Characterization of four very virulent Argentinian strains of Marek's disease virus and the influence of one of those isolates on synergism between Marek's disease vaccine viruses

Updating of epidemiological and pathological features of Marek's disease in laying hens and broilers

Marek's disease (MD) is a huge problem for researchers due to the significant losses in bird flocks, but more importantly, the virus's mutagenic potential. The purpose of this study was to describe non-classical gross lesions observed in broilers and laying hens that suggest the disease emergence and re-emergence. The survey was conducted on 10 broiler and 4 laying hen flocks. All of the dead birds were necropsied in order to obtain a comprehensive diagnosis of lesions, analysing both macroscopic and microscopic alterations. Marek's disease occurred in 80% of cases in broilers and 100% of cases in layer hens. The disease struck 26-day-old broilers and hens at 2 weeks of age, causing a total mortality of 6% and 15%, respectively. There were no clinical indications of the classical neurological form of the disease in either rearing type, and tumour lesions were mostly detected in the liver, spleen, and ovarian follicles in layers, and in the proventriculus in broilers. These findings demonstrated that MD is widespread and that its resurgence is primarily manifested in visceral rather than neurological manifestations. Despite MD immunization, biosecurity remains critical.

Development and Evaluation of the Protective Efficacy of Novel Marek's Disease Virus Rispens Vector Vaccines Against Infectious Bursal Disease

Infectious bursal disease (IBD) is a major disease affecting the poultry industry and is caused by infection with IBD virus (IBDV). To develop a novel vaccine to prevent IBD in chickens, recombinant Marek's disease virus Rispens viruses carrying the VP2 gene of IBDV driven by five different promoters (Rispens/IBD) were constructed using homologous recombination and a bacterial artificial chromosome (BAC). Rispens/IBD driven by the chicken beta-actin (Bac) promoter (Rispens/Bac-IBD), Rous sarcoma virus promoter, or simian virus 40 promoter were administered to 1-day-old SPF chicks, and the protective efficacy against IBDV was evaluated by challenging chicks with virulent IBDV. As a result, Rispens/Bac-IBD showed the best protection (87%). Next, we constructed the virus driven by the Bac-derived Coa5 promoter (Rispens/Coa5-IBD) for a secondary in vivo trial using commercial layer chickens since Rispens/Bac-IBD was thought to be genetically unstable. Rispens/Coa5-IBD showed stability in vitro and exhibited better antibody production and protection during challenge against virulent IBDV at both 5 (95%) and 7 wk of age (91%) compared with that of Rispens/Bac-IBD (90% at 5 wk of age and 84% at 7 wk of age). Thus, Rispens/Coa5-IBD may be a novel promising vaccine against IBD and virulent Marek's disease.

Combination of Two Marek's Disease Virus Vectors Shows Effective Vaccination Against Marek's Disease, Infectious Bursal Disease, and Newcastle Disease

Herpesvirus of turkeys (HVT) is a widely used vector for poultry vaccines. However, different HVTs expressing different foreign antigens cannot always be used simultaneously because of the risk of recombination and interference. In this study, we inoculated a mixture of an HVT-expressing the antigen of Newcastle disease virus (NDV; HVT/ND) and Marek's disease virus (MDV) serotype 1 Rispens virus expressing the antigen of infectious bursal disease virus (IBD; Ripens/IBD) into chickens. This mixture showed 94%, 100%, or 94% protection against MDV, IBDV, or NDV challenge, respectively. In conclusion, the combination of Rispens/IBD and HVT/ND is effective for vaccination against MDV, IBDV, and NDV without significant interference.

Molecular evolution of Marek's disease virus (MDV) field strains in a 40-year time period

Marek's disease (MD) presents a serious threat in poultry production. The disease has been limited for over 40 yr by protective vaccination. The widely applied vaccination against MD is also one of the factors causing evolutionary pressure onto field Marek's disease virus (MDV) virulent strains. Molecular evolution of MDV genes involved in oncogenesis may increase the pathogenicity of MDV virulent strains. The goal of the presented study was to sum up the molecular evolution of MDV field strains isolated in the last 40 yr in Poland. In total, 85 field MDV strains collected between 1974 and 2012 were propagated in chicken embryo fibroblasts. After DNA extraction, three sets of primers were designed for PCR complementary to the MDV076 (RLORF7) region encoding the meq oncogene as well to the MDV077 (23 kDa protein binding alpha-enolase) and MDV077.5 (RLORF6) genes. The obtained 85 MDV076, 60 MDV077, and 58 MDV077.5 cloned fragments were sequenced and aligned with the sequences of reference MDV strains showing different pathogenicity levels. The retrieved nucleotide (nt) and deduced amino acid sequences of RLORF7, 23 kDa protein, and LORF6 of Polish field strains showed several mutations and substitutions homologous to those observed in reference strains with a determined pathogenicity. The observed changes indicated the continuous evolution of field MDV strains. The RLORF7 nt sequence of analyzed MDV isolates showed similarity to virulent and very virulent MDV reference strains. The obtained 23 kDa and LORF6 nt sequences provided more important data and were more similar to mildly pathogenic strains than to virulent and very virulent MDV. The specific nt motifs in all three genes may indicate an increase of MDV virulence and were found in strains starting from 2006. According to the obtained results, the strains isolated in 2012 are similar to the very virulent plus MDV group. The study showed that RLORF7, 23 kDa protein, and RLORF6 fragments harbor sequence motifs that may have some association with MDV pathogenicity level. However, the exact role of the investigated regions in pathogenicity should be further examined by knock-out MDV strains. Also, the true MDV pathotype may only be determined by traditional in vivo experiments.

Correlation between Marek's disease virus pathotype and replication

Marek's disease (MD) virus (MDV) is an alphaherpesvirus that causes MD, a lymphoproliferative disease in chickens. Pathotyping has become an increasingly important assay for monitoring shifts in virulence of field strains; however, it is time-consuming and expensive, and alternatives are needed to provide fast answers in the face of current outbreaks. The purpose of this study was to determine whether differences in virus replication between pathotypes that have been reported using a small number of virulent (v) and very virulent plus (vv+) MDV strains could be confirmed with a large collection of MD viruses. Based on pilot study data, bursa, brain, and lung samples were collected at 9 and 11 days postinoculation (dpi) from birds challenged with 1 of 15 MDV strains. The correlation between virus replication and virulence was confirmed between vMDV strains and higher virulent strains, but in most cases, there was no significant difference between very virulent (vv) and vv+MDV groups. At both 9 and 11 dpi, chickens infected with vv and vv+MDV had significantly lower body weights and relative thymus and bursa weights compared with chickens challenged with vMDV. However, similar to virus quantity, there was no significant difference between weights in birds challenged with vv or vv+MDV. The significant differences observed in maternal antibody negative (ab-) chickens were not significant in maternal antibody positive (ab+) chickens, demonstrating the requirement of ab- birds for this type of comparison. These data do not support the use of virus replication or organ weights as an alternative to pathotyping for discrimination between all three virulent MDV pathotypes but may be useful for determining a virus replication threshold to choose which field strains meet a minimum virulence to be pathotyped by traditional methods.

Generation, transmission and infectiosity of chicken MDV aerosols under experimental conditions

To further investigate the airborne infection mechanism of Marek's disease virus (MDV), a MDV aerosol infection model was established, and the generation, transmission and infectiosity of MDV aerosols were monitored in this study. Two positive/negative pressure isolators, in which SPF chickens were raised, were connected with a closed conduit. Two repetitive trials, Trial 1 (T1) and Trial 2 (T2) were carried out for objective assessment. Air samples were collected using the AGI-30 sampler. Viral DNA in air samples and feather follicle samples were detected using real-time quantitative PCR (QRT-PCR). MDV in air and blood samples was detected by indirect immunofluorescence assay (IFA). In chickens of isolator A (MDV inoculation group), MDV was detected in feather follicles in 100% of the tested chickens at 6 days post inoculation (dpi) in both trials; and MDV was isolated from blood samples at 9-10 dpi. MDV DNA was detected in air samples from isolator A at 12 dpi in T1 and 14 dpi in T2 and concentration of aerosolized MDV DNA was peaked at 3.84 × 10(6)copies/m(3) air at 40 dpi in T1, and 6.17 × 10(5)copies/m(3) air at 38 dpi in T2, respectively. Infectious MDV (cell culture) was isolated from isolator A at 17 in T1 and 19 dpi in T2, respectively. MDV aerosol in Isolator B was almost same as isolator A. Viremia was detected in isolator B at 26-30 dpi. The incidence of viremia in isolator B reached 70% at 3 months post inoculation. These results demonstrated that infected chicken could discharge virus, the MDV could form aerosols and infect neighboring chickens. Understanding the mechanism of generation and infection of MDV aerosols is helpful to prevent and control MD.

Isolation and analysis of a very virulent Marek's disease virus strain in China

Background

A severe MD was broken out at a farm in Shandong, China, despite FC126 vaccination of the chickens at 1-day-old. The mortality of the flocks reached up to 38.3%. The infected chickens were found to have MD pathological changes, including enlargement of spleens, livers and kidneys, and tumors occured on organs later. Samples were collected from the chickens for diagnosis.

Methods

The collected samples were inoculated into primary duck embryo fibroblast (DEF) cells, and the MDV strain named SD2012-1 was isolated. In order to identify the isolate, amplification by PCR and sequencing of oncogenic Meq and vIL-8 gene were processed, the obtained sequences were compared with the sequences of reference strains, and SD2012-1 was used to challenge immunized SPF chickens.

Results

A very virulent MDV isolate strain, SD2012-1, was isolated from a chicken flock in Shandong Province, China, the isolate had the characteristics of very virulent MDV-1, nucleotide and deduced amino acid sequence comparisons of Meq and vIL-8 gene of SD2012-1 with those of reference strains showed SD2012-1 had high homology with MDV strains isolated from China, SD2012-1 could break through the protection provided by HVT vaccine and HVT + SB-1 vaccine immunization and caused the mortality of SPF chickens over 60%. The immune failure occured at the farm could be due to the improper selection of vaccines. SD2012-1 produced death later and the gross postmortem lesions of chickens died early and later were different.

Conclusions

MDV strain SD2012-1 isolated from Shandong Province, China was found to have the characteristics of very virulent MDV-1, which could break through the protection provided by HVT vaccine and HVT + SB-1 vaccine, the virus seemed to have a long latent period, and cause different gross postmortem lesions of chickens between chickens died early and later. A better immunization way should be chosen to prevent infection of this MDV strain in field.

New insight into the structure, development, functions and popular disorders of bursa Fabricii

Humoral immune responses in birds, contrary to mammals, depend on the normal functioning of bursa Fabricii. Recent studies have delivered new information about the structure, development and origin of cells that compose the bursa environment. Several viral infections affect bursa, causing lymphocyte depletion or excessive proliferation. This review summarizes data on the development and histology of healthy bursa and introduces some common disorders that affect this organ.

Pathotyping of Australian isolates of Marek's disease virus and association of pathogenicity with meq gene polymorphism

We report the pathotyping of six Australian isolates of Marek's disease virus-1 (MDV1) isolated between 1992 and 2004 and association of virulence with meq gene polymorphism. Unvaccinated and herpesvirus of turkeys (HVT)-vaccinated specific pathogen free chickens were challenged at day 5 with 500 plaque forming units of Marek's disease virus. The isolates induced gross Marek's disease lesions in 53 to 94% of unvaccinated chickens, and HVT induced a protective index ranging from 38 to 100% by 56 days post challenge. This experiment provides evidence that current Australian isolates of MDV1 vary significantly in pathogenicity. However, there was no clear evidence that the most virulent recent isolates were more pathogenic than isolates from the 1980s or that any of the isolates belong to the highest pathotype category of very virulent plus. Evidence is presented that virulence can be predicted by measurements taken as early as 13 days post challenge. The meq gene sequences of five of the isolates used in the experiment were determined. When compared with the very virulent US isolate Md5, there was a 177 base-pair insertion and distinct point mutations in each of the five isolates. There were no individual mutations in the meq sequences that correlated with levels of virulence. However, amino acid alignment of the five Australian and 14 international isolates revealed that the number of repeat sequences of four prolines (PPPP repeats) in the meq gene (overall range 2 to 8) was strongly associated with virulence across all isolates, with the most pathogenic isolates having the fewest number of repeats. The results suggest that the presence of the 177 base-pair insertion alone is not an indicator of attenuation. Rather, the number of PPPP repeats, independent of the presence of the insertion, is a better indicator of pathogenicity.

Concomitant turkey herpesvirus-infectious bursal disease vector vaccine and oil-adjuvanted inactivated Newcastle disease vaccine administration: consequences for vaccine intake and protection

Hatchery vaccination protocols in day-old chicks are designed to provide early priming and protection against several poultry diseases including, but not limited to, Marek's disease (MD), infectious bursal disease (IBD), and Newcastle disease (ND). The constraint of concomitant administration of live MD and IBD vaccines plus ND inactivated oil-adjuvanted vaccines (IOAVs) requires improvements in vaccine technology. Single-needle concomitant subcutaneous (SC) application of IBD/MDV and killed NDV vaccine and the use of viral vectors for expression of immunogenic proteins are a current trend in the industry. The objective of this work was to assess the compatibility of a turkey herpesvirus (HVT)-infectious bursal disease (vHVT-IBD) vector vaccine applied simultaneously with IOAV and to evaluate the consequences for vaccine intake, the need for additional immunizations with the respective vaccines, and protection. Five separate trials were performed using double- and/or single-needle injectors. The levels and persistence of vaccine intake, serologic response, vHVT-IBD virus combination with the MD Rispens strain, and/or live NDV vaccination were also assessed. Histopathology and PCR at injection sites showed adequate vaccine intake detected up to 44 days postvaccination. Serologic evidence of vaccine priming was observed, and all vaccinated groups differed (P < 0.05) from the control at different time points. MD, NDV, and IBD protection results after concomitant double-shot single-needle vaccination were near 85%, 95%, and 100%, respectively. Taken together the results indicate no deleterious effects on the efficacy of the vHVT-IBD vaccine monitored by vaccine intake, serologic and challenge results, and combinations after concomitant live/killed vaccination, suggesting the suitability of its use in hatchery vaccination. All types of injectors used as well as injection techniques, vaccines injected separately or together, gave the same results.

Sequence analysis of the Meq gene in the predominant Marek's disease virus strains isolated in China during 2006-2008

The main aim of the present study were to investigate sequence diversity in the Meq gene of Marek's disease viruses (MDV) isolated in China and to determine the most prevalent MDV strains. The 19 MDV strains were isolated from dead or diseased chickens from different chicken farms in China during 2006-2008, and the Meq gene was sequenced from each of these strains. Sequence analysis showed that all of the isolates contained an open reading frame of 1020 nucleotides, which encoded a 339 amino acid peptide. Compared with reference MDV strains, 12 of the 19 MDV isolates possessed two amino acid substitutions, (T → A) at position 139 and (P → R) at position 176, one isolate shared sequence similarity with the attenuated strain CVI988, and five of the other six isolates exhibited one amino acid change (P → T) at position 177 or 176. The 19 MDV isolates shared between 99.0 and 100% nucleotide sequence homology, and between 97.7 and 100% amino acid sequence homology. The nucleotide and amino acid sequence identity between the 19 MDV isolates and the 25 reference MDV strains varied from 97.6 to 100% and 94.4 to 100%, respectively. Based on the phylogenetic relationships between Meq gene sequences, Chinese MDV isolates constituted a separate clade to MDV reference strains, demonstrating that a different genotype of MDV was prevalent in China between 2006 and 2008.

Pathological and immunohistochemical study of chickens with co-infection of Marek's disease virus and chicken anaemia virus

Chicken anaemia virus (CAV) is the most important confounding pathogen in Marek's disease virus (MDV) infection. The effect of CAV co-infection at 4 weeks of age after inoculation of virulent MDV (vMDV, KS strain) or very virulent MDV (vvMDV, Md/5 strain) in 1-day-old chicks was investigated by pathological and immunohistochemical studies. CAV increased the mortality rates induced by vMDV or vvMDV. The packed cell volume was reduced significantly in vMDV-CAV infection; however, no reduction or non-significant reduction was observed in vMDV infection. Bone marrow hypoplasia was related to CAV co-infection and none of the birds inoculated with vMDV or vvMDV had hypoplasia. Severe atrophy of the thymus and bursa of Fabricius was observed in the vvMDV-CAV and vvMDV groups. Complete regeneration of the thymus cortex and bursa of Fabricius in the vMDV group was noted and was in contrast to sequential lymphoid depletion after CAV inoculation in the vMDV-CAV group. The spleen was either regenerated, lymphoid depleted or had lymphoproliferative lesions. Lymphoid depletion in the spleen was not detected in the vMDV group; however, it was prominent in the vMDV-CAV and vvMDV-CAV groups during the first 2 weeks after CAV inoculation. CAV inclusions and antigens were detected in the thymus cortex and spleen of vMDV-CAV and vvMDV-CAV groups during the experiment. Severe depletion of CD8(+) T cells was observed in depleted spleen and thymus. The neoplastic foci appeared around splenic arterioles and venules, and stained mainly by CD4 antibody; however, CD8(+) T cells were singly dispersed or were present in clusters. It could be concluded that CAV was responsible for bone marrow hypoplasia, severe anaemia and hindrance of lymphoid organ regeneration in MDV-CAV co-infection.

Vaccination against Marek's disease reduces telomerase activity and viral gene transcription in peripheral blood leukocytes from challenged chickens

We investigated whether telomerase activity and viral gene transcription were associated with protection against the RB-1B strain of Marek's disease virus (MDV) in chickens vaccinated with Rispens CVI988 or the herpes virus of turkey (HVT). Telomerase activity in peripheral blood leukocytes (PBLs) seemed to be an appropriate marker of lymphoma and levels of viral transcription were correlated with the virulence of MDV strains. Vaccinated protected birds had lower levels of telomerase activity and RB-1B viral gene transcription than unvaccinated chickens infected with RB-1B. The decrease in RB-1B viral transcription was more marked in chickens vaccinated with CVI988 than in those vaccinated with HVT. Indeed, RB-1B viral transcription was not detectable after 14 days post-challenge. In conclusion, telomerase activity and gene transcription in challenge MDV strains are potential new reliable criteria of protection in vaccinated chickens.

Evaluation of Marek's disease field isolates by the "best fit" pathotyping assay

Although determination of the pathotype is central to the study of Marek's disease (MD) field isolates, methods are not standardized and results from different laboratories may not compare well with the original Avian Disease and Oncology Laboratory assay. This study was designed to investigate the validity of the "best fit" pathotyping assay, a simplified method recently described for testing of field isolates of MD virus (MDV). Twenty serotype 1 MDV strains were isolated from 12 breeder and commercial flocks in eight regions of the Russian Federation and were pathotyped by the best fit assay using vaccinated and non-vaccinated chickens from Schelkovo specific pathogen free breeders. Lesion responses induced by field isolates were compared with those induced by reference strains JM/102W, Md5, and 648A representing pathotypes v, vv and vv+, respectively. Based on comparison with reference strains, we determined the pathotype of eight isolates as vv+, 11 isolates as vv and one isolate as v. Lesion responses induced by the three reference strains consistently differentiated the respective pathotypes in non-vaccinated chickens and in chickens vaccinated with FC126 (serotype 3) alone or with a bivalent FC126 + 301B/1 vaccine (serotypes 3 and 2, respectively). Variation between reference strain responses in replicate trials was minimal. In some cases, calculation of the proportional distance between pairs of reference strains aided in the classification of field isolates. These results indicate that the "best fit" pathotyping assay can be conducted with local chicken strains and, in the absence of statistical analysis, provides pathotype designations that are consistent with those obtained by the Avian Disease and Oncology Laboratory method. In addition, the pathogenicity of Russian isolates appeared comparable with that of United States isolates.

Classification of Marek's disease viruses according to pathotype: philosophy and methodology

The concept of pathotype in Marek's disease (MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv+ pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable.The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens (15x7 ab+), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A (American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease (both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria (such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit" comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria (see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.