elimination.
For the vaccination trial, the slope of the summary statistic regression for log2 antibody titer per day for the calves in the unvaccinated control group that did not seroconvert was used to calculate the number of days required for a decrease of 1 in log2 reciprocal antibody titer (ie, 1 dilution) in titer and estimate the apparent mean antibody half-life for each pathogen. Thus, the apparent antibody half-life for each pathogen was the reciprocal of the rate of decay of the antibody titer for that pathogen. For example, the apparent antibody half-life, or time required to decrease the log2 receiprocal antibody titer by 1 dilution, would be 20 days for a pathogen with a rate of decay of 0.05 days−1 (ie, 1/0.05 days) All analyses were performed with commercially available statistical sofware,o and values of P ≤ 0.05 were considered significant.
Results
BRD RETROSPECTIVE CASE-CONTROL STUDY
Of the 76 calves in the retrospective case-control study, 38 (50%) seroconverted to ≥ 1 BRD pathogen between sampling intervals. The majority of those calves seroconverted to M haemolytica (25/38 [66%]) and BCV (21/38 [55%]), with only 2 (5%) calves seroconverting to PI3 and 1 (3%) calf each seroconverting to BRSV and BVDV1. All the calves that seroconverted to M haemolytica did so between 5 weeks and 3 months of age. Of the 21 calves that seroconverted to BCV, 5 (24%) seroconverted between enrollment and 2 weeks of age, 3 (14%) seroconverted between 2 and 5 weeks of age, and 13 (62%) seroconverted between 5 weeks and 3 months of age. The 2 calves that seroconverted to PI3 did so between enrollment and 2 weeks of age, and those that seroconverted to BRSV and BVDV1 did so between 5 weeks and 3 months of age. Thirteen of 38 (34%) calves seroconverted to more than 1 pathogen (10 calves seroconverted to M haemolytica and BCV, 1 calf seroconverted to PI3 and M haemolytica, 1 calf seroconverted to BVDV1 and BCV, and 1 calf seroconverted to BRSV and BCV). Seroconversion to any of the BRD pathogens was not significantly associated with a calf being classified as a case. The median log2 reciprocal antibody titers against BRSV, BVDV1, BVDV2, BHV1, PI3, BCV, and M haemolytica over time were summarized graphically (Figure 1).
Median log2 reciprocal antibody titers against BRSV (solid gray line with circles), BVDV types 1 (BVDV1; dashed gray line with squares) and 2 (BVDV2; solid gray line with squares), BHV1 (dashed gray line with triangles), PI3 (dotted gray line with diamonds), BCV (dashed black line with white circles), and Mannheimia haemolytica leukotoxin (solid black line with triangles) over time for 76 Holstein calves < 3 months of age from 5 commercial dairy farms in Minnesota and Ontario, Canada, that were (cases; n = 38) and were not (controls; 38) treated for BRD. The line for each pathogen represents the median for all 76 calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
Median log2 reciprocal antibody titers against BRSV (solid gray line with circles), BVDV types 1 (BVDV1; dashed gray line with squares) and 2 (BVDV2; solid gray line with squares), BHV1 (dashed gray line with triangles), PI3 (dotted gray line with diamonds), BCV (dashed black line with white circles), and Mannheimia haemolytica leukotoxin (solid black line with triangles) over time for 76 Holstein calves < 3 months of age from 5 commercial dairy farms in Minnesota and Ontario, Canada, that were (cases; n = 38) and were not (controls; 38) treated for BRD. The line for each pathogen represents the median for all 76 calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
Median log2 reciprocal antibody titers against BRSV (solid gray line with circles), BVDV types 1 (BVDV1; dashed gray line with squares) and 2 (BVDV2; solid gray line with squares), BHV1 (dashed gray line with triangles), PI3 (dotted gray line with diamonds), BCV (dashed black line with white circles), and Mannheimia haemolytica leukotoxin (solid black line with triangles) over time for 76 Holstein calves < 3 months of age from 5 commercial dairy farms in Minnesota and Ontario, Canada, that were (cases; n = 38) and were not (controls; 38) treated for BRD. The line for each pathogen represents the median for all 76 calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
The rate of decay of antibody titers against BRSV for a subset of the BRD cases differed significantly from that of the control calves and was dependent on the initial antibody titer against BRSV (Figure 2). The rate of decay of antibody titer for BRD cases with an initial log2 receiprocal anti-BRSV antibody titer ≥ 8 (ie, the median) did not differ significantly from that of the control calves. However, the rate of decay of antibody titer for BRD cases with an initial log2 receiprocal anti-BRSV antibody titer ≤ 5.5 (ie, the 10th percentile) was significantly slower than that for control calves.
Estimated rate of decline in antibody titer against BRSV over time for the calves of Figure 1 with calves separated by group (case or control) and initial log2 reciprocal antibody titer against BRSV (minimum [3.5; cases, dashed black line with circles; controls, solid black line with circles], 10th percentile [5.5; cases, dotted dark gray line with squares; controls, solid dark gray line with squares] median [8.0; cases, dashed gray line with triangles; controls, solid gray line with triangles], 90th percentile [9.5; cases, dashed gray line with squares; controls, solid gray line with squares]; and maximum [11.0; cases, dashed gray line with diamonds; controls, solids gray line with diamonds]). The rates of antibody decay for case calves with initial anti-BRSV antibody titers at the 10th percentile (5.5) or the minimum (3.5) were significantly slower than those for the corresponding control calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
Estimated rate of decline in antibody titer against BRSV over time for the calves of Figure 1 with calves separated by group (case or control) and initial log2 reciprocal antibody titer against BRSV (minimum [3.5; cases, dashed black line with circles; controls, solid black line with circles], 10th percentile [5.5; cases, dotted dark gray line with squares; controls, solid dark gray line with squares] median [8.0; cases, dashed gray line with triangles; controls, solid gray line with triangles], 90th percentile [9.5; cases, dashed gray line with squares; controls, solid gray line with squares]; and maximum [11.0; cases, dashed gray line with diamonds; controls, solids gray line with diamonds]). The rates of antibody decay for case calves with initial anti-BRSV antibody titers at the 10th percentile (5.5) or the minimum (3.5) were significantly slower than those for the corresponding control calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
Estimated rate of decline in antibody titer against BRSV over time for the calves of Figure 1 with calves separated by group (case or control) and initial log2 reciprocal antibody titer against BRSV (minimum [3.5; cases, dashed black line with circles; controls, solid black line with circles], 10th percentile [5.5; cases, dotted dark gray line with squares; controls, solid dark gray line with squares] median [8.0; cases, dashed gray line with triangles; controls, solid gray line with triangles], 90th percentile [9.5; cases, dashed gray line with squares; controls, solid gray line with squares]; and maximum [11.0; cases, dashed gray line with diamonds; controls, solids gray line with diamonds]). The rates of antibody decay for case calves with initial anti-BRSV antibody titers at the 10th percentile (5.5) or the minimum (3.5) were significantly slower than those for the corresponding control calves.
Citation: American Journal of Veterinary Research 76, 3; 10.2460/ajvr.76.3.239
Although the initial antibody titers against BVDV2, BHV1, PI3, and M haemolytica were significantly associated with the rates of antibody decay for those pathogens, the respective rates of antibody decay did not differ significantly between BRD cases and control calves. The same was true for antibody titers against BCV, but the assumption of normality was violated for that model, and the application of various transformations to the data failed to improve the model fit. Normality was achieved only when 2 initial log2 receiprocal anti-BCV antibody titers > 20 (ie, outliers that were > 2 SD from the mean, which might have been indicative of an active BCV infection in the calf or its dam) were removed from the analysis. However, with the outliers removed from the analysis, the initial antibody titer was not significantly associated with the rate of antibody decay. Because of this finding and their undue influence on the model, the outliers were removed from the final model for BCV. For BVDV1, the rate of antibody decay did not differ significantly between case and control calves, and the initial anti-BVDV1 antibody titer was not associated with the rate of antibody decay.
The initial antibody titers against BRSV (P = 0.03) and BHV1 (P = 0.02) for case calves were significantly lower than those for the control calves (Table 1).
Least squares mean initial log2 reciprocal antibody titers against BRSV, BVDV types 1 (BVDV1) and 2 (BVDV2), BHV1, PI3, BCV, and Mannheimia haemolytica leukotoxin for Holstein dairy calves < 3 months old from 5 commercial farms in Minnesota and Ontario, Canada, that were (cases; n = 38) and were not (controls; 38) treated for BRD.
| Pathogen | Cases | Controls | Difference (SE) between controls and cases | P value |
|---|---|---|---|---|
| BRSV | 7.5 | 8.2* | 0.68 (0.30) | 0.03 |
| BVDV1 | 10.1 | 10.2 | 0.16 (0.29) | 0.58 |
| BVDV2 | 9.4 | 9.7 | 0.35 (0.36) | 0.33 |
| BHV1 | 5.7 | 6.5* | 0.73 (0.31) | 0.02 |
| PI3 | 9.5 | 9.7 | 0.14 (0.29) | 0.62 |
| BCV | 10.1 | 10.7 | 0.61 (0.43) | 0.16 |
| M haemolytica | 16.0 | 16.4 | 0.32 (0.22) | 0.16 |
The least squares mean initial log2 reciprocal antibody titers were calculated by the use of the respective antibody titers against each pathogen measured in serum samples obtained between 1 and 7 days of age while controlling for the actual age of each calf at sample acquisition and the inclusion of a random effect in the model to account for the clustering of calves within herds.
Value differs significantly (P ≤ 0.05) from that for the cases.
When age at sample acquisition was controlled, the results of the multivariable logistic regression indicated that the odds of being a case were not significantly associated with the initial antibody titer against any pathogen except BHV1 (P = 0.04). The odds of being a case increased by 1.5 (95% confidence interval, 1.0 to 2.1) for each decrease of 1 in the initial log2 reciprocal anti-BHV1 antibody titer.
VACCINATION TRIAL
The rate of decay of antibody titers against BRSV (P = 0.35), BVDV1 (P = 0.32), BVDV2 (P = 0.81), BHV1 (P = 0.21) or PI3 (P = 0.82) was not associated with treatment group (2 week, 5 week, 2 dose, and unvaccinated control) or any other covariate included in the model. The apparent mean half-life was 25 days for antibodies against BRSV, 19 days for antibodies against BVDV1, 18 days for antibodies against BVDV2, 22 days for antibodies against BHV1, 21 days for antibodies against PI3, 35 days for antibodies against BCV, and 32 days for antibodies against M haemolytica.
Discussion
Results of the present retrospective case-control study of BRD in dairy calves < 3 months old on 5 farms in Minnesota and Ontario, Canada, indicated that the initial serum antibody titers against BRSV and BHV1 (serum samples obtained between 1 and 7 days of age) were significantly lower for calves that were treated for BRD than for calves that were not treated for BRD, which suggested that BRSV and BHV1 were associated with clinical BRD in the calves on those farms. Despite serologic evidence that calves on those farms were also exposed to BCV and M haemolytica, exposure to those 2 pathogens was not associated with treatment for clinical BRD. The antibody half-lives calculated for the dairy calves of the present vaccination trial were shorter than those reported by investigators of another study11 that involved beef calves.
In the present case-control study, when the initial antibody titer against BRSV was low, the rate of anti-BRSV antibody decay for calves that were treated for BRD (cases) was slower than that for calves that were not treated for BRD (controls), which suggested that exposure to BRSV was a risk factor for the development of clinical BRD, and calves with a low concentration of maternally derived antibodies against BRSV might have been generating endogenous anti-BRSV antibodies in response to a natural infection. However, the isotype of the anti-BRSV antibodies was not determined in the present study; thus, their origin (maternally derived or endogenous) is unknown. The findings of the present study supported those of other studies5,9,10 that suggest young calves usually do not seroconvert (develop a ≥ 4-fold increase in antibody titer against a specific pathogen) following viral antigen exposure via MLV or killed vaccines when maternal antibody concentration is high, but are capable of seroconversion in the absence of maternal antibodies. Unlike those other studies,5,9,10 the calves of the present study were not deprived of specific antibodies, and initial anti-BRSV antibody titers as high as 1:64 did not prevent the calves treated for BRD from mounting an apparent humoral response. A similar interaction between the effect of disease and initial antibody titer on the rate of decay of that antibody titer was reported by investigators of another study12 in which dairy calves were vaccinated with a killed BVDV vaccine at 15 days of age and an MLV BVDV vaccine at 45 days of age.
Detection of seroconversion or a differential decline in antibody titers is more difficult for calves with high concentrations of maternal antibody titers than for calves with low concentrations of maternal antibody titers. The antibody concentration must double to achieve an increase of 1 in log2 reciprocal antibody titer. Therefore, calves with high concentrations of maternal antibodies must produce more endogenous antibodies to achieve an increase of 1 in log2 reciprocal antibody titer than calves with low concentrations of maternal antibodies. Consequently, endogenous antibody production (eg, seroconversion) might be detectable in calves with low concentrations of maternal antibody titers but not in calves with high concentrations of maternal antibody titers, even though the actual amount of endogenous antibody produced was the same for both groups of calves.
The initial antibody titers against BRSV and BHV1 for the calves of the present case-control study were associated with treatment for BRD before 3 months of age. Calves with high initial antibody titers against BRSV and BHV1 were less likely to be treated for BRD before 3 months of age than were calves with low initial antibody titers against those 2 pathogens. Results of studies conducted in Europe have implicated BRSV,21,22 PI3,22 and BHV123 in the development of BRD in young dairy calves. In North America, BVDV24 and M haemolytica,24 Pasteurella multocida,24,25,26 and Mycoplasma spp24 have been associated with BRD in young dairy calves. In another study,27 a small group of dairy calves fed maternal colostrum or a colostrum replacement product had high concentrations of antibody titers against BRSV, BVDV1, BVDV2, BHV1, and PI3, and the mean time for those calves to become seronegative for those pathogens ranged from 3.75 to 6.5 months of age. However, that study27 did not assess the association between antibody titers and clinical BRD. To our knowledge, little is known about the pathogens that commonly cause BRD in young calves on commercial dairy farms in North America.
Results of multiple studies5,6,9,10 indicate that young calves vaccinated when maternal antibodies are present fail to seroconvert against the vaccine pathogens. Investigators of other studies report that vaccination of young calves increases the apparent half-life of maternal antibodies11 and the time required until antibody titers were no longer detectable.12 Those studies differed from the present study in that the study calves were much older (60 to 90 days) when vaccinated11 and more likely to develop a humoral response, and the sample size was larger,11,12 which improved the researchers’ ability to detect a statistical difference.
In the present vaccination trial, calves vaccinated with a multivalent MLV vaccine did not develop a detectable antibody response, and cell-mediated immunity was not evaluated. In a concurrent study,p the cell-mediated responses of calves from the same population that were vaccinated in the same manner as the calves of the vaccination trial were evaluated with a BHV1-specific CD25 expression flow cytometry assay. In that study,p calves vaccinated at 2 weeks and at both 2 and 5 weeks had cell-mediated responses at 54 to 66 days of age that were significantly higher than those of unvaccinated calves and calves vaccinated at 5 weeks of age. However, the association of a cell-mediated response with clinical BRD was not assessed in that study,p and the antibody titers did not differ significantly among the 4 treatment groups.
In the present study, the rate of antibody decay in young dairy calves was not associated with exposure to most of the putative pathogens of BRD. Calves with low initial concentrations of maternal antibodies that were treated for BRD had a lower rate of anti-BRSV antibody decay than did similar calves that were not treated for BRD. Calves with high initial antibody titers against BRSV and BHV1 were less likely to be treated for BRD during the first 3 months after birth than were calves with low initial antibody titers against those 2 pathogens. These results suggested that BRSV and BHV1 were associated with the development of clinical BRD in this population of young dairy calves, whereas BVDV1, BVDV2, PI3, BCV, and M haemolytica were not associated with clinical BRD in the same population. Vaccination of dairy calves with an MLV vaccine against BRSV, BVDV1, BVDV2, BHV1, and PI3 at 2 weeks of age and at both 2 and 5 weeks of age had no apparent effect on the rate of decline of antibody titers against any of the vaccine pathogens. High concentrations of maternal antibodies in those calves likely impeded the humoral response elicited by vaccination. In young dairy calves, a humoral response subsequent to vaccination is more likely to be detected in calves with low concentrations of maternal antibodies than in calves with high concentrations of maternal antibodies.
Acknowledgments
This manuscript represents a portion of a thesis submitted by Dr. Windeyer to the Department of Population Medicine, Ontario Veterinary College, University of Guelph, as partial fulfillment of the requirements for a Doctor of Veterinary Sciences degree.
Supported in part by Pfizer Animal Health Canada and the University of Guelph.
The authors thank William Sears for statistical assistance.
ABBREVIATIONS
| BCV | Bovine coronavirus |
| BHV1 | Bovine herpesvirus type 1 |
| BRD | Bovine respiratory disease |
| BRSV | Bovine respiratory syncytial virus |
| BVDV | Bovine viral diarrhea virus |
| MLV | Modified-live virus |
| PI3 | Parainfluenza virus type 3 |
Footnotes
BoviShield Gold 5, Pfizer Animal Health Canada, Kirkland, QC, Canada.
BD Vacutainer, Becton, Dickinson and Co, Franklin Lakes, NJ.
NADL strain in Mardin-Darby bovine kidney cells, Institute of Armand Frappier, Montreal, QC, Canada.
NVSL 125c strain in Mardin-Darby bovine kidney cells, USDA National Veterinary Services Laboratory, Ames, Iowa.
Cooper-1 strain ATCC VR 864 in Mardin-Darby bovine kidney cells, American Type Culture Collection, Manassas, Va.
VIDO strain in Georgia bovine kidney cells, Vaccine and Infectious Disease Organization, Saskatoon, SK, Canada.
SF4 strain in bovine turbinate cells, USDA National Veterinary Services Laboratory, Ames, Iowa.
VIDO strain in Mardin-Darby bovine kidney cells, Vaccine and Infectious Disease Organization, Saskatoon, SK, Canada.
Nunc Maxisorp flat bottom 96-well plates, ThermoFisher Scientific, Rochester, NY.
Clones 601, 603, and 605, provided by Dr. S. Srikumaran, Washington State University, Pullman, Wash.
Sigma, St Louis, Mo.
Clone BG18, 1/2,000 dilution in wash buffer, 50 μL/well, Sigma, St Louis, Mo.
Kirkegaard Perry Laboratories, Gaithersburg, Md.
BioTek, Winooski, Vt.
SAS, version 9.2, SAS Institute Inc, Cary, NC.
Schrupp M, Bandrick M, Molitor T, et al. Effect of vaccinating pre-weaned dairy calves on cell mediated immune function (abstr), in Proceedings. Minn Dairy Health Conf 2009;85.
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