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CVI988／Rispens疫苗免疫效力试验

应用荷兰农业部提供的鸡马立克氏病（ＭＤ）ＣＶＩ９８８／Ｒｉｓｐｅｎｓ Ⅰ型致弱种毒, 在农业部批准的符合ＧＭＰ 要求的生产车间研制出鸡马立克氏病ＣＶＩ９８８／Ｒｉｓｐｅｎｓ 疫苗。将按国际标准检验合格的三批疫苗及进口商品ＣＶＩ９８８／Ｒｉｓｐｅｎｓ 疫苗接种１ 日龄ＳＰＦ 雏鸡, 于７ 日龄经腹腔攻击鸡马立克氏病强毒（北京－ １ 株） 血毒, 全部鸡只隔离饲养观察至６０ 日龄并作全群剖检。经测定： 非免疫攻毒组１００％ 发病,健康对照组全部阴性, 三批国产ＣＶＩ９８８／Ｒｉｓｐｅｎｓ 疫苗保护指数分别为９０－０, ９０－０, ９３－３ , 进口商品苗保护率为９３－３ 。结果表明国产和进口ＣＶＩ９８８／Ｒｉｓｐｅｎｓ疫苗均能提供对ＭＤ 较高的免疫保护力, 国产疫苗的保护效果达到了国际同类产品的先进水平。     

鸡马立克氏病CVI988/Rispens疫苗免疫效力试验

本试验用北京市农林科学院畜牧兽医研究所制备的ＣＶＩ９８８／Ｒｉｓｐｅｎｓ疫苗和进口的ＣＶＩ９８８／Ｒｉｓｐｅｎｓ疫苗免疫１日龄来航鸡,７日龄以ＭＤＶ北京－１株血毒进行攻击,６０日龄全群剖检。经免疫效力试验两次测定,３批北京所制备的ＣＶＩ９８Ｒｉｓｐｅｎｓ疫苗产品保护指数分别为试验的９０．０、９０．０、９３．３和试验（２）的１００．０、１００．０、９４．５与进口商品ＣＶＩ／９８８Ｒｉｓｐｅｎｓ苗的保     

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.     

Recent increase of Marek's disease in Korea related to the virulence increase of the virus

The incidence of Marek's disease (MD), an important neoplastic disease of chickens, suddenly increased in 1997 in Korea. Most MD cases of this country were detected in chickens over 20 wk of age. Five MD viruses were isolated from field flocks in which severe MD losses had occurred, and one of the viruses was studied to compare its pathotype with the prototype JM strain. The isolate KOMD-IC induced severe depression not only in body weight but also in relative bursal weight, and the depression by KOMD-IC was more severe than that induced by JM strain. In addition, the incidence of MD tumor caused by KOMD-IC was higher than that caused by the JM strain. The protective capacity of several MD vaccines was studied against challenge with KOMD-IC. The protective levels of several MD vaccines such as herpesvirus of turkeys (HVT), HVT plus SB1, and Rispens were usually lower against challenge with KOMD-IC than those challenged with JM strain, even if the chickens vaccinated with serotype 1 were not completely protected against challenge with KOMD-IC. The above results indicate that the virulence of KOMD-IC isolated recently was increased, and the increase of MD outbreak in Korea may be related to the virulence increase of the virus. Various MD vaccine programs were applied to reduce MD loss to a broiler breeder farm where severe MD loss had occurred. Serotype 1 vaccine could dramatically decrease the mortality due to MD, and the best results were obtained from the flocks vaccinated with bivalent vaccine of Rispens and HVT.     

CVI 988/Rispens冷冻苗鸡胚胎免疫的免疫病理学机制

用CCI 988/Rispens冷冻苗免疫18日龄的鸡胚,观察淋巴器官的超微结构变化,揭示胚胎免疫的免疫病理学机制.结果显示:胚胎免疫组和胚胎免疫攻毒组的淋巴细胞活性增加,细胞核仁数目和胞浆内线粒体数目增多;非免疫攻毒组的淋巴细胞感染病毒,引起细胞核膜和线粒体受损,细胞有明显的退行性变化,表现为细胞大小不一,常染色质增加,异染色质减少.由此可见,18日龄胚胎免疫对淋巴器官的早期发育具有明显的免疫促进作用,可使雏鸡提前产生有效的免疫力.     

鸡马立克病疫苗株CVI988/Rispens meq基因编辑及缺失毒株的构建与鉴定

meq是鸡马立克病病毒（MDV）最重要的致瘤基因，在马立克病（MD）肿瘤发生中发挥关键作用。同时，它在疫苗株和强毒株之间具有明显的序列差异性。本文利用CRISPR/Cas9基因编辑技术，以MDV疫苗株CVI988/Rispens meq基因为靶点，设计合成gRNA，克隆构建pX459-gRNA质粒，转染CEF并感染CVI988/Rispens，然后对meq基因编辑的病毒噬斑进行克隆纯化，经过PCR扩增、测序分析及IFA鉴定，成功构建1株meq基因编辑的缺失毒株CVI988Δmeq-C7，为后续筛选和鉴定抗MD疫苗株MEQ单抗及鉴别诊断研究奠定了基础。     

Early posthatch protection against Marek's disease in chickens vaccinated in ovo with a CVI988 serotype 1 vaccine

CVI988, a serotype 1 Marek's disease virus (MDV), was used as an in ovo vaccine in specific-pathogen-free chickens to determine if this virus induces early posthatch protection against Marek's disease as has been shown previously for turkey herpesvirus. MDV CVI988 was injected at embryonation day (ED) 17 (group 1) or at hatch (group 2). A third group (group 3) was left unvaccinated. At 1, 2, 3, 4, 5, and 7 days of age, chickens from each group were sampled and examined as follows: a) single-cell suspensions of spleen were inoculated onto chicken embryo fibroblast monolayers to isolate the virus; b) sections of bursal tissues were stained by indirect immunofluorescence assays with anti-pp38 monoclonal antibody to identify viral antigen expression; and c) chickens were exposed intra-abdominally to MDV RB1B, a virulent serotype 1 MDV. Results revealed that in chickens given MDV CVI988 at ED 17, virus and virus-encoded protein were not detected until chickens were 3 and 2 days old after hatching, respectively. Results also indicated that during the first 4 days after hatch, the chickens given MDV CVI988 at ED 17 were better protected against virulent MDV than those given MDV CVI988 at hatch (P < or = 0.001). These results suggested that MDV CVI988 proteins were adequately expressed in the embryo to initiate prehatch immunologic response. Additional efforts with more sensitive techniques than used in this study are needed to identify the nature of viral expression in embryos.     

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.     

Avian influenza adenovirus-vectored in ovo vaccination: target embryo tissues and combination with Marek's disease vaccine

We investigated embryo tissues targeted by replication competent adenovirus (Ad)-free recombinant Ad expressing a codon-optimized avian influenza (AI) H5 gene from A/turkey/WI/68 (AdH5) when injected into 18-day embryonated eggs. We also evaluated the effects of concurrent in ovo vaccination with the experimental AdH5 vaccine and commercially available Marek's disease virus (MDV) vaccine combinations Rispens/turkey herpesvirus (HVT) or HVT/SB-1. Computed tomography indicates that in ovo injection on day 18 of incubation places the solution in the amnion cavity, allantoic cavity, or both. Ad DNA was consistently detected in the chorioallantoic membranes as well as in the embryonic bursa of Fabricius, esophagus, and thymus 3 days postinoculation. H5 expression in these tissues also was detected by immunofluorescence assay. These results indicate possible swallowing of vaccine virus contained in the amnion. In contrast, vaccine localization in the allantoic fluid would have allowed bursal exposure through the cloaca. When the AdH5 vaccine was used in combination with MDV, chickens responding to the AdH5 vaccine had similar AI antibody levels compared with AdH5-only-vaccinated birds. However, combined vaccinated groups showed reduced vaccine coverage to AI, suggesting some level of interference. The combination of AdH5 with MDV Rispens/HVT affected the vaccine coverage to AI more severely. This result suggests that the replication rate of the more aggressive Rispens strain of serotype 1 may have interfered with the Ad-vectored vaccine. Increasing the Ad concentration produced similar AI antibody titers and AI vaccine coverage when applied alone or in combination with the HVT/SB-1 vaccine. Ad DNA was detected in hatched chickens 2 days after hatch but was undetectable on day 9 after hatch. MDV DNA was detected in feather follicles of all vaccinated birds at 12 days of age. Thus, Ad-vector vaccination does not interfere with the efficacy of MDV vaccination by using any of the commonly used vaccine strains.     

Studies on a vaccine against infectious bursal disease

An infectious bursal disease vaccine, registered for use in breeder flocks, was studied for efficacy on the day-old offspring of vaccinated hens and for virulence in susceptible day-old and 6-week-old chickens. When given to susceptible day-old chicks and 6-week-old cockerels, the vaccine was found to induce atrophy and pathology of the bursa of Fabricius similar to that observed in field infections. Chicks vaccinated at day-old had markedly lowered titres in the haemagglutination inhibition test to Newcastle disease virus, when this was given 2 weeks later, but the response of the 6-week-old cockerel was similar to that of control birds. Maternal antibody induced by the vaccine protected chicks against infection at day-old.     

Differential quantification of cloned CVI988 vaccine strain and virulent RB-1B strain of Marek’s disease viruses in chicken tissues, using real-time PCR

The ‘gold standard’ vaccine against Marek’s disease in poultry is the CVI988/Rispens virus, which is not easily distinguishable, antigenically or genetically, from virulent Marek’s disease herpesvirus. Accurate differential measurement of the CVI988 vaccine and virulent viruses is important to investigate mechanisms of vaccinal protection. Minimal sequence differences between CVI988 and virulent MDV strains restrict the application of molecular diagnostic methods such as real-time PCR to distinguish between these viruses. The use of bacterial-artificial-chromosome (BAC) cloned CVI988 virus, which carries the BAC vector sequences in place of the U_s2 gene, allows its differential quantification from virulent strains using real-time PCR assays that target the BAC vector sequence and the U_S2 gene respectively. These novel assays allowed investigation of replication of both serotype-1 vaccine virus (cloned CVI988) and challenge virus (RB-1B strain) in tissues of individual chickens in an experimental vaccination-challenge model of Marek’s disease.     

The use in practice of inactivated oil emulsion vaccine against infectious bursal disease in broiler breeders and its influence on the progeny: a comparative field trial

Field trials were conducted to establish the effect of the use of an inactivated oil emulsion vaccine against Infectious Bursal Disease (IBD OEV) in broiler breeder hens, and its effects on their progeny. The performance of 18 broiler flocks, which were the progeny of the IBD OEV vaccinated breeder hens, but which were not vaccinated with a live vaccine against IBD, was equal to that of broiler flocks which were vaccinated with a live IBD vaccine and originated from parent stock that had been vaccinated only against IBD with a live vaccine. In none of the 18 flocks, progeny of IBD OEV vaccinated parents, was IBD diagnosed. In a second stage, 15 broiler flocks were included in the trial: these were derived partly from IBD OEV vaccinated parents, and partly from parents that received only live IBD vaccine at 8-10 days of age. No cases of IBD occurred and all flocks were positive for IBD precipitins at slaughter age. Vaccination with a live vaccine against IBD at the age of 8-10 days had no influence on NCD antibody development after a NCD vaccination at 7 days. No immunosuppressive effect from this type of live live IBD vaccine could be determined under field conditions.     

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.     

Vaccination against infectious anemia of poultry (CAA)--results of field studies]

Infectious anemia of poultry is a disease of high economical significance. Connatal infection of chicks with the chicken anemia agent (CAA) via the embryonated egg causes anemia along with severe immunosuppression, thus rendering the chicken susceptible for secondary infections. In order to prevent infection of young chicks, it is necessary to induce immunity against CAA in parent flocks, with the aim to prevent connatal spread of the infection and provide maternal protection for baby chicks. In this publication, the efficacy and use of a live CAA vaccine is reported. From autumn 1986 until summer 1990, 3 experimental vaccine charges were applied in 85 broiler parent flocks with totally 3.1 million chickens. In addition, totally 293,000 broiler breeder and 171,000 layer breeder chicken were vaccinated in 1989/90. The vaccine was administered between the 13th and 19th week of life by drinking water without adverse effect to the birds. Chicken anemia symptoms were observed only at the begin of laying period in two parent flocks. These flocks had been vaccinated in the 17th and 19th week, respectively. The offsprings of all other vaccinated parent flocks remained free of chicken anemia. Day-old chicks derived from vaccinated parent flocks were protected against CAA challenge infection. It is emphasized, that vaccination should be performed within the 13th to 15th week of life, because according to our observations, this will lead to an immediate seroconversion.     

An enzyme-linked immunosorbent assay (ELISA) for detection of Marek's disease virus-specific antibodies and its application in an experimental vaccine trial

An enzyme-linked immunosorbent assay (ELISA) for the detection of Marek's disease virus (MDV)-specific antibodies was developed. Chicken embryo cells (CEC) or chicken kidney cells (CKC) were infected with MDV vaccine strain CVI988/Rispens, and infected-cell lysates were prepared at day 5 post-infection by freeze-thawing. Uninfected-cell lysates served as negative controls. Sera were used at a 1 : 100 dilution and were added in parallel to wells containing the infected and uninfected cell lysates. The optical densities at 492 nm (OD(492 nm)) were measured after detection of bound chicken antibodies with anti-chicken IgG peroxidase conjugate and colour reactions using o-phenylenediamine (OPD) as a substrate. The best results concerning the signal-to-noise ratio were obtained by using CKC cells rather than CEC for antigen preparation. The OD(492 nm) of plasma or serum samples with infected CKC was <0.02 when samples of unvaccinated and unchallenged maternal antibody-negative white leghorn chickens were tested. Sera and plasma samples of positive control birds exhibited OD(492 nm) of <0.01 when tested with uninfected CKC. The assay was used to monitor a trial that compared experimental BAC DNA vaccines and a commercial vaccine. Sustained seroconversion and antibody titers that were constantly rising until day 84 after vaccination (71 days after challenge) was observed only when chickens did not develop Marek's disease. In contrast, chickens developing the disease mounted marginal and short-lived antibody titers only. We conclude that the developed ELISA may be a valuable tool for the evaluation of the efficacy of MDV vaccination under experimental but possibly also under field conditions.     

Subclinical infectious bursal disease in an integrated broiler production operation.

A field study was designed to determine the prevalence of subclinical infectious bursal disease (IBD) in broiler chickens from a commercial poultry company. Bursae of Fabricius (BF) from two vaccinated and three nonvaccinated broiler flocks were evaluated histologically, and antibody profiles of these broiler and matched parent breeder flocks were established. Lesions of IBD, including lymphoid necrosis, stromal edema, and infiltrates of heterophils and macrophages, were first detected in BF at 24 days of age in both vaccinated and nonvaccinated chickens. At 41 days, all BF had lesions characteristic of IBD, including severe lymphoid depletion, proliferation of epithelial cells, and mild fibroplasia. Although mean maternal antibody levels (measured by enzyme-linked immunosorbent assay) in broilers were apparently protective through day 12, IBD antibodies decreased to nonprotective levels (below 1,000) by day 16 or 20. Titers began to increase by day 28 or 32 because of field exposure. Sentinel birds, placed with broiler flocks, also developed IBD antibody titers. Broiler breeders had low and nonuniform antibody titers. Prevalence of field IBD exposure was high, and existing vaccination programs were not effective.