New serotype 2 and attenuated serotype 1 Marek's disease vaccine viruses: comparative efficacy

Attempts were made, through selection of optimum viral strains, to develop improved vaccines against Marek's disease (MD). Seven attenuated serotype 1 strains and 22 avirulent serotype 2 strains, both alone and in combination with the FC126 strain of serotype 3, were screened for protective efficacy against challenge with virulent and very virulent MD viral strains. The three viruses selected as most promising were evaluated alone and in various combinations and compared with commercially available vaccines, including FC126, bivalent (FC126 + SB-1), and CV1988/C, in 12 separate assays. Two of these new viruses--301B/1 (serotype 2) and Md11/75C/R2 (serotype 1)--were exceptionally protective compared with prototype vaccine strains. Four new monovalent and polyvalent vaccines based on these two isolates protected chickens better than FC126 alone or CV1988/C alone. Three of these new vaccines provided better protection than the bivalent (FC126 + SB-1) vaccine. Protective synergism was noted commonly between viruses of serotypes 2 and 3 but only sporadically between serotypes 1 and 2 or between serotypes 1 and 3. Strain CVI988/C was protective but was no better than FC126 alone, and it was less effective than bivalent (FC126 + SB-1) vaccine, even when used as a bivalent vaccine with FC126 or SB-1.     

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)     

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.     

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.     

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.     

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.     

马立克氏疫苗病毒在鸡传代细胞上的培养工艺

为探索马立克氏病（Marek＇s disease,MD）疫苗病毒在DF-1细胞系上的最佳培养工艺以及由此方法制备的马立克氏病疫苗对鸡的保护效力,通过进行培养基和血清选择、病毒在DF-1细胞上接种稀释度和接种方式选择以及滴度测定,并制备以DF-1细胞为基质的CVI988和FC-126病毒疫苗,按中华人民共和国兽用生物制品规程（2000）检验合格后与其他商品苗进行保护效率对比试验。结果表明,试验成功探索了CVI988和FC-126病毒在DF-1细胞系上的培养工艺,以DF-1细胞为基质制备的疫苗也取得了和其他商品苗保护率相当的良好效果,因此推断DF-1细胞可以作为马立克氏病疫苗病毒的传代细胞培养候选基质。     

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.     

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     

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.     

Pathogenicity studies with plaque-purified preparations of Marek's disease virus strain CVI-988

The pathogenicity of Marek's disease (MD) strain CVI-988 vaccine, eight plaque-purified preparations originating from this strain, and the vaccine HVT FC126 (based on herpesvirus of turkeys) was determined by intramuscular administration of high virus doses to day-old specific-pathogen-free Rhode Island Red (RIR) chickens, which are extremely MD-susceptible. Paralysis and neuritis were observed in 88% of RIR chickens inoculated with MDV CVI-988 at the cell-passage level of the commercial vaccine. HVT FC126 caused paralysis in two of 39 RIR chickens tested, of which one had an endoneural lymphoma, and another three had endoneural inflammation. Five plaque-purified MDV CVI-988 virus preparations at various cell-culture-passage levels caused no lesions. Of another three clones, two caused inflammatory B-type lesions in the nerves of 1/10 chickens, and the third clone caused inflammatory nonneoplastic MD lesions in the liver of 1/11 chickens.     

Optimization of the protocols for double vaccination against Marek's disease by using commercially available vaccines: evaluation of protection, vaccine replication, and activation of T cells

Revaccination against Marek's disease is a widespread practice in some countries. The rationale of this practice is unknown, and there is no consensus in the protocols. Recently, we have demonstrated that administration of the first vaccine at 18 days of embryonation followed by a more protective second vaccine at hatch (18ED/1d) reproduced systematically the benefits of revaccination under laboratory conditions. Here, we have used the same model to optimize the revaccination protocols by using currently available vaccines and to determine whether two features associated with Marek's disease vaccine-induced protection (activation of T cells and replication of vaccine virus) are involved in the revaccination protocols. Protection conferred by three revaccination protocols (turkey herpesvirus [HVT] 18ED/HVT+SB-1 1d, HVT 18ED/CVI988 1d, and HVT+SB-1 18ED/ CVI988 1d) was evaluated. Revaccination protocols also were compared with single vaccination protocols (HVT 18ED, HVT+SB-1 18ED, HVT+SB-1 1d, CVI988 18ED, and CVI988 1d). Our results demonstrated that it is possible to improve efficacy of the currently available vaccines by using them in revaccination programs. Administration of HVT 18ED/CVI988 1d and HVT+SB-1 18ED/CVI988 1d were the two protocols that conferred the highest protection against a very early challenge (2 days of age) with very virulent plus Marek's disease virus strain 648A. In a separate experiment, we evaluated vaccine replication and activation of T cells in single and revaccination protocols. Our results demonstrated that replication of the second vaccine, although decreased compared with single vaccination, could be detected at 3 days (HVT, CVI988) or at 6 days (SB-1). Administration of the first vaccine (HVT) at 18ED resulted in a high percentage of activated T cells. Administration of a second vaccine (either HVT-SB-1 or CVI988) at 1d resulted in increased intensity of MHC-II stain in activated T cells.     

Effect of in ovo vaccine delivery route on herpesvirus of turkeys/SB-1 efficacy and viremia

A study was designed to ascertain the influence of in ovo site of inoculation and embryonic fluid type on the development of Marek's disease (MD) vaccine viremia and efficacy against MD challenge. The experiments were divided into in vitro and in vivo phases. In the in vitro phase, herpesvirus of turkeys/SB-1 vaccine was combined with basal medium eagle (BME) medium (control), amniotic fluid, or allantoic fluid and subsequently titrated on secondary chick embryo fibroblast cultures. There were no significant differences in titer between the virus inoculum carried in BME and the virus inoculum combined with either the allantoic fluid or the amniotic fluid. In the in vivo phase, five routes of inoculation, amniotic, intraembryonic, allantoic, air cell, and subcutaneous at hatch, were compared for generation of protection against virulent MD challenge. Comparisons were made in both specific-pathogen-free and commercial broiler embryos/chicks and, for the amniotic and allantoic routes, injection at either day 17 or day 18 of embryonation. Reisolation of the vaccine virus at day 3 of age was also done for all routes with the exception of the air cell route. Vaccine virus was recovered from all birds tested that were injected in ovo via the amniotic and intraembryonic routes and the subcutaneously at hatch route but was isolated only sporadically from birds inoculated via the allantoic route. Vaccination protective efficacy against virulent MD for all birds vaccinated in ovo via the amniotic or intraembryonic routes and birds vaccinated subcutaneously at hatch was over 90% regardless of day of in ovo injection or bird type. Protective efficacy for vaccines delivered in ovo by either the allantoic or the air cell routes was less than 50% regardless of day of injection or bird type. Therefore, in ovo MD vaccines must be injected either via the amniotic route or the intraembryonic route for optimal performance.     

Derivation,safety and efficacy of a Marek's disease vaccine developed from an Australian isolate of very virulent Marek's disease virus

OBJECTIVE: To develop a serotype 1 Marek's disease (MD) vaccine from a very virulent MDV (vvMDV) pathotype and demonstrate safety and efficacy against early challenge with very virulent field strains in the presence of maternal antibody. STUDY DESIGN: Strain BH 16 was isolated and attenuated by serial cell culture passage. One of two cloned passages was selected for vaccine development following early laboratory-scale protection trials in commercial birds. Comparative protection trials were carded out on the BH 16 vaccine and on a CVI 988 Rispens vaccine using commercial and SPF chickens. Challenge viruses used were either a low passage strain BH 16 virus, the Woodlands No. 1 strain or MPF 57 strain of MDV. The BH 16 vaccine was back-passaged in SPF chickens six times and virus recovered from the final passage and the original vaccine virus were tested for safety. The immunosuppressive potential of the BH 16 and Rispens vaccines was also assessed in parallel. RESULTS: The BH 16 and Rispens vaccines induced comparable levels of protection when used as monovalent or multivalent vaccines, although protection achieved with the monovalent vaccines was lower. No gross tumour formation was evident in any birds receiving the BH 16 vaccine or bird-passaged virus, although microscopic lesions were present in 2/12 birds that received the bird-passaged virus. In tests for immunosuppression, there was no histological evidence of damage to either the bursa of Fabricius or the thymus. CONCLUSION: The BH 16 vaccine was shown to be safe and at least as protective as the Rispens vaccine against three highly virulent MD challenge viruses.     

Derivation, safety and efficacy of a Marek's disease vaccine developed from an Australian isolate of very virulent Marek's disease virus

Objective To develop a serotype 1 Marek's disease (MD) vaccine from a very virulent MDV (vvMDV) pathotype and demonstrate safety and efficacy against early challenge with very virulent field strains in the presence of maternal antibody.
Study design Strain BH 16 was isolated and attenuated by serial cell culture passage. One of two cloned passages was selected for vaccine development following early laboratory-scale protection trials in commercial birds. Comparative protection trials were carried out on the BH 16 vaccine and on a CVI 988 Rispens vaccine using commercial and SPF chickens. Challenge viruses used were either a low passage strain BH 16 virus, the Woodlands No. 1 strain or MPF 57 strain of MDV. The BH 16 vaccine was back-passaged in SPF chickens six times and virus recovered from the final passage and the original vaccine virus were tested for safety. The immunosuppressive potential of the BH 16 and Rispens vaccines was also assessed in parallel.
Results The BH 16 and Rispens vaccines induced comparable levels of protection when used as monovalent or multi-valent vaccines, although protection achieved with the mono-valent vaccines was lower. No gross tumour formation was evident in any birds receiving the BH 16 vaccine or bird-passaged virus, although microscopic lesions were present in 2/12 birds that received the bird-passaged virus. In tests for immunosuppression, there was no histological evidence of damage to either the bursa of Fabricius or the thymus.
Conclusion The BH 16 vaccine was shown to be safe and at least as protective as the Rispens vaccine against three highly virulent MD challenge viruses.     

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.     

鸡马立克氏病Z4＋Fc126双价活疫苗配比试验

本研究用不同剂量的Ｚ４和Ｆｃ１２６配制成鸡马立克氏病双价活疫苗做配比试验。２个试验８种配比的Ｚ４和Ｆｃ１２６双介活疫苗免疫易感鸡，攻毒保护试验结果表明，Ｚ４无论与Ｆｃ１２６细胞结合毒还是与Ｆｃ１２６冻干毒配成双价活疫苗，均能给免疫鸡群提供坚强的免疫保护力，Ｚ４与Ｆｃ１２６间存在很强的免疫协同作用，即使Ｚ４剂量小至仅占疫苗病毒ＰＦＵ总数的１０％，这种协同作用仍能使免疫鸡群产生较强的免疫保护力。     

The role of the reticuloendothelial system in the immunopathology of Marek's disease

The role of macrophages in immunity against Marek's disease (MD) was studied. Chickens of one group were subjected to depletion of macrophages using repeated doses of Francil amorphous silica and those of another group were subjected to activation of macrophages using repeated doses of brewer's thioglycollate broth. Chickens of a third group were vaccinated with herpesvirus of turkeys FC 126 vaccine followed by depletion of macrophages. Chickens of these three groups, as well as groups of healthy unvaccinated and healthy vaccinated chickens, were challenged with virulent MD virus. A sixth group of healthy uninfected chickens was kept as a control. The results, based on clinical signs, gross and histopathological studies and agar gel precipitation test (AGPT) for antibodies, indicated that activation of macrophages enhanced immunity against MD and depletion of macrophages had the opposite effect. The protective effect of vaccination against MD was also lowered by depletion of macrophages. The results of AGPT indicated retardation of MD virus replication by macrophage activation and the reverse on depletion.     

Comparison of blood and feather pulp samples for the diagnosis of Marek's disease and for monitoring Marek's disease vaccination by real time-PCR

Comparison of blood and feather pulp (FP) samples for the diagnosis of Marek's disease (MD) and for monitoring Marek's diseases vaccination in chickens (serotypes 2 and 3 vaccines) by real time-PCR was evaluated. For diagnosis of MD, quantification of serotype 1 Marek's disease virus (MDV) DNA load was evaluated in 21 chickens suffering from MD. For each chicken, samples of blood and FP were collected and MDV DNA load was quantified. Solid tumors are the sample of choice for MD diagnosis by real time-PCR and, hence, 14 solid tumors were included in the study as positive controls. Load of MDV DNA in FP was equivalent to that detected in solid tumors (threshold cycle [Ct] ratio above 1.7). MDV DNA load in blood samples was lower than in solid tumors and FP samples. Nonetheless, there was a statistically significant correlation of the results obtained from FP and blood (r = 0.92). Results of the Pearson correlation test showed that Ct ratio values of 1.7 in FP correspond to Ct ratio values of 1.2 in peripheral blood. For monitoring vaccines, serotypes 2 and 3 MDV DNA load was evaluated in blood and FP samples of vaccinated chickens. Serotype 2 MDV DNA load was evaluated in samples of blood and FP from 34 chickens vaccinated with SB-1 strain. Serotype 3 MDV DNA load was evaluated in blood and FP samples from 53 chickens vaccinated with HVT strain. For both serotypes, frequency of positive samples and load of vaccine DNA was higher in FP than in blood samples. There was not a statistically significant correlation between the load of SB-1 DNA (r = 0.17) or HVT DNA (r = -0.04) in FP and blood. Our results show that the load of serotypes 1, 2, and 3 DNA is higher in FP than in blood. Diagnosis of MD could be done using both FP and blood samples. Monitoring of MD vaccination by real time-PCR required the use of FP samples. There were a high percentage of false negative samples when using blood to detect serotypes 2 and 3 MDV by real time-PCR.     

Efficacy and safety of a cell-culture live virus vaccine for hemorrhagic enteritis of turkeys: laboratory studies

Poults free from hemorrhagic enteritis (HE) antibody were vaccinated by gavage at 1 day or 2 weeks of age with a live HE vaccine virus that had been propagated in a Marek's disease (MD)-induced B-lymphoblastoid cell line of turkey origin. Vaccinated and unvaccinated poults were challenged with a virulent HE virus at various times postvaccination. One hundred tissue-culture-infectious doses of the vaccine virus per poult were sufficient to induce a serological response as well as to protect poults against HE lesions and mortality. Vaccinated poults were protected against the disease as early as 1 week and as late as 8 weeks PV. The vaccine was efficacious by several routes of application. The vaccine virus spread horizontally from vaccinated to contact-exposed poults, as indicated by seroconversion and resistance of contact-exposed poults to challenge. The vaccine had no detectable harmful effects on the humoral immune response to particulate antigens or on weight gain of vaccinated poults. The vaccine proved to be free from MD virus, as indicated by the absence of MD lesions and antibody in 8-week-old chickens inoculated intra-abdominally with the vaccine at hatching. These findings indicate that the cell-culture-propagated HE vaccine is efficacious and safe.