Evaluation of a quadrivalent inactivated vaccine for the protection of cattle against diseases due to common viral infections

INTRODUCTION In those countries where the raising of cattle has a high economic priority, requiring intensive methods of animal husbandry, respiratory disease in young and adult cattle and infectious reproductive disorders in breeding animals are a major cause of economic loss. There are several viruses that commonly infect the bovine respiratory and/or reproductive tract. These comprise bovine herpesvirus 1 (BHV-1) commonly known as infectious bovine rhinotracheitis (IBR) virus, parainfluenza type 3 (PI3) virus, bovine respiratory syncytial virus (BRSV), adenovirus serotypes 1–6, reovirus serotypes 1–3, bovine virus diarrhoea virus (BVDV) and rhinoviruses. Of these, however, IBR virus, PI3 virus, BVDV and BRSV are generally considered as important in the aetiology of bovine respiratory disease. IBR virus and BVDV are also significant causes of transplacental infections and reproductive losses. Experimentally, however, only IBR virus consistently produces febrile respiratory disease although natural infections by PI3 virus and BRSV are well-documented causes of severe respiratory disease in cattle. BVDV is a common cause of abortions and infertility, and some strains of BVDV could also cause severe disease characterised by pyrexia, a marked thrombocytopaenia, profuse diarrhoea, dehydration and even deaths. Hence the naturally occurring respiratory disease and bovine pneumonia are considered to be a multi-aetiology syndrome and the best understood interaction in the disease process is that between the primary IBR virus infection and secondary Pasteurella spp. infection in shipping fever. PI3 virus and BVDV have also been implicated in pneumonic pasteurellosis. It is conceivable that primary viral infections cause significant damage to ciliary epithelia, and thus to the mucus escalator, leaving a way open for secondary bacterial and/or mycoplasma infection of the lower respiratory tract. Hence prevention of the primary viral infections through vaccination will significantly reduce the incidence of respiratory disease and the associated economic loss. This paper documents the quality of short and long-term protection afforded by a quadrivalent inactivated bovine vaccine containing IBR virus, PI3 virus, BRSV and BVDV. The experimental approach used is considered novel in that the efficacy against each of the 4 constituent viruses was assessed by sequential challenges. Also the vaccine is the 1st example of a polyvalent cattle vaccine containing BRSV antigen from a persistently infected, viable bovine cell line.


INTRODUCTION
In those countries where the raising of cattle has a high economic priority, requiring intensive methods of animal husbandry, respiratory disease in young and adult cattle and infectious reproductive disorders in breeding animals are a major cause of economic loss.There are several viruses that commonly infect the bovine respiratory and/or reproductive tract.These comprise bovine herpesvirus 1 (BHV-1) commonly known as infectious bovine rhinotracheitis (IBR) virus, parainfluenza type 3 (PI3) virus, bovine respiratory syncytial virus (BRSV), adenovirus serotypes 1-6, reovirus serotypes 1-3, bovine virus diarrhoea virus (BVDV) and rhinoviruses 15 .Of these, however, IBR virus, PI3 virus, BVDV and BRSV are generally considered as important in the aetiology of bovine respiratory disease.IBR virus and BVDV are also significant causes of transplacental infections and reproductive losses.Experimentally, however, only IBR virus consistently produces febrile respiratory disease although natural infections by PI3 virus and BRSV are well-documented causes of severe respiratory disease in cattle 4,15,18 .BVDV is a common cause of abortions and infertility, and some strains of BVDV could also cause severe disease characterised by pyrexia, a marked thrombocytopaenia, profuse diarrhoea, dehydration and even deaths 3,6 .Hence the naturally occurring respiratory disease and bovine pneumonia are considered to be a multi-aetiology syndrome and the best understood interaction in the disease process is that between the primary IBR virus infection and secondary Pasteurella spp.infection in shipping fever 19 .PI3 virus and BVDV have also been implicated in pneumonic pasteurellosis 7,17 .It is conceivable that primary viral infections cause significant damage to ciliary epithelia, and thus to the mucus escalator, leaving a way open for secondary bacterial and/or mycoplasma infection of the lower respiratory tract 15 .Hence prevention of the primary viral infections through vaccination will significantly reduce the incidence of respiratory disease and the associated economic loss.This paper documents the quality of short and long-term protection afforded by a quadrivalent inactivated bovine vaccine containing IBR virus, PI3 virus, BRSV and BVDV.The experimental approach used is considered novel in that the efficacy against each of the 4 constituent viruses was assessed by sequential challenges.Also the vaccine is the 1st example of a polyvalent cattle vaccine containing BRSV antigen from a persistently infected, viable bovine cell line 12 .
Infectivity titrations.For BHV-1, PI3 and BVDV NS infectivity titrations, using stored frozen specimens, serial 10-fold dilutions of virus were inoculated (200 µ / well and 4 wells/dilution) onto 1-3-dayold BEL cell monolayers in 96-well microtitration plates.The plates were incubated at 37 °C in an atmosphere of 5 % CO2 in air.Cytopathic effects (CPE) were read 5-7 days later.However, for noncytopathic ruminant pestiviruses (NCRPVs) BEL cell monolayers were superinfected with cytopathic BVDV (CP-BVDV) strain C86 after 5 days incubation, at a multiplicity of infection of 0.1-0.01 and plates reincubated at 37 °C for 2-3 days.Monolayers completely or partially (at infectivity end point) free of CPE were scored as NCRPV infected as described before 13 .CPE-refractory wells were used to calculate the tissue culture median infective dose (TCID50) by a standard method 16 .
Virus isolation.BRSV isolations from NS and LW were performed piecemeal using fresh samples as they became available and BVDV isolations from BC and serum from stored frozen specimens.Isolation was on 1-3-day-old BEL cell monolayers in 24-well plates, inoculating 200 µ of an undiluted sample per well and 1 well per sample.After inocula adsorption (1 hour, 37 °C) each well was re-fed with 800 µ of HSMM and plates incubated at 37 °C in an atmosphere of 5 % CO2 in air and CPE read 5 days later.All samples were re-passaged similarly on fresh monolayers of BEL cells using 200 µ of each passage 1 sample.The 2nd-passage samples were read for CPE after 5 days incubation at 37 °C and results scored as definite findings.
Virus-neutralising antibody titrations.For VN antibody titrations, inactivated (56 °C, 30 minutes) sera were serially diluted in HSMM in 96-well microtitration plates followed by addition to each well of an equal volume (50 µ ) of virus suspension in HSMM containing 100-200 TCID50 of virus.Mixtures were incubated at 37 °C for 1 hour when 100 µ of freshly dispersed suspension of 2-4 × 10 4 BEL cells in HSGM (see above) was added to each well and plates incubated at 37 °C as before.Each test also included titration of (i) a positive standard antiserum to constituent vaccinal viruses from a hyperimmunised calf and (ii) the dose of virus contained in the reaction.CPE-based VN tests were read 5 days afterwards and titres expressed as the reciprocal of the highest serum dilution in log2 scale completely neutralising a virus.End points for neutralisation of NCRPVs were developed and scored after superinfection of monolayers with CP-BVDV on day 5 as described previously 13 .Values with half digits represent a dilution giving complete neutralisation in 1 of 2 monolayers but in both monolayers at lower dilutions.Values shown as <2.0 denote no neutralisation by 4-fold diluted sera.

Animal procedures
Nasopharyngeal swabs (NS) were taken using a 12 cm cotton-tipped plastic swab and extracting each swab in 2 m HSMM.
Lung washings (LW) were collected from unanesthetised calves by irrigating lungs with 40 m of HSMM using a coaxial soft plastic catheter and recovering between 20-25 m for each lavage sample.
Buffy coat cells from blood samples taken in equal volume of isotonic 0.7 % (w/v) tri-sodium citrate solution were washed in phosphate-buffered saline (PBS, pH 7.4) and layered onto 4 m of 63 % (v/v) percol™ (density 1.13 g/m , Amersham Pharmacia Biotech AB, Uppsala, Sweden) cushion in PBS and centrifuged (2000 g, 10 minutes at room temperature) and collecting leukocyte band at sample-cushion interface was collected.This leukocyte fraction was suspended in HSMM diluent at a concentration of 10 6 cells/m and frozen at -70 °C until tested. 13,14allenge inocula All 4 challenge inocula described below were sterile, mycoplasma free, 2nd to 4th passage stocks grown in BEL cells propagated in HSGM and maintained in HSMM.All viruses except BRSV were grown at 37 °C; BRSV stocks were grown at 35 °C.

BHV-1
Strain 532 was isolated in-house from a nasopharyngeal swab from a field calf (No. 532) with a severe febrile rhinotracheitis.This isolate at a titre of 7.7 log10 TCID 50 per millilitre was administered intranasally (IN, 2 m ) and intratracheally (IT, 5 m ) to a 3-month-old Friesian-Hereford calf (No. 31), 5 days afterwards, the calf had a high fever (41.5 °C) and severe rhinotracheitis accompanied by anorexia, severe depression, tachypnoea and dyspnoea when the calf was lavaged (lung washings) and virus re-isolated in BEL cells.The challenge inoculum consisted of an equal volume pool of the nasal swab isolate from calf 532 and the lung isolate from calf 31.For challenge each calf was administered 2 m IN containing 6.0 ± 0.3 log10 TCID 50 .

PI3
The challenge inoculum consisted of a pool of in-house lung isolates from 5-month-old calves (Nos 444 and 465) that had to be euthanased due to severe tachypnoea, dyspnoea, pyrexia (39.5-40.5 °C) and anorexia over several days.The 2nd passage BEL cell stocks of strains 444 and 465 had titres of 8.3 and 8.7 log10 TCID50 per millilitre, respectively.For challenge an equal volume pool of the stocks was diluted to contain 6.0 log10 TCID 50 per millilitre and each calf was given 2 m IN and 5 m IT.

BVDV
Calves were challenged with a mixture of 11 isolates of NCRPVs, 3 of which were authentic border disease virus (BDV) strains.The isolation history or source and titre of the constituent strains in the pool are shown in Table 1.The inoculum contained 7.2 log10 TCID50 per millilitre.

BRSV
The study involved an equal volume pool of 4 field isolates of BRSV (strain AC2C13, Compton, ZW and PL-4).Prior to incorporation into the challenge pool, each isolate was individually passaged once in 2-3-week-old BRSV seronegative and NCRPV free Friesian bull calves.For this, each calf was inoculated IN and IT with 4 and 5 m , respectively, of BEL cell grown virus suspension and lung washings collected 5 and 7 days afterwards and then passaged twice (strain AC2C13 and Compton) or thrice (ZW and PL-4) in BEL cell monolayers at 35 °C in the presence of mycoplasma removal agent (ICN Biomedical, Thame, UK).The latter was a precautionary measure and the incubation period for the serial passages was 5-7 days.The pooled challenge inoculum contained BRSV at a titre of 5.4 log10 TCID50 per millilitre and for challenge each calf was inoculated IN with 4 m and IT with 5 m .

Efficacy determination
The study involved 3 matched groups, each with 5 or 6 seronegative (all 4 vaccine viruses), 2-week-old Frisean-Hereford cross bull calves.The basic intramuscular (IM) vaccination regime of 2 inoculations a month apart was staggered between the long-term (≥6 months) and the shortterm (≤4 months) by about 5 months so that the 2 vaccinated and the unvaccinated control groups could be challenged at the same time.Challenges were performed sequentially at intervals of 3-8 weeks in the order: IBR virus, PI3 virus, BVDV and BRSV as described above under challenge inocula.Following each challenge, calves were monitored for clinical reactions (day -2 to day +21), virus shedding in nasal mucus (day 0 to day 14) and bled for serology at intervals until challenge and afterwards.Additionally, for the BVDV challenge, the incidence of viraemia (both serum and leukocytes) was assessed from day 1 through to day 14 and after BRSV challenge and shedding in lungs was assessed by virus isolation from lung washings on alternate days for 2 weeks.

Clinical monitoring
Pyrexia (rectal temperature of ≥39.5 °C) was scored as 1 (39.5-40.0°C), 2 (40.1-40.5 °C), 3 (40.6-41.0°C)and 4 (>41.0°C).Respiration rate per minute of 40 and above was scored as 1 (40-50), 2 (51-60), 3 (61-70) and 4 (>70).Calves that were depressed but eating were given a score of 1, those that were depressed with reduced appetite were scored as 2 and anorexia was given a score of 4. Hyperpnoea was given a score of 2 and dyspnoea a score of 4. Nasal discharge was scored as 1 for marked serous, mild mucopurulent as 2, marked mucopurulent as 4 and ocular discharge as 1 for lachrymation, 2 for mild mucopurulent and 4 for marked mucopurulent.Mild conjunctivitis was given a score of 2 and marked conjunctivitis a score of 4. Oral ulcers were scored individually until healing and the sizebased scores given were 1 for 1-5 mm, 2 for 6-10 mm and 3 for over 10 mm in diameter.Vomiting of food plus clear fluid was given a score of 1, with bile as 2 and with blood as 3. Diarrhoea was scored as 1 if soft with mucus, 2 if watery, mucoid and profuse and 3 if haemorrhagic.Injection site lesions were scored as 1 for swelling of 5-20 mm, 3 for swelling of 21-50 mm, 5 for swelling larger than 50 mm and abscess as 10.
Calves were clinically examined for all the individual signs at the same time once daily.

Against IBR virus
The 1st of 4 sequential challenges was with 6.0 log10 TCID 50 of IBR virus administered intranasally, 24 (group 1, long-term immunity) or 4 (group 2, short-term immunity) weeks after the recommended vaccination regime.Group 2 calves were vaccinated 5 months after group 1 calves using the same batch of +4 to +8 °C stored vaccine.It resulted in marginally but not significantly higher circulating group mean VN antibody titre in the younger group 1 calves and activity did not significantly decline until 6 months later (Table 2).Concurrently monitored group 2 and 3 (unvaccinated control) calves, respectively, remained seronegative until after vaccination or challenge.
At the level of average amount of virus shed daily by each group shown as group mean titres in Table 2, differences between vaccinated groups 1 and 2 and unvaccinated calves in group 3 were between 80-40 000-fold.Values shown in italics in Table 2 represent an average amount of virus shed by a calf in a group calculated by averaging the amount shed by an individual and then using these values to obtain the group mean shown as mean titre per calf.These values were 4.3 ± 0.5, 5.1 ± 0.5 and 6.4 ± 0.2 log10 TCID 50 per millilitre, respectively, for groups 1, 2 and 3. Hence both sets of means showed a significant (P = <0.05)protection against virus shedding for a period up to 6 months.Furthermore, results in Table 2 show a significantly shortened shedding period due to the vaccine (P = 0.01-0.02,respectively, for groups 2 and 1).
Although the daily clinical monitoring was from day -2 through to day +21, cumulative daily group mean scores for only day -2 through to day +14 are shown since the main reaction occurred between days 2 and 8 after challenge (Table 2).Between days 2 and 8 the daily group mean scores in group 1 calves were 13.6, 14.2, 16.6, 4.5, 2.1, 2, 1.2 and 2.0-fold lower compared to those in group 3 control calves.During this period the group mean scores in group 2 calves were 17.0, 10.6, 14.7, 5.2, 3.0, 1.4, 1.9 and 1.7-fold lower compared to the control group 3 calves.The group mean score per calf, calculated for days 1 to 14 were 3.0 ± 0.6, 3.1 ± 0.8 and 9.9 ± 2.3, respectively, for groups 1, 2 and 3. Hence calves in both vaccinated groups were significantly (P = <0.05)protected.In keeping with the challenge virus shedding data, control calves seroconverted with VN antibody titres of between 5 and 7 whereas the VN titre in all vaccinated calves rose above 9.0 (Table 2).Individually, the serum VN titre rise in vaccinated calves ranged in excess of between 8 and 128-fold (data not shown).Results in Table 2 support the claim that a normal course of 2 intramuscular vaccinations would significantly protect seronegative cattle up to a period of 6 months.

Against PI3 virus
Findings for the PI3 virus component are shown in Table 3.After the course of 2 IM vaccinations, all calves became positive for circulating VN antibody but overall the response was about 4-fold higher in the older (group 2) calves while unvaccinated control calves remained seronegative until after challenge.The latter was administered intranasally as well as intratracheally 3 weeks after the

IBR virus challenge.
Although the challenge inoculum contained low passage lung isolates from 2 pneumonic calves, it did not result in marked clinical signs despite the fact that the inoculum had replicated in all calves (results for up to 14 days shown in Table 3) However, during the main virus growth phase between days 1 and 8 there was 50-10 000-fold less virus recovered from vaccinated groups than that from unvaccinated group 3 calves (Table 3).Unsurprisingly, virus titres in nasal mucus between vaccinated groups 1 and 2 were not significantly different (P = 0.6036).
The significant protective effect due to the vaccine was also evident when comparing the average amount of virus shed by a calf for which the values were 2.5, 2.3 and 4.3 log10 TCID50 per millilitre swab, respectively, for groups 1, 2 and 3 (values in italics).Furthermore, both vaccinated groups shed virus for a much (P = 0.005, group 1 and 0.003, group 2) shorter period than unvaccinated calves and in this respect vaccinated group 2 was at the borderline of significance (P = 0.058) compared to vaccinated group 1.As expected from the virus shedding data all calves in the study seroconverted but overall, the level of seroconversion was at least 4-fold lower in the control group than that in the vaccinated groups.Based on virus shedding data, a conclusion from results in Table 3 is that the vaccine would significantly protect seronegative cattle for a period of up to at least 6 months.

Against ruminant pestiviruses
Findings for the BVDV component (Strain C86) are shown in Although calves in both vaccinated groups also became infected, the amount shed during the peak period (days 2-8) was significantly (10-300-fold P = 0.02-0.03)lower than that shed by the unvaccinated calves.While the shedding by both vaccinated groups was significantly (P = 0.005 group 1 and 0.007 group 2) shorter (3.5 ± 1.6 and 1.6 ± 0.5 vs 7.7 ± 0.8 days), more recently (group 2) vaccinated calves were significantly (P = 0.049) better protected.
Both leukocytes and serum were tested for the presence of virus.Isolation results are shown in Table 5.Although all vaccinated calves had infected leukocytes for 1-4 days, none of the animals yielded virus from their sera.By contrast, all unvaccinated calves were viraemic in both fractions, normaly for 6-9 days in leukocytes (mean 7.5 ± 1.2 day) and for 5-8 days in serum (mean 6.2 ± 1.2 days) between days 1 and 10 (Table 5) but not afterwards (individual results not shown).In serum the titres ranged between 1.7 and 2.4 log10 TCID 50 per m and daily individual mean titres were between 1.9 and 2. shown).While the vaccine did not prevent infection of leukocytes, the duration of leukocyte viraemia in vaccinated groups was significantly (P = 0.004, group 1; and 0.007, group 2) short-lived compared to the unvaccinated group.Despite considerable local and systemic virus replication, the level of seroconversion in control calves after challenge was surprisingly moderate with circulating VN antibody titres in the range of 4-6 whereas the response in all vaccinated calves was beyond the highest (9.0 log2) dilution in the titration.Data in Tables 4 and 5 allow the conclusion that the vaccine would significantly protect seronegative calves for a period of at least 6 months.The fact that there was no serum viraemia in vaccinated calves allows the conclusion that the vaccine would also significantly prevent transplacental infection.
Against BRSV Findings for the BRSV component are shown in Table 6.Unlike other components the VN response to BRSV in both vaccinated groups 1 and 2 was just detectable.Nonetheless in group 1 calves activity persisted until 31 weeks in all 6 calves and thereafter in 2-3 of 6 calves up to 36 weeks but not at challenge 42 weeks after vaccination.All 5 group 2 calves were VN antibody positive at challenge 18 weeks after vaccination.However, no activity was detected in any of 6 unvaccinated calves until after challenge.
In daily sampling of nasopharyngeal swabs for 10 days after BRSV challenge, none of the calves in vaccinated groups 1 and 2 shed virus.During this period between 16.7 and 83.3 % of unvaccinated calves excreted virus, the daily average of excretors was 48.3 ± 25.4 % ( washings was on alternative days for ethical reasons.During this period no BRSV was recovered from lung washings of any calves in vaccinated groups 1 and 2. However, at this time between 33.3 and 100 % of unvaccinated calves shed virus in lungs and daily average of excretors was 88.9 ± 27.2 % (Table 6).Clinically, however, the challenge infection did not result in significant respiratory disease.
Results in Table 6 allow the conclusion that the recommended course of 2 IM vaccinations would provide significant protection for the claimed period of 6 months.

DISCUSSION
The principle of multivalent inactivated cattle vaccines is not new but there is scanty published data indicating the quality of protection that should be expected from such vaccines.No doubt the available multivalent vaccines do meet the licensing requirements of countries where such products are marketed, but it is important that the data for experimental validation of efficacy and safety are more widely and readily available to practising clinicians and other interested parties not directly connected with the product licensing authorities.The present inactivated vaccine was formulated to reduce the impact of 4 most commonly implicated viruses in the aetiology of bovine respiratory disease and also the reproductive disease due to IBR virus and BVDV.It is important to point out that controlled efficacy determinations made are of naïve target species and challenge infections were sequential in order to mimic the field situation.The latter, however, was at long enough intervals in order to avoid interference due to the preceding challenge.The fact that all 3 experimental groups were challenged at the same time allowed a direct comparison of short-term (≥1 month) and longterm (≥6 months) immunity.
Immune stimulation due to the vaccine was measured by the virus neutralisation (VN) test since it is the most virus specific serological response and VN is a recognised defence mechanism in many viral infections.After the recommended course of 2 IM vaccinations high VN activity was recorded against all vaccine components except BRSV.In natural hosts, BRSV is known to produce a low-grade VN antibody response 1 .Furthermore, the VN activity to BRSV was comparable to that after live virus challenge 18   (iv) Pyrexia and respiratory disease score after challenge.

Group and treatment
Group mean score on day  2).In keeping with the general experience 8,9 , vaccination was not expected to completely prevent challenge virus growth and shedding from the upper respiratory tract; nonetheless, the amount and the duration of virus shedding in nasal mucous was significantly lower in vaccinated calves than that in the control group (Table 2).As expected, therefore, all calves showed a rise in VN antibody titre that was markedly higher in vaccinated calves than unvaccinated calves due to prior sensitization.In common with other experimental efficacy studies 15,2 the PI3 virus challenge by IN plus IT routes caused only mild clinical signs despite the fact that the low passage lung isolates used originated from calves with pneumonia.Nonetheless, the challenge virus recovery from nasal mucus (Table 3) showed a highly significant protection due to the vaccine administered 7 or 27 weeks previously.The fact that the challenge virus grew in all calves was also reflected in the VN antibody response that as expected , was significantly higher in vaccinated groups than the control group (Table 3).
Cattle infected with BVDV can present a variety of clinical signs both enteric and respiratory and usually as a trivial illness of a few days' duration. 5However, the major economic impact of BVDV is fertility failure, abortions and other consequences of transplacental infection.In the present study only mild respiratory signs were observed in the unvaccinated calves despite an intranasal challenge consisting of 8 BVDV (types 1a, 1c, 1d and a strain not belonging to 1a,b,c, or d) and 3 BDV strains.Results shown in Table 4 do, however, demonstrate short-term (2 months and 2 weeks) and long-term (7 months and 2 weeks) efficacy against mild clinical signs.A far clearer measure of both the short and the long-term efficacy was evident at the level of virus isolation from nasal mucous, leukocytes and serum shown in Tables 4 and 5.In this regard it should be pointed out that it is not possible to prevent re-infection of cattle with BVDV even after a short period and even with a homologus strain 11,14 .A noteworthy finding shown in Table 5 is a complete lack of viraemic serum in vaccinated calves although the calves had infected leukocytes albeit for a significantly shorter duration than uninfected calves.In our experience 14 and that of others 11 it is not possible to completely prevent leukocyte-associated viraemia despite immune stimulation.An important conclusion, however, is that the lack of serum viraemia suggest that the quadrivalent vaccine like the monovalent vaccine 14 containing the same BVDV strain (C86) will also significantly prevent transplacental BVDV infection, abortion and fertility losses due to BVDV infection.
A major drawback in assessing vaccines against BRSV is the inability to experimentally reproduce respiratory disease 12,18 .The present study also did not record clinical reactions typically seen in natural BRSV infections.Nonetheless, virus isolation for up to 10 days from nasal swabs and lung washings from all 6 control calves with accompanying seroconversion indicates that the challenge was infectious.This is further supported by the fact that all calves in both vaccinated groups showed an anamnestic response for VN antibody despite no obvious virus shedding in nasal mucus or lung washings (Table 5).While the latter observation indicates a complete protection against challenge virus shedding from the upper and the lower respiratory tracts of calves in both vaccinated groups, data do not explain the booster sero-response in the absence of detectable virus growth.This has been recorded before 10 and the likely explanation is that the vaccinated calves did undergo a lowgrade infection which remained undetected and/or the low amounts of shed virus was inactivated since BRSV is very labile.Nonetheless, results shown in Table 6 do support the conclusion that a course of 2 IM inoculations with the this vaccine protected calves against BRSV up to 9 months.This conclusion, however, would have been far more robust had the BRSV challenge produced a significant respiratory disease.Hence for the BRSV component in the present quadrivalent vaccine and for that matter any other BRSV vaccines, field trials are essential to further augment experimental data.
A matter of practical concern for any inactivated vaccine is the incidence and the severity of clinical reactions, particularly those at the injection site induced by the vaccine.In this regard we have, in experimental safety studies and in field trials, not observed clinical reactions that have given us cause for concern (data not shown).Usually the type and the nature of the adjuvant determines the persistence and the severity of injection site reactions.Findings for the aluminium salt adjuvant in the present quadrivalent vaccine indicate that it is innocuous and does not normally cause a severe injection site reaction other than a mild inflammatory reaction for a few days.
In summary, the experimental data in naive target species for the efficacy of the quadrivalent inactivated vaccine containing IBR virus, PI3 virus, BVDV, BRSV and aluminium salts demonstrated a significant protection for each component for at least 6 months.It is also noteworthy that for 3 (IBR virus, PI3 virus and BVDV) of the vaccine components, VN antibody persists for over 12 months (data not shown).Also under normal circumstances the vaccine will not induce significant adverse clinical reactions systemically or at the injection site.
upon 2 serial passages, each for 4 days at 37 °C.c Daily mean excretors = sum of per cent of shedding calves divided by numbers of days of shedding.

and titres used in a single pool to challenge calves. Strain Passage level Source
All strains were in-house isolates except Moredun 2 and R 2727.The former was kindly supplied by P F Nettleton, Moredun Institute, Edinburgh, and the latter by S Edwards, VLA, Weybridge.Typing based on N-Pro sequence homology by H-J Thiel and P Becher, Institute for Virology, Justus-Liebig University, Giessen, Germany.
a Genotype a b

Table 2 : Responses to IBR virus component. (i) Serum VN antibody activity. Group and treatment
Titre in log2 scale; values shown a <2 denote no activity detected in 4-fold diluted serum.
a Geometric group mean VN antibody titre b at time (weeks) b

Table 4 : Responses to BVD virus component. (i) Serum VN antibody activity. Group and treatment
a Geometric group mean VN antibody titre b at time (weeks)

m serum on day after challenge Mean titre
Mean titre per calf: sum of individual mean titres (sum of daily virus recovered divided by numbers of days virus shed) divided by number of animals in group. b

Table 6 : Response to BRSV component. (i) Serum VN antibody activity. Group and treatment
Daily mean excretors = sum of per cent of shedding calves divided by number of days of shedding. a