Protective efficacy of Marek's disease virus (MDV) CVI-988 CEF65 clone C against challenge infection with three very virulent MDV strains

Comparative 50% protective dose (PD50) assays were performed using a plaque-purified preparation of Marek's disease virus (MDV) strain CVI-988 at the 65th chicken embryo fibroblast (CEF) passage level (MDV CVI-988 CEF65 clone C) and three commercial MD vaccines: herpesvirus of turkeys (HVT) FC126, MDV CVI-988 CEF35, and a bivalent vaccine composed of HVT FC126 and MDV SB-1. In addition, comparative PD50 assays were performed in groups of chickens with maternal antibody to each of the three vaccines. Three representatives of the newly emerged biovariant very virulent (vv) MDV strains-RB/1B, Tun, and Md5-were employed as challenge virus. The experiments made feasible the differentiation between virulent MDV and vvMDV strains, within serotype 1. Vaccination with CVI-988 clone C vaccine resulted in PD50 estimates of about 5 plaque-forming units (PFUs) against challenge infection with each of the three vvMDV strains. The PD50 estimate of CVI-988 clone C vaccine was 12-fold below the PD50 of HVT FC126. The protective synergism of bivalent vaccine, composed of HVT and SB-1, was confirmed by groups given the lowest vaccine doses. The bivalent vaccine, however, resulted in incomplete protection in groups given the highest vaccine doses. Homologous maternal antibodies to serotype 1 caused a fivefold increase in the PD50 estimate of CVI-988 clone C. Heterologous maternal antibodies against HVT did not interfere with efficacy of CVI-988 clone C vaccination. However, the combination of maternal antibodies against both HVT and SB-1 (serotypes 2 and 3) showed a strong adverse effect on CVI-988 clone C vaccine.(ABSTRACT TRUNCATED AT 250 WORDS)     

鸡马立克氏病活疫苗免疫效力比较试验

用ＨＶＴ冻干苗、ＨＶＴ细胞结合苗、ＣＶＩ９８８细胞结合苗、ＳＢ１＋ＦＣ１２６双价活疫苗、３０１Ｂ／１＋ＦＣ１２６双价活疫苗和Ｚ４＋ＦＣ１２６双价活疫苗等６种鸡马立克氏病（ＭＤ）疫苗免疫ＳＰＦ白来航鸡或普通伊莎鸡，用鸡马立克氏病病毒（ＭＤＶ）强毒ＧＡ株、京－１血毒以及鸡马立克氏病超强毒ｖｖＭＤＶ－Ｍｄ５毒株分别攻击进行免疫效力比较试验。试验表明，ＭＤ单价苗的免疫效力强弱顺序依次是ＣＶＩ９８８、ＨＶＴ细胞结合苗和ＨＶＴ冻干苗，这３种ＭＤ单价苗均能给免疫鸡群提供有效的免疫保护力。ＳＢ１＋ＦＣ１２６、Ｚ４＋ＦＣ１２６和３０１Ｂ／１＋ＦＣ１２６等３种ＭＤ双价苗免疫效力显著高于ＭＤ单价苗，均能给免疫鸡群提供较强的免疫保护力，并能有效地抵抗ｖｖＭＤＶ－Ｍｄ５毒株的致瘤作用。Ｚ４＋ＦＣ１２６和３０１Ｂ／１＋ＦＣ１２６ＭＤ双价苗免疫效力无显著差异     

A comparative evaluation of the protective efficacy of rMd5deltaMeq and CVI988/ Rispens against a vv+ strain of Marek's disease virus infection in a series of recombinant congenic strains of White Leghorn chickens

Marek's disease (MD) is a lymphoproliferative disease of domestic chickens caused by a highly infectious, oncogenic alpha-herpesvirus known as Marek's disease virus (MDV). MD is presently controlled by vaccination. Current MD vaccines include attenuated serotype 1 strains (e.g., CVI988/Rispens), avirulent serotype 2 (SB-1), and serotype 3 (HVT) MDV strains. In addition, recombinant MDV strains have been developed as potential new and more efficient vaccines to sustain the success of MD control in poultry. One of the candidate recombinant MDV strains, named rMd5deltaMeq, was derived from Md5, a very virulent strain of MDV lacking the MDV oncogene Meq. Our earlier reports suggest that rMd5deltaMeq provided protection equally well or better than commonly used MD vaccines in experimental and commercial lines of chickens challenged with very virulent plus (vv+) strains of MDV. In this study, maternal antibody-positive (trial 1) and negative (trial 2) chickens from a series of relatively MD resistant lines were either vaccinated with the rMd5deltaMeq or CVI988/Rispens followed by infection of a vv+ strain of MDV, 648A, passage 10. This report presents experimental evidence that the rMd5deltaMeq protected significantly better than the CVI988/Rispens (P < 0.01) in the relatively resistant experimental lines of chickens challenged with the vv+ strain of MDV. Together with early reports, the rMd5deltaMeq appeared to provide better protection, comparing with the most efficacious commercially available vaccine, CVI988/Rispens, for control of MD in lines of chickens regardless of their genetic background.     

Difference between influences of homologous and heterologous maternal antibodies on response to serotype-2 and serotype-3 Marek's disease vaccines

Marek's disease (MD) vaccines representing serotypes 2 (SB-1 strain of MD virus) and 3 (FC-126 strain of turkey herpesvirus) were administered to 1-day-old MD-susceptible chicks that either were free of antibodies or carried maternally derived antibodies against SB-1, FC-126, or a serotype-1 MD virus, CU-2 strain. Homologous antibodies delayed the development of vaccine virus viremias and inhibited vaccinal immunity, as judged by protection against challenge with the virulent JM-10 strain of MD virus 7 days postvaccination. Heterologous antibodies had little effect on vaccine responses. Antibodies were shown to interfere with both cell-associated and cell-free vaccine virus.     

Field trials to test the efficacy of polyvalent Marek's disease vaccines in broilers

The efficacies of trivalent (Md11/75C + SB-1 + HVT), bivalent (SB-1 + HVT), and turkey herpesvirus (HVT) vaccines against Marek's disease (MD) were compared in commercial broiler flocks in four trials involving 11 farm locations and 486,300 chickens. In all four trials, chickens receiving polyvalent vaccines had lower leukosis (MD) condemnation rates than chickens vaccinated with HVT alone; when data were summarized for each vaccine type in each trial, condemnation rates for the bivalent- or trivalent-vaccinated groups were 56-96% (mean 78%) lower than those for HVT-vaccinated chickens. Polyvalent vaccination was clearly mor efficacious than HVT in 8 of 11 individual farms, although it did not always reduce leukosis condemnations to acceptable levels. Body weights of chickens vaccinated with polyvalent vaccines did not differ consistently from those vaccinated with HVT. Chickens inoculated with the trivalent vaccine had slightly lower overall leukosis condemnation rates (0.24%) than those inoculated with the bivalent vaccine (0.45%) in trials 1-3, where direct comparisons were made. Bivalent vaccines containing either 1,500 or 200 plaque-forming units of SB-1 virus were equally effective; thus, HVT may need to be supplemented with only small amounts of SB-1 to obtain the benefits of protective synergism. SB-1 virus did not appear to carry over from polyvalent-vaccinated flocks to subsequent HVT-vaccinated flocks in the same houses, even when old litter was used.     

Comparative efficacy of BAC-derived recombinant SB-1 vaccine and the parent wild type strain in preventing replication, shedding and disease induced by virulent Marek’s disease virus

A widely used vaccine against Marek’s disease (MD) in poultry is the virus SB-1, which is antigenically-related to the causative agent, Marek’s disease herpesvirus. We recently cloned the SB-1 genome as an infectious bacterial artificial chromosome, BAC, (pSB-1). The protective efficacies and replication kinetics of pSB-1 and the parent strain (SB-1) were compared in an experimental model of MD induced by a virulent strain, RB-1B. Although vaccine virus replication and shedding was lower for pSB-1 than for SB-1, both vaccines reduced replication and shedding of RB-1B, and were equally effective in protecting chickens against MD. With the cloning of pSB-1, we have now generated full length genomic clones of MD vaccine virus strains belonging to each of the three serotypes. Vaccine viruses derived from each of these clones demonstrated protective efficacies at levels similar to those produced by the respective parent viruses, demonstrating their suitability to be used as vaccine candidates.     

Field trials with a bivalent vaccine (HVT and SB-1) against Marek's disease

White leghorn chickens on five farms were given a bivalent Marek's disease (MD) vaccine consisting of turkey herpesvirus (HVT) and SB-1 (a nononcogenic MD virus); other chickens received only HVT. The farms had histories of "vaccination failures," presumably owing to an exceptionally virulent challenge MD virus. The bivalent vaccine uniformly protected chickens better than HVT alone between 12 and 16-20 weeks of age, when serious MD losses occurred. During that period, total mortality in groups given both viruses ranged from 0.39 to 1.26% (mean 0.86%), whereas that in HVT-vaccinated groups not exposed to SB-1 varied from 1.92 to 7.44% (mean 3.43%). Chickens in pens or rows with close contact to those given bivalent vaccine also had low MD mortality rates (0.46-1.06%, mean 0.77%), probably from the spread of SB-1.     

New serotype 2 and attenuated serotype 1 Marek's disease vaccine viruses: selected biological and molecular characteristics

Two new Marek's disease vaccine viruses, Md11/75C/R2 (serotype 1) and 301B/1 (serotype 2), were evaluated in chickens with maternal antibodies (ab+) or without maternal antibodies (ab-). Strain Md11/75C/R2 was mildly pathogenic in ab--chickens, but this pathogenicity was markedly reduced in ab+ chickens. Md11/75C/R2 spread less by contact and replicated better, both in vivo and in vitro, than CVI988/C, another serotype 1 vaccine virus. Strain 301B/1 was similar to SB-1, another serotype 2 vaccine virus: both were nonpathogenic for ab--chickens, spread readily by contact, and replicated well in vivo. In vitro, 301B/1 grew more rapidly and produced larger plaques than SB-1. Notable characteristics of strain CVI988/C included absence of pathogenicity, poor replicative ability, and the absence of one epitope detected by a common serotype-1-specific monoclonal antibody. All four viruses could be distinguished from each other by restriction enzyme analysis of viral DNA. We conclude that Md11/75C/R2, although exceptionally protective, may require further attenuation. On the other hand, 301B/1, which in other studies induced higher levels of protection than SB-1, is nonpathogenic and may be considered for use as a commercial vaccine.     

Attenuated revertant serotype 1 Marek's disease viruses: safety and protective efficacy.

In earlier studies, a revertant serotype 1 Marek's disease virus (MDV), clone Md11/75C/R2, was found to be a highly protective vaccine virus but was mildly pathogenic for susceptible chickens. The term "revertant" indicates that the virus, after attenuation, gained virulence following backpassage in chickens. The present study is an attempt to develop a more attenuated but still protective vaccine virus from Md11/75C/R2. Forty-two derivative viruses or clones from Md11/75C/R2 were evaluated. Two of these, designated clones R2/23 and R2/29, induced viremia but little or no pathology in preliminary trials and were selected for further study. In a series of nine trials, both clones provided protection against challenge with very virulent MDV strains that was superior to that induced by turkey herpesvirus (HVT) and was not significantly different (P greater than 0.05) from that induced by a bivalent (HVT + SB-1) vaccine. Both clones appeared fully attenuated based on pathogenicity tests in susceptible antibody-negative chickens. Both clones gained virulence on backpassage in chickens, but this seemed of little concern because neither virus spread by contact to other chickens. Although the two clones were very similar, clone R2/23 appeared to have a slightly lower pathogenic potential following backpassage and thus best meets the combined criteria of safety and efficacy.     

The efficacy of recombinant fowlpox vaccine protection against Marek's disease: its dependence on chicken line and B haplotype

Earlier studies have shown that the B haplotype has a significant influence on the protective efficacy of vaccines against Marek's disease (MD) and that the level of protection varies dependent on the serotype of MD virus (MDV) used in the vaccine. To determine if the protective glycoprotein gene gB is a basis for this association, we compared recombinant fowlpox virus (rFPV) containing a single gB gene from three serotypes of MDV. The rFPV were used to vaccinate 15.B congenic lines. Nonvaccinated chickens from all three haplotypes had 84%-97% MD after challenge. The rFPV containing gB1 provides better protection than rFPV containing gB2 or gB3 in all three B genotypes. Moreover, the gB proteins were critical, since the B*21/*21 chickens had better protection than chickens with B*13/*13 or B*5/*5 using rFPV with gB1, gB2, or gB3. A newly described combined rFPV/gB1gEgIUL32 + HVT vaccine was analyzed in chickens of lines 15 x 7 (B*2/*15) and N (B*21/*21) challenged with two vv+ strains of MDV. There were line differences in protection by the vaccines and line N had better protection with the rFPV/gB1gEgIUL32 + HVT vaccines (92%-100%) following either MDV challenge, but protection was significantly lower in 15 X 7 chickens (35%) when compared with the vaccine CVI988/Rispens (94%) and 301B1 + HVT (65%). Another experiment used four lines of chickens receiving the new rFPV + HVT vaccine or CVI988/Rispens and challenge with 648A MDV. The CVI 988/Rispens generally provided better protection in lines P and 15 X 7 and in one replicate with line TK. The combined rFPV/gB1gEgIUL32 + HVT vaccines protected line N chickens (90%) better than did CVI988/Rispens (73%). These data indicate that rFPV + HVT vaccines may provide protection against MD that is equivalent to or superior to CVI988/ Rispens in some chicken strains. It is not clear whether the rFPV/gB1gEgIUL32 + HVT vaccine will offer high levels of protection to commercial strains, but this vaccine, when used in line N chickens, may be a useful model to study interactions between vaccines and chicken genotypes and may thereby improve future MD vaccines.     

Induction of strong protection by vaccination with partially attenuated serotype 1 Marek's disease viruses

Studies were conducted to better understand the relationship among Marcek's disease (MD) vaccine strains between induction of protective immunity and the degree of attenuation (or virulence). To obtain viruses at different stages of attenuation, very virulent plus MD strains 584A and 648A and selected clones of these strains were serially passaged in chicken and duck cells. These viruses were considered fully attenuated after passage for 70-100 times in chicken embryo cell cultures until they no longer induced gross lesions in susceptible, maternal antibody-negative (ab-) chickens. Lower passages of the same strains were considered partially attenuated, provided their virulence was less than that of the parent strain. Four of five partially attenuated preparations derived from MD virus strains 584A and 648A or the previously attenuated Md11 strain induced 28%-62% higher levels of protection in maternal antibody-positive (ab+) chickens against virulent MD challenge than the fully attenuated counterpart viruses. The partially attenuated 584A/d2/3 strain replicated in chickens but was totally nonprotective. Data from two subsequent trials in ab+ chickens confirmed that protection induced by the partly attenuated (passage 80) preparations was 79% and 118% higher, respectively, than that induced by the fully attenuated (passage 100) preparations of strain 648A. However, in one trial with ab- chickens, no difference in protection between partially and fully attenuated virus was observed. Strong protection (up to 85%) against highly virulent challenge also was provided by preparations of 648A at passages 40-60, which were moderately oncogenic when used alone. Partially attenuated strains tended to replicate to higher titers in both ab+ and ab- chickens compared with fully attenuated vaccines. Also, ab+ and ab- chickens vaccinated with partially attenuated strains developed three- to nine fold more extensive microscopic lesions in peripheral nerves at 14 and 22 days after virulent challenge than chickens vaccinated with fully attenuated strains. When measured in ab+ chickens, loss of lesion induction by 648A was achieved 30 passages earlier (at passage 70) than when measured in ab- chickens. Thus, maternal antibodies appeared to abrogate the pathogenicity of some partially attenuated strains. These studies establish for MD the principle that at least some partially attenuated MD viruses may replicate better and induce stronger immunity against virulent challenge than fully attenuated preparations of the same strain, at least when tested in ab+ chickens. Moreover, depending on passage level, partially attenuated vaccine strains may be relatively innocuous for ab+ chickens, causing few or no lesions.     

Isolation of very virulent Marek''s disease viruses from vaccinated chickens in Australia

Very virulent Marek's disease viruses (vvMDV), defined as isolates against which the herpesvirus of turkey (HVT) vaccine provide poor protection, have been isolated from poultry flocks in both the United States and Europe. Twenty-one samples from vaccinated Australian flocks, experiencing problems with excessive Marek's disease (MD), were tested for the presence of transmissible MD viruses (MDV). Of the 16 samples which contained a transmissible agent, 14 were pathogenic in chickens, based on the development of MD lesions or depression of the bursa/body weight ratio. Of the pathogenic isolates which have been successfully typed 10 were serotype 1, and one was serotype 2 MDV. Pathogenicity of isolates varied. Several isolates caused tumours in 20-30% of both vaccinated and unvaccinated chickens. Two isolates, MPF6 and MPF23, caused tumours in more than 50% of chickens. When MPF6 and MPF23 were tested in vaccine trials bivalent vaccine gave no better protection against development of MD lesions than a monovalent vaccine. Isolate MPF23 was so pathogenic that lesions were produced in all chickens, regardless of the vaccine protocol used. Therefore vvMDV have been isolated in Australia, and unlike the vaccines tested overseas, bivalent Australian vaccines do not appear to provide greater protection against these vvMDV.     

Serotype 1 viruses modified by backpassage or insertional mutagenesis: approaching the threshold of vaccine efficacy in Marek's disease

Improved vaccines to control Marek's disease (MD) in chickens are desired by the poultry industry but have been difficult to develop. Studies were conducted to evaluate strategies for deriving MD vaccines of high protective efficacy, irrespective of virulence. Candidate viruses from parent strains representing v and vv+ pathotypes were modified by cell culture passage, backpassage in chickens, or insertional mutagenesis following cocultivation with retroviruses. Ten strains considered most likely to exhibit high protective efficacy were selected for further study. The ability of these modified viruses to protect commercial or maternal antibody-positive (ab+) chickens against virulent MD virus (MDV) challenge was compared with that of strain CVI988, the standard commercial MD vaccine. Modified strains were also evaluated for the ability to induce lymphomas or other pathologic changes in ab+ and antibody-negative (ab-) chickens. Two of the 10 modified viruses, strains RM1 and CVI988/BP5, provided high levels of protection against highly virulent MDV challenge. The magnitude of protection was greater than that of one laboratory and two commercial preparations of CV1988, but was approximately equal to that of two other commercial preparations of CVI988 in laboratory and field tests. Three of the strains, including RMI and CVI988/BP5, induced lymphoid organ atrophy in ab-chicks but not in ab+ commercial chicks, a property designated here as L phenotype. Seven strains, including two L+ strains, were mildly oncogenic for ab- chicks, a property designated here as O phenotype. Five of these strains caused no tumors in ab+ chickens. The two fully attenuated strains induced neither lymphomas nor lymphoid organ atrophy. The L and O phenotypes appeared not to be linked, and both (especially the L phenotype) appeared associated with high levels of protection. These studies also illustrated differences in the protective efficacy of different preparations of CVI988 vaccine, indicating the need to choose carefully the most protective strains as controls for efficacy studies. A new vv+ strain, designated as 686, is described and appears useful as a challenge virus; it is the most virulent of the 48 field isolates of MDV thus far pathotyped in this laboratory. These findings support the conclusion that new virus strains with high levels of protective immunity comparable to that of CVI988 can be developed. However, the question of whether strains can be developed that exceed the efficacy of current CVI988-based vaccines remains unanswered. After more than 30 years of unsuccessful endeavor by many laboratories toward this goal, it now may be useful to consider whether the efficacy of MD vaccines is limited by some type of biologic threshold.     

鸡马立克氏病双价疫苗的免疫效力试验

用“814”、B-1/att弱毒疫苗和HVT疫苗匹配组成双价疫苗,进行实验室和田间免疫效力试验。实验室试验表明,由“814”和HVT匹配构成的双价疫苗,免疫效果很好,优于单价疫苗,尤其明显优于HVT单价疫苗。此双价疫苗的田间试验,也取得了令人满意的结果。     

Effect of diluting Marek's disease vaccines on the outcomes of Marek's disease virus infection when challenged with highly virulent Marek's disease viruses

Dilution of Marek's disease (MD) vaccines is a common practice in the field to reduce the cost associated with vaccination. In this study we have evaluated the effect of diluting MD vaccines on the protection against MD, vaccine and challenge MD virus (MDV) kinetics, and body weight when challenged with strains Md5 (very virulent MDV) and 648A (very virulent plus MDV) by contact at day of age. The following four vaccination protocols were evaluated in meat-type chickens: turkey herpesvirus (HVT) at manufacturer-recommended full dose; HVT diluted 1:10; HVT + SB-1 at the manufacturer-recommended full dose; and HVT + SB-1 diluted 1:10 for HVT and 1:5 for SB-1. Vaccine was administered at hatch subcutaneously. One-day-old chickens were placed in floor pens and housed together with ten 15-day-old chickens that had been previously inoculated with 500 PFU of either Md5 or 648A MDV strains. Chickens were individually identified with wing bands, and for each chicken samples of feather pulp and blood were collected at 1, 3, and 8 wk posthatch. Body weights were recorded at 8 wk for every chicken. Viral DNA load of wild-type MDV, SB-1, and HVT were evaluated by real time-PCR. Our results showed that dilution of MD vaccines can lead to reduced MD protection, reduced relative body weights, reduced vaccine DNA during the first 3 wk, and increased MDV DNA load. The detrimental effect of vaccine dilution was more evident in females than in males and was more evident when the challenge virus was 648A. However, lower relative body weights and higher MDV DNA load could be detected in chickens challenged with strain Md5, even in the absence of obvious differences in protection.     

不同鸡MD疫苗免疫鸡及强毒攻击后羽囊排毒规律

分别以７种鸡ＭＤ疫苗免疫ＳＰＦ鸡和狼山鸡,用琼脂扩散试验（ＡＧＰ）检查鸡群ＭＤＶ强毒攻击后不同时期的羽囊抗原,结果表明,免疫组鸡羽囊排毒高峰推迟,排毒率下降,排毒高峰维持时间短,不同疫苗免疫不同品种鸡后排毒情况有差异,ＣＶＩ９８８和两种二价苗效果优于ＨＶＴ苗。     

Antibody response of chickens to serotype 1, 2, or 3 Marek's disease vaccines based on ELISA with infected cells as antigen

An enzyme-linked immunosorbent assay (ELISA) was applied to evaluate the antibody response of commercial White Leghorn chickens to vaccination against Marek's disease (MD) at hatch (day 0) with serotype-1 (Rispens), -2 (SB-1), or -3 (turkey herpesvirus, HVT) vaccine virus and to challenge on day 21 with MD virus. Antigens for the test were whole chicken embryo fibroblast cells infected with Rispens, SB-1, or HVT. The chickens were progeny of stock that had been vaccinated with HVT, and on day 21 the nonvaccinated group had higher levels of maternal antibodies to HVT than to other antigens (P < 0.05). Only SB-1 vaccine had induced antibodies by day 21, and this was detected only against homologous antigens. On day 49, all three vaccines had induced higher levels of antibodies to homologous than to heterologous antigens. Marek's Disease virus (MDV) induced antibodies to all three antigens, but challenging vaccinated chicks did not significantly increase levels of antibodies on day 81 to any of the three antigens. It was concluded that an ELISA using whole cells as antigens would have potential value for monitoring the antibody response induced by MD vaccines and virulent MDV.     

抗菌药对马立克氏病活疫苗毒性的作用

用不同剂量的青霉素钾、硫酸链霉素、硫酸卡那霉素、硫酸庆大霉素和恩诺沙星等５种常见抗菌药加入ＨＶＴ冻干菌、ＣＶＩ９８８细胞结合苗和Ｚ４＋ＦＣ１２６双价活疫苗中,作用１ｈ后,分别取样计数,研究其对马立克氏病（ＭＤ）疫苗的毒性作用。结果表明：５种抗菌药加入ＭＤ疫苗稀释液后,ｐＨ值变化较大,ＭＤ疫苗空斑数显著减少,以至使ＭＤ疫苗失效。     

Protection of pigs against challenge with virulent Streptococcus suis serotype 2 strains by a muramidase-released protein and extracellular factor vaccine

The efficacy of a muramidase-released protein (MRP) and extracellular factor (EF) vaccine in preventing infection and disease in pigs challenged either with a homologous or a heterologous Streptococcus suis serotype 2 strain (MRP+EF+) was compared with the efficacy of a vaccine containing formalin-killed bacterin of S. suis serotype 2 (MRP+EF+). The enhancement of the immune response by different adjuvants (a water-in-oil emulsion [WO] and an aluminium hydroxide-based adjuvant [AH]) and their side effects were also studied. The MRP and EF were purified by affinity chromatography. Pigs were vaccinated twice at three weeks and six weeks of age and challenged intravenously with virulent S. suis serotype 2 strains (MRP+EF+) at eight weeks of age. At challenge, the pigs vaccinated with MRP+EF/WO had high anti-MRP and anti-EF titres and were protected as effectively as pigs vaccinated with WO-formulated vaccines with bacterin. Eight of the nine pigs survived the challenge and almost no clinical signs of disease were observed. The titres obtained with the MRP+EF/AH vaccine were low and only two of the five pigs were protected. Pigs vaccinated with either MRP or EF were less well protected; three of the four pigs died after challenge but the clinical signs of disease were significantly less severe than those observed in the placebo-vaccinated pigs. The protective capacity of the bacterin/AH vaccine was very low, and the mortality among these pigs was as high as in the placebo-vaccinated pigs (80 per cent). Postmortem histological examination revealed meningitis, polyserositis and arthritis in the clinically affected pigs. The results demonstrate that a subunit vaccine containing both MRP and EF, formulated with the WO adjuvant, protected pigs against challenge with virulent S. suis type 2 strains.     

Embryo vaccination against Marek's disease with serotypes 1, 2 and 3 vaccines administered singly or in combination

Marek's disease virus (MDV) vaccines of serotypes 1 and 2 administered in 18-day-old embryonated eggs induced better protection against post-hatch challenge at 3 days with virulent MDV than vaccines given at hatch. Embryonal vaccination with a polyvalent vaccine containing equal quantities of serotypes 1 and 2 of MDV and serotype 3 virus (turkey herpesvirus, HVT) was also significantly more effective than post-hatch vaccination. These and earlier results indicate that protective efficacy of single or combined Marek's disease vaccine serotypes against post-hatch challenge at 3 days can be substantially improved if the vaccines are injected into 18-day embryos rather than at hatch. Injection of vaccines of serotypes 1 or 2 into embryonated eggs or hatched chicks did not cause detectable gross or microscopic lesions in chickens. Vaccine viruses of serotypes 1 and 2 could be isolated from spleen cells of chickens 1 week post-vaccination, and the titer of recoverable viruses was higher in chickens that received the vaccines at the 18th day of embryonation than in chickens vaccinated at hatch. Although embryo vaccination with HVT usually provided better protection than post-hatch vaccination against early post-hatch challenge with variant pathotypes of MDV, the protection was poor regardless of vaccination protocol. If challenge with variant pathotypes of MDV was delayed until embryonally or post-hatch HVT-vaccinated chickens were 21 days of age, protection of chickens by HVT was not enhanced. Thus, resistance induced by embryonal vaccination with HVT was qualitatively similar to that induced by post-hatch vaccination with this virus.