Evaluation and application of an indirect ELISA for the detection of antibodies to bovine respiratory syncytial virus in milk, bulk milk, and serum

Associations between Bovine Coronavirus and Bovine Respiratory Syncytial Virus Infections and Productivity, Health Status and Occurrence of Antimicrobial Resistance in Swedish Dairy Herds

Bovine respiratory syncytial virus (BRSV) and bovine coronavirus (BCoV) affect dairy herds worldwide. In this study, effects on herd health, morbidity, and antimicrobial resistance (AMR) were assessed. Herds were considered free of infection (FREE), recently infected (RI) or past steadily infected (PSI) based on antibody testing of milk from primiparous cows. Data from farm records, national databases, and AMR of fecal Escherichia coli from calves were used as outcome variables. Compared to BRSV FREE herds: BRSV PSI herds had significantly higher odds of cough in young stock, a higher proportion of quinolone-resistant E. coli (QREC), but a lower proportion of cows with fever. BRSV RI herds had significantly higher odds of diarrhea in calves and young stock, a higher proportion of QREC and higher odds of multidrug-resistant E. coli. Compared to BCoV FREE herds: BCoV PSI herds had significantly higher odds of cough in all ages, and of diarrhea in young stock and cows, and a higher proportion of cows with fever. BCoV RI herds had significantly higher odds of diarrhea in young stock and cows and of cough in all ages. The results support previous research that freedom from BRSV and BCoV is beneficial for animal welfare and farm economy and possibly also mitigates AMR.

Seroprevalence of selected endemic infectious diseases in large-scale Estonian dairy herds and their associations with cow longevity and culling rates

Herd culling rates and longevity represent herd health and welfare status as well as farm economic performance. The contribution of endemic circulation of the main cattle pathogens to herd performance has not been previously analysed. The aim of this study was to estimate the herd prevalence of selected endemic bovine pathogens among large commercial dairy herds and to analyse their associations with herd culling rates and longevity. Bulk tank milk (BTM) samples and 10 heifer serum samples were collected from 120 Estonian dairy herds with at least 100 cows, between August 2019 and July 2020. All samples were tested for antibodies against bovine herpesvirus 1 (BHV-1), bovine viral diarrhoea virus (BVDV), bovine respiratory syncytial virus (BRSV), Mycoplasma bovis, Mycobacterium avium spp. paratuberculosis (MAP) and Salmonella Dublin using commercial ELISA. Data on herd size, milk yield, culling rate (CR) and mean age of the culled cows (MAofCC) were collected from the Estonian Livestock Performance Recording Ltd. The apparent herd and animal prevalences were calculated, and linear regression models were used to identify associations between the herd status of six tested pathogens and CR and MAofCC. The herd seroprevalences for antibodies based on BTM and heifer serum sample testing were BHV-1 56.7 % (95 % CI 47.3; 65.7), Mycoplasma bovis 48.3 % (95 % CI 39.1; 57.6), MAP 2.5 % (95 % CI 0.5; 7.1) and S. Dublin 24.2 % (95 % CI 16.8; 32.8) in all tested herds. Excluding vaccinated herds, herds prevalence for BVDV was 27.0 % (95 % CI 19.0; 36.3) and for BRSV 94.7 % (95 % CI 88.1; 98.3). Herd seropositive status for BRSV was associated with lower MAofCC, and herds with BSRV-seropositive youngstock had increased CR. Herds with positive BTM test results for S. Dublin culled cows at an older average age (Coef = 3.79 months, 95 % CI 0.52; 7.07, p = 0.023). MAP-positive herds had somewhat lower herd MAofCC (Coef = -6.18 months, 95 % CI -12.98; 0.63, p = 0.075). There was also a tendency of BVDV-negative herds to have a lower CR than BVDV-positive herds (Coef = -3.03 %, 95 % CI -6.54; 0.49, p = 0.090), and vaccination against BVDV tended to be protective against high CR (Coef = -6.26 %, 95 % CI -12.61; 0.09, p = 0.053 compared to infected herds). This study shows that Estonian large-scale dairy herds are endemically infected with several important cattle pathogens. Most of the studied pathogens influence longevity and culling rates, thus entailing health and economic consequences.

Foot-and-Mouth Disease Surveillance Using Pooled Milk on a Large-Scale Dairy Farm in an Endemic Setting

Pooled milk is used for the surveillance of several diseases of livestock. Previous studies demonstrated the detection of foot-and-mouth disease virus (FMDV) in the milk of infected animals at high dilutions, and consequently, the collection of pooled milk samples could be used to enhance FMD surveillance. This study evaluated pooled milk for FMDV surveillance on a large-scale dairy farm that experienced two FMD outbreaks caused by the A/ASIA/G-VII and O/ME-SA/Ind-2001d lineages, despite regular vaccination and strict biosecurity practices. FMDV RNA was detected in 42 (5.7%) of the 732 pooled milk samples, and typing information was concordant with diagnostic reports of clinical disease. The FMDV positive milk samples were temporally clustered around reports of new clinical cases, but with a wider distribution. For further investigation, a model was established to predict real-time RT-PCR (rRT-PCR) C_T values using individual cattle movement data, clinical disease records and virus excretion data from previous experimental studies. The model explained some of the instances where there were positive results by rRT-PCR, but no new clinical cases and suggested that subclinical infection occurred during the study period. Further studies are required to investigate the effect of vaccination on FMDV excretion in milk, and to evaluate more representative sampling methods. However, the results from this pilot study indicate that testing pooled milk by rRT-PCR may be valuable for FMD surveillance and has provided evidence of subclinical virus infection in vaccinated herds that could be important in the epidemiology of FMD in endemic countries where vaccination is used.

Evaluation of a multiplex immunoassay for bovine respiratory syncytial virus and bovine coronavirus antibodies in bulk tank milk against two indirect ELISAs using latent class analysis

Bovine respiratory syncytial virus (BRSV) and bovine coronavirus (BCV) are responsible for respiratory disease and diarrhea in cattle worldwide. The Norwegian control program against these infections is based on herd-level diagnosis using a new multiplex immunoassay. The objective of this study was to estimate sensitivity and specificity across different cut-off values for the MVD-Enferplex BCV/BRSV multiplex, by comparing them to a commercially available ELISA, the SVANOVIR^® BCV-Ab and SVANOVIR^® BRSV-Ab, respectively. We analyzed bulk tank milk samples from 360 herds in a low- and 360 herds in a high-prevalence area. As none of the tests were considered perfect, estimation of test characteristics was performed using Bayesian latent class models. At the manufacturers’ recommended cut-off values, the median sensitivity for the BRSV multiplex and the BRSV ELISA was 94.4 [89.8–98.7 95% Posterior Credibility Interval (PCI)] and 99.8 [98.7–100 95% PCI], respectively. The median specificity for the BRSV multiplex was 90.6 [85.5–94.4 95% PCI], but only 57.4 [50.5–64.4 95% PCI] for the BRSV ELISA. However, increasing the cut-off of the BRSV ELISA increased specificity without compromising sensitivity. For the BCV multiplex we found that by using only one of the three antigens included in the test, the specificity increased, without concurrent loss in sensitivity. At the recommended cut-off this resulted in a sensitivity of 99.9 [99.3–100 95% PCI] and specificity of 93.7 [88.8–97.8 95% PCI] for the multiplex and a sensitivity of 99.5 [98.1–100 95% PCI] and a specificity of 99.6 [97.6–100 95% PCI] for the BCV ELISA.

The bovine paranasal sinuses: Bacterial flora, epithelial expression of nitric oxide and potential role in the in-herd persistence of respiratory disease pathogens

The bovine paranasal sinuses are a group of complex cavernous air-filled spaces, lined by respiratory epithelium, the exact function of which is unclear. While lesions affecting these sinuses are occasionally reported in cattle, their microbial flora has not been defined. Furthermore, given that the various bacterial and viral pathogens causing bovine respiratory disease (BRD) persist within herds, we speculated that the paranasal sinuses may serve as a refuge for such infectious agents. The paranasal sinuses of clinically normal cattle (n = 99) and of cattle submitted for post-mortem examination (PME: n = 34) were examined by microbial culture, PCR and serology to include bacterial and viral pathogens typically associated with BRD: Mycoplasma bovis, Histophilus somni, Mannheimia haemolytica and Pasteurella multocida, bovine respiratory syncytial virus (BRSV) and bovine parainfluenza-3 virus (BPIV-3). Overall, the paranasal sinuses were either predominantly sterile or did not contain detectable microbes (83.5%: 94.9% of clinically normal and 50.0% of cattle submitted for PME). Bacteria, including BRD causing pathogens, were identified in relatively small numbers of cattle (<10%). While serology indicated widespread exposure of both clinically normal and cattle submitted for PME to BPIV-3 and BRSV (seroprevalences of 91.6% and 84.7%, respectively), PCR identified BPIV-3 in only one animal. To further explore these findings we investigated the potential role of the antimicrobial molecule nitric oxide (NO) within paranasal sinus epithelium using immunohistochemistry. Expression of the enzyme responsible for NO synthesis, inducible nitric oxide synthase (iNOS), was detected to varying degrees in 76.5% of a sub-sample of animals suggesting production of this compound plays a similar protective role in the bovine sinus as it does in humans.

Bovine respiratory syncytial virus and bovine coronavirus antibodies in bulk tank milk - risk factors and spatial analysis

Bovine respiratory syncytial virus (BRSV) and bovine coronavirus (BCoV) are considered widespread among cattle in Norway and worldwide. This cross-sectional study was conducted based on antibody-ELISA of bulk tank milk (BTM) from 1347 herds in two neighboring counties in western Norway. The study aims were to determine the seroprevalence at herd level, to evaluate risk factors for BRSV and BCoV seropositivity, and to assess how these factors were associated with the spatial distribution of positive herds. The overall prevalence of BRSV and BCoV positive herds in the region was 46.2% and 72.2%, respectively. Isopleth maps of the prevalence risk distribution showed large differences in prevalence risk across the study area, with the highest prevalence in the northern region. Common risk factors of importance for both viruses were herd size, geographic location, and proximity to neighbors. Seropositivity for one virus was associated with increased odds of seropositivity for the other virus. Purchase of livestock was an additional risk factor for BCoV seropositivity, included in the model as in-degree, which was defined as the number of incoming movements from individual herds, through animal purchase, over a period of five years. Local dependence and the contribution of risk factors to this effect were assessed using the residuals from two logistic regression models for each virus. One model contained only the x- and y- coordinates as predictors, the other had all significant predictors included. Spatial clusters of high values of residuals were detected using the normal model of the spatial scan statistic and visualized on maps. Adjusting for the risk factors in the final models had different impact on the spatial clusters for the two viruses: For BRSV the number of clusters was reduced from six to four, for BCoV the number of clusters remained the same, however the log-likelihood ratios changed notably. This indicates that geographical differences in proximity to neighbors, herd size and animal movements explain some of the spatial clusters of BRSV- and BCoV seropositivity, but far from all. The remaining local dependence in the residuals show that the antibody status of one herd is influenced by the antibody status of its neighbors, indicating the importance of indirect transmission and that increased biosecurity routines might be an important preventive strategy.

Comparison of Bovine coronavirus-specific and Bovine respiratory syncytial virus-specific antibodies in serum versus milk samples detected by enzyme-linked immunosorbent assay

Bovine coronavirus (BCV; Betacoronavirus 1) and Bovine respiratory syncytial virus (BRSV) are significant causes of enteric and respiratory disease in beef and dairy cattle throughout the world. Indirect enzyme-linked immunosorbent assays are widely used to detect serum antibodies for herd monitoring and prevalence studies. In dairy herds, milk is more readily collected than serum. Hence, in order to investigate the test agreement between serum and milk, both serum and milk samples from 105 cows in 27 dairy herds were analyzed in parallel for presence of immunoglobulin G antibodies to BCV and BRSV. The Bland-Altman analyses of data demonstrated good agreement between serum and milk antibody titers for both viruses. The results indicate milk samples are sufficient for surveillance of antibodies to BCV and BRSV.

A longitudinal study of the dynamics of bovine corona virus and respiratory syncytial virus infections in dairy herds

The objective of this study was to examine the dynamics of bovine corona virus (BCV) and bovine respiratory syncytial virus (BRSV) infections in dairy herds over a 3-year period. The status of 79 dairy herds located in two Northern and two Southern Regions of Sweden were surveyed by measuring antibody concentrations to BCV and BRSV in pooled milk samples from primiparous cows, and in bulk-tank milk twice annually. In the Southern Regions the percentage of antibody-positive herds remained persistently high (75-100%), whereas in herds based in the Northern Region, the percentage of positive herds for BCV and BRSV was 38-80% and 0-80%, respectively, with antibody levels to BRSV decreasing over time. Pooled milk samples of 'home-bred' primiparous animals were found to be most useful in terms of monitoring herd status but could gradually be replaced by bulk-tank sampling once freedom from infection was established.

Risk factors for seropositivity to bovine coronavirus and bovine respiratory syncytial virus in dairy herds

A cross-sectional study was conducted to evaluate the association between herd-level characteristics, management routines and farm-level antibody status of bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) in 257 Swedish dairy herds. The possible spatial clustering of positive herds compared with negative herds was also investigated. For each herd, a pooled milk sample from five primiparous cows was analysed for the presence of antibodies to BCV and BRSV. Herd-level information was obtained by a questionnaire. Logistic regression was used to evaluate the association between predictors and antibody status to BCV and BRSV. Large herd size, being located in southern Sweden, and not providing boots for visitors were found to be associated with being antibody-positive to BCV and BRSV. A short distance to the nearest cattle herd was an additional risk factor for BCV. One of the studied areas was suitable for spatial analysis. Positive herds were not spatially autocorrelated when compared with negative herds as estimated by the K-function regarding both BCV and BRSV. This indicates that local factors such as daily visiting milk trucks and wild animals were unlikely to be important sources of infection in this area. Moran's I statistics and semi-variogram showed no evidence of spatial autocorrelation in the residuals, indicating that remaining unidentified factors are not spatially dependent in the areas under study.

The relationship between antibody status to bovine corona virus and bovine respiratory syncytial virus and disease incidence, reproduction and herd characteristics in dairy herds

Background

Bovine respiratory syncytial virus (BRSV) and bovine corona virus (BCV) affects cattle worldwide. Our objective was to evaluate the effects of these infections on general health and reproduction parameters measurable on herd level and to explore the association between antibody status and some herd characteristics.

Methods

We collected a pooled milk sample from five primiparous cows from 79 Swedish dairy herds in September 2006. The samples were analysed for immunoglobulin G antibodies to BCV and BRSV with indirect enzyme-linked immunosorbent assays. Herd level data from 1 September 2005 to 30 August 2006 were accessed retrospectively. The location of the herds was mapped using a geographical information system.

Results

Ten herds were antibody negative to both viruses and were compared with 69 herds positive to BCV or BRSV or both. Positive herds had a higher (P = 0.001) bulk tank milk somatic cell count (BMSCC) compared with negative herds. The medians for all other analyzed health and reproductive parameters were consistently in favour of the herds negative to both viruses although the differences were not statistically significant. A higher proportion (P = 0.01) of herds used professional technicians for artificial insemination, rather than farm personnel, amongst the 33 herds negative to BCV compared with the 46 positive herds.

Conclusions

Our result shows that herds that were antibody positive to BCV and/or BRSV had a higher BMSCC compared with herds negative to BCV and BRSV. There was also tendency that negative herds had a better general herd health compared with positive. A higher proportion amongst the BCV negative herds used external technicians for AI instead of farm personnel, indicating that it is possible to avoid infection although having regular visits. Negative herds were located in close proximity to positive herds, indicating that local spread and airborne transmission between herds might not be of great importance and that herds can stay free from these infection transmission although virus is circulating in the area.

Associations between bovine coronavirus and bovine respiratory syncytial virus infections and animal performance in Swedish dairy herds

To assess the economic impact of bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) infections, accurate estimates of their associated effects on animal performance are needed. This study aimed to quantify the variation in individual test-day milk yield and somatic cell count, risk of reproductive failure after first service of dairy cows, and risk of death of calves and heifers according to the BCV and BRSV status of the herd. Three types of status were defined for BCV and BRSV infections, based on 1) the dynamics over a 7-mo period of BCV- and BRSV-specific antibody levels in pooled milk of primiparous cows; 2) the possible occurrence of presumably BCV- and BRSV-related clinical outbreaks; and 3) the combination of both pieces of information. A total of 36,184 test days, 2,716 cows with a first service, and 4,104 calves and heifers in 65 Swedish herds were included in the analyses. Animal performance associated with BCV and BRSV infections was quantified using hierarchical mixed generalized and survival models, after adjustment for covariates known to influence the performance under study. A significant reduction in milk yield was observed for cows in presumably BRSV recently infected herds, as well as in herds having a presumably BRSV-related clinical outbreak (of 0.57 and 0.91 kg/d, respectively), compared with cows in presumably infection-free herds. There was also a significant increase in somatic cell count (of 12,000 cells/mL) for cows located in herds with a BRSV outbreak. The risk of failure after first service, as well as the risk of death in calf and heifer, was numerically higher in BRSV-infected herds, although this was not statistically significant. In contrast, BCV infection herd status, as defined in the present study, was not significantly associated with any production losses in animals from infected herds compared with those in infection-free herds.

Spatial patterns of bovine corona virus and bovine respiratory syncytial virus in the Swedish beef cattle population

BACKGROUND

Both bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) infections are currently wide-spread in the Swedish dairy cattle population. Surveys of antibody levels in bulk tank milk have shown very high nationwide prevalences of both BCV and BRSV, with large variations between regions. In the Swedish beef cattle population however, no investigations have yet been performed regarding the prevalence and geographical distribution of BCV and BRSV. A cross-sectional serological survey for BCV and BRSV was carried out in Swedish beef cattle to explore any geographical patterns of these infections.

METHODS

Blood samples were collected from 2,763 animals located in 2,137 herds and analyzed for presence of antibodies to BCV and BRSV. Moran's I was calculated to assess spatial autocorrelation, and identification of geographical cluster was performed using spatial scan statistics.

RESULTS

Animals detected positive to BCV or BRSV were predominately located in the central-western and some southern parts of Sweden. Moran's I indicated global spatial autocorrelation. BCV and BRSV appeared to be spatially related: two areas in southern Sweden (Skaraborg and Skåne) had a significantly higher prevalence of BCV (72.5 and 65.5% respectively); almost the same two areas were identified as being high-prevalence clusters for BRSV (69.2 and 66.8% respectively). An area in south-east Sweden (Kronoberg-Blekinge) had lower prevalences for both infections than expected (23.8 and 20.7% for BCV and BRSV respectively). Another area in middle-west Sweden (Värmland-Dalarna) had also a lower prevalence for BRSV (7.9%). Areas with beef herd density > 10 per 100 km(2) were found to be at significantly higher risk of being part of high-prevalence clusters.

CONCLUSION

These results form a basis for further investigations of between-herds dynamics and risk factors for these infections in order to design effective control strategies.

Reduced likelihood of bovine coronavirus and bovine respiratory syncytial virus infection on organic compared to conventional dairy farms

The prevalence of antibodies to bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) infections was studied in 20 conventional and 20 organic dairy herds. The organic farms had produced ‘certified’ milk for at least 2 years. On two occasions, with a 1-year interval, 699 serum samples from 624 peri-parturient cows were tested by ELISA for antibodies to BCV and BRSV. Accompanying data relating to the sampled animals were collected in order to identify potential factors associated with increased antibody prevalence. The antibody prevalence was high at both sampling times with approximately 85% and 80% of animals positive for antibodies to BCV and to BRSV, respectively. Conventional herds had a significantly higher mean antibody prevalence to BCV and BRSV than the organically managed herds (P < 0.01). Animal age was significantly associated with increased antibody prevalence (P < 0.001). The findings of this study suggest that organic farm management may be effective in reducing the seroprevalence of these viruses relative to conventional farming methods.

Quantitative evaluation of genetic and environmental parameters determining antibody response induced by vaccination against bovine respiratory syncytial virus

The parameters controlling IgG antibody responses induced by vaccination against bovine respiratory syncytial virus (BRSV) were investigated in 463 Holstein-Charolais crossbred cattle. Pre- and post-vaccination sera were tested by enzyme linked immunosorbent assays (ELISA) for BRSV-specific IgG and IgG2. Year-of-birth, age, sex and pre-existing antibody were significant sources of variation for IgG responses. Pre-vaccination, progeny with a higher proportion of Holstein genes had higher total BRSV-IgG. By Day 35 post-vaccination, heritabilities peaked at 0.26 for total BRSV-IgG and 0.36 for BRSV-IgG1. There was no evidence for interbreed differences between Holstein and Charolais calves, post-vaccination. These results suggest that calf-sire has a major heritable influence on serum IgG levels following BRSV immunisation.

Use of bulk milk for detection of Neospora caninum infection in dairy herds in Thailand

The relationship between the level of Neospora caninum antibodies in bulk milk and the seroprevalence in lactating cows was investigated. Bulk milk was also used to estimate the prevalence of N. caninum infection in dairy herds in the northeast and north Thailand. Bulk milk and individual serum from all lactating cows in 11 herds as well as 220 bulk milk samples from nine milk collection centres were analysed for presence of N. caninum antibodies using an iscom ELISA. In the 11 herds the bulk milk absorbances ranged between 0.04 and 0.89 and the seroprevalences varied between 0 and 46%. Five herds had milk absorbances below 0.20, among those were the two herds housing only seronegative lactating cows. In the remaining three herds with such low bulk milk absorbances one or two cows (5-14%) were seropositive. Six of the investigated herds had bulk milk absorbances above 0.20. In the two herds with the highest bulk milk absorbances more than 30% of the cows were seropositive. Using an absorbance of 0.20 to discriminate between negative and positive herds, 102 (46%) of 220 bulk milk samples were judged positive. There was no significant difference in mean bulk milk absorbance between the milk collection centres within each region. However, the proportion of herds with bulk milk absorbances > or =0.50 in the north was statistically (P < 0.01) higher than that in the northeast. It was concluded that bulk milk antibody testing can be used to identify N. caninum-infected herds and that N. caninum is a common infection in dairy herds in Thailand.

Evaluation of two enzyme-linked immunosorbent assays for detecting Salmonella enterica subsp. enterica Serovar Dublin antibodies in bulk milk

Two enzyme-linked immunosorbent assays (ELISAs) for the detecting Salmonella enterica subsp. enterica serovar Dublin antibodies in bulk milk were developed and evaluated for potential use in control programs. The ELISAs were based on either lipopolysacharide (LPS ELISA) or flagellar antigen (GP ELISA). Sensitivity was determined with 79 case herds with a wide range of clinical signs. Specificity was determined with 125 Dutch and 200 Swedish control herds. The relation between antibodies in bulk milk, antibodies in serum, and the level of milk production of individual cows was studied with 61 case herds. The optimal optical density (OD) values of the LPS ELISA and the GP ELISA were determined to be 0.2 and 0.5, respectively. The sensitivities of the LPS ELISA and the GP ELISA were 54 and 63%, respectively, with a specificity of 98% for both ELISAs with samples from the Dutch control herds. The specificities for samples from the Swedish herds were 100% for the LPS ELISA and 95% for the GP ELISA. The sensitivity of the combination of tests was 65% when samples were run in parallel, and the specificity was 100% when samples were run in series, irrespective of whether the samples came from Dutch or Swedish control herds. The variance (R(2)) in the OD value for bulk milk samples could be explained by the percentage of seropositive lactating cows in a herd with the LPS ELISA for 51% of the samples and with the GP ELISA for 72%. The variance in the OD value was best explained by the combination of the percentage of seropositive lactating cows in the herd and the mean log(10) serum antibody titer for that herd (R(2) = 62% for the LPS ELISA and R(2) = 75% for the GP ELISA). Case herds more often tested negative by the ELISA with bulk milk when the percentage of seropositive lactating cows was less than 5%. It is concluded that both ELISAs with bulk milk can be used in control programs to distinguish between infected and noninfected herds. Specificity can be increased by using the two tests in combination. Sensitivity was relatively low for both single tests and both tests combined.

Predicting herd protection against foot-and-mouth disease by testing individual and bulk tank milk samples

Four groups of cattle were tested for antibodies against foot-and-mouth disease (FMD) virus type O(1) over three 70 day vaccination cycles using the liquid-phase-blocking-ELISA (LPBE). First lactation cows showed the lowest titres and group protection levels (GPLs) against FMD virus strains with 'r' values < or =0.5 while second lactation animals gave the highest results. When mean serum titres for each group and sampling date were plotted against GPL a strong correlation was found. Revaccination was indicated at a mean titre of approximately log10 2.88 (1:760; R=0.93; n=86) if the herd was threatened by field strains with an 'r' value of 0.25, or log10 2.62 (1:420; R=0.83; n=48) if this ratio was 0.5. Significant overall correlation (R=0.53; n=624) was obtained between serum titres and milk IgG(1) results derived from the modified specific isotype assay (SIA). Milk titres equivalent to 1:100, 1:200, 1:400 and 1:800 were 1:3.8, 1:6.3, 1:10.4 and 1:17.1, respectively, in first lactation cows. Bulk tank milk samples demonstrated a repeating pattern of results corresponding to the vaccination cycle with no titre lower than log10 1.05 (1:11). Colostrum from first lactation animals showed mean SIA results of log10 4.06 (1:11,480) and early milk titres only levelled off approximately 11 days post partum (dpp).

An experimental study of a concurrent primary infection with bovine respiratory syncytial virus (BRSV) and bovine viral diarrhoea virus (BVDV) in calves

Experimental infections with bovine respiratory syncytial virus (BRSV) and bovine viral diarrhoea virus (BVDV) were performed to study the effect of concurrent BRSV and BVDV infections. Twelve seronegative calves, in 3 groups, were inoculated on a single occasion with pure BRSV (group A), BRSV and noncytopathogenic BVDV (group B) or mock infected (group C). Mild respiratory symptoms were recorded 4 to 5 days post inoculation (dpi) in group A and group B calves. One calf in group A was severely affected and required medical treatment. In group B, fever (40.7-41.4 degrees C) was prominent 7 to 8 dpi. Only calves in group B were BVDV positive in purified lymphocytes at 2 to 14 dpi and showed increased serum interferon levels, with a peak at 4 dpi, indicating BVDV to be responsible for inducing the rise. BRSV was detected in lung lavage fluids up to 7 dpi for group A calves, compared to 11 dpi for group B and calves in this group also seroconverted later displaying lower BRSV titers. The time lag before an antibody response and the titers recorded in group B, indicated that the duration of BVDV infection in lymphocytes negatively influenced the capacity to mount a BRSV antibody response.

Prevalence of antibodies to bovine virus diarrhoea virus and other viruses in bulk tank milk in England and Wales

Bulk tank milk samples from 1070 dairy herds in England and Wales were tested by ELISA for antibodies to bovine virus diarrhoea virus (BVDV). A subset of 341 herds was tested by ELISA for antibodies to bovine herpesvirus 1 (BHV-1), bovine respiratory syncytial virus (BRSV) and bovine coronavirus (BCV). None of the herds had less than 40 dairy cows and none had been vaccinated against BVDV. The prevalence of BVDV antibody-positive herds in the national population was estimated at 95 per cent and approximately 65 per cent of the herds had a high level of bulk tank antibody suggestive of recent infection with BVDV. Dairy herds in East Anglia and the south-east of England had a significantly lower risk of being BVDV antibody-positive than herds in the rest of England and Wales. However, these regional differences tended to diminish with increasing herd size. Around 69 per cent of the herds were BHV-1 antibody-positive and all the herds were antibody positive to BRSV and BCV. Comparison with earlier serological surveys revealed that there had been little change in the prevalence and distribution of BVDV antibody-positive herds in England and Wales over the last 20 years, but that there had been an increase in the prevalence of BHV-1 antibody-positive herds.

Detection of bovine herpesvirus 1 glycoprotein E antibodies in individual milk samples by enzyme-linked immunosorbent assays

The purpose of this study was to determine whether individual milk samples can replace serum samples for the detection of bovine herpesvirus 1 (BHV1) glycoprotein E (gE)-specific antibodies. Serum and milk samples were collected at the same time from cattle in BHV1-free herds, cattle in unvaccinated herds, and cattle in herds that were vaccinated twice with a BHV1 marker vaccine. The samples were tested in two gE enzyme-linked immunosorbent assay (ELISA) systems. In comparison to serum, the results showed that the gE-blocking ELISA was highly sensitive for testing milk samples (0.96). In contrast, the gE ELISA was less sensitive (0.79). The specificities of the gE-blocking ELISA and the gE ELISA for testing milk samples were very high (1.00 and 0.99, respectively). The presented results indicate that individual milk samples, which can be collected relatively easily and inexpensively, can be used instead of individual serum samples in the gE-blocking ELISA for the screening of cattle for BHV1 gE antibodies.

Bovine respiratory syncytial virus

Since the first report of BRSV in the 1970s, the understanding of this agent and its respective disease has increased dramatically. Current evidence supports a major role for this virus in bovine respiratory disease. Advances in diagnostics have increased the ability to demonstrate this virus in field outbreaks of respiratory disease. The clinical signs and pathologic features have been well described, and vaccines are available to aid in prevention and control. Still, many questions remain to be answered with respect to BRSV. It appears there may be antigenic subgroups of BRSV, but the epidemiologic significance and relevance to immunization of this remains unknown. The question of differences in virulence among isolates of this virus has yet to be addressed. From an epidemiologic standpoint, the means by which BRSV perpetuates in the cattle population has yet to be elucidated. Although progress has been made in understanding the pathogenesis and immune response to BRSV, the mechanism of disease production and immune protection is incomplete. Lastly, efficacy testing of existing vaccines need to continue, as well as the development of new vaccines and new approaches to vaccination.

Standardization of enzyme-linked immunosorbent assays (ELISAs) for quantitative estimation of antibodies specific for infectious bovine rhinotracheitis virus, respiratory syncytial virus, parainfluenza-3 virus, and bovine viral diarrhea virus

Commercial enzyme-linked immunosorbent assays (ELISAs) for detection of serum antibodies to bovine viral diarrhea virus (BVDV), parainfluenza-3 virus (PI3V), respiratory syncytial virus (RSV), and infectious bovine rhinotracheitis virus (IBRV) were standardized to give a quantitative result when testing was performed at a single optimum dilution. For each test, serum samples were titrated and their end point titers calculated by an algebraic method directly from a plot of each titration series and also from a regression line fitted to this plot. The corrected optical density (COD) of each sample when tested at dilutions of 1/25, 1/50, and 1/100 was expressed as a percentage of the COD of a positive reference serum included on each plate, this value was the sample/positive (S/P) ratio. For each test, the linear relationship between the S/P ratio obtained at a dilution of 1/25, 1/50, and 1/100 and the end point titer calculated by each method was determined. In each case, the best linear relationship existed when samples were tested at a dilution of 1/100 (r = 0.973 for BVDV, 0.962 for PI3V, 0.961 for RSV, 0.947 for IBRV). From the equation of these lines, an increase in the S/P ratio between acute and convalescent serum samples of 31%, 23%, 21%, and 35% would correspond to a 4-fold rise in ELISA titer to BVDV, PI3V, RSV, and IBRV, respectively. ELISA titers calculated from S/P ratios at 1/100 were significantly related to virus neutralization titers to BVDV, RSV, and IBRV and to hemagglutination inhibition titers to PI3V (P < < 0.001 in all cases). Samples with low S/P ratios had the greatest intraassay and interassay variation. Intraassay reproducibility ranged from 3.5% to 22.3% (coefficient of variation), with a median value of 9.5%. Interassay reproducibility was lower, ranging from 6.0% to 50.6%, with a median of 17.4%.

Development of tests for antibodies against foot-and-mouth disease virus in cattle milk

The liquid-phase blocking ELISA (LPBE) and a specific isotype assay (SIA) for bovine IgG1 were modified to detect antibodies against FMDV isolate O1 Manisa in cattle milk. Samples from vaccinated animals were mostly indistinguishable from negative control cattle in the LPBE but 90% of milks from convalescent animals (which had also been vaccinated several times previously) gave positive results. The SIA was able to detect 95% of cattle vaccinated up to 12 months previously, and 100% of the recovered animals examined. Both tests could, therefore, be used for surveillance purposes following an outbreak of FMD to identify herds of cattle which had been infected, but the SIA would be required to show the presence of vaccinated animals. Samples from convalescent cattle showed the highest correlation between antibody levels in milk and in sera. Agreement was also high among recently vaccinated animals, but declined with increasing time since immunisation, though there was still a strong correlation of IgG1 levels after 6 months.

Severe respiratory disease in dairy cows caused by infection with bovine respiratory syncytial virus

Outbreaks of severe respiratory disease caused by bovine respiratory syncytial virus (BRSV) were recorded in dairy herds throughout Sweden in 1988 and subsequently. The virus was demonstrated in nasopharyngeal swab material from animals in the acute stage of the disease by culture, the polymerase chain reaction (PCR) and by immunofluorescence. Serological data from the herds investigated showed that the cows had seroconverted to BRSV rather than to bovine coronavirus, bovine viral diarrhoea virus or parainfluenza-3 virus. It was predominantly dairy herds in isolated areas that contracted a severe primary BRSV infection, often after the purchase of new animals. A nationwide survey for BRSV antibodies in bulk milk samples showed the highest prevalence, of 84 to 89 per cent, in the southernmost regions of Sweden and the lowest prevalence, of 41 to 51 per cent, in the north of the country. The prevalence of BRSV was highest in areas with the highest populations of cattle.