Influence of vaccination with avirulent herpesvirus on subsequent infection of chickens with virulent Marek's disease herpesvirus.

Vaccination of chickens with turkey herpesvirus (HVT) or attenuated Marek's disease herpesvirus (aMDHV) blocked infection with virulent MDHV (VMDHV) for approximately 5 weeks after contact exposure. However, there was no apparent blockage of infection when challenge virus was administered intraabdominally (IA). Evidence for infection with VMDHV was based on viral isolation by in vivo assay or by detecting precipitins to "A" antigen associated with virulent virus. The HVT stimulated production of neutralizing antibody against VMDHV in a high percentage of chickens, whereas the aMDHV was a comparatively poor inducer of such antibody. Despite this difference, both of the vaccinal viruses conferred protection against development of Marek's disease.     

Molecular characteristics of Polish field strains of Marek's disease herpesvirus isolated from vaccinated chickens

Background

Twenty-nine Marek''s disease virus (MDV) strains were isolated during a 3 year period (2007-2010) from vaccinated and infected chicken flocks in Poland. These strains had caused severe clinical symptoms and lesions. In spite of proper vaccination with mono- or bivalent vaccines against Marek''s disease (MD), the chickens developed symptoms of MD with paralysis.Because of this we decided to investigate possible changes and mutations in the field strains that could potentially increase their virulence. We supposed that such mutations may have been caused by recombination with retroviruses of poultry - especially reticuloendotheliosis virus (REV).

Methods

In order to detect the possible reasons of recent changes in virulence of MDV strains, polymerase chain reaction (PCR) analyses for meq oncogene and for long-terminal repeat (LTR) region of REV were conducted. The obtained PCR products were sequenced and compared with other MDV and REV strains isolated worldwide and accessible in the GeneBank database.

Results

Sequencing of the meq oncogene showed a 68 basepair insertion and frame shift within 12 of 24 field strains. Interestingly, the analyses also showed 0.78, 0.8, 0.82, 1.6 kb and other random LTR-REV insertions into the MDV genome in 28 of 29 of strains. These genetic inserts were present after passage in chicken embryo kidney cells suggesting LTR integration into a non-functional region of the MDV genome.

Conclusion

The results indicate the presence of a recombination between MDV and REV under field conditions in Polish chicken farms. The genetic changes within the MDV genome may influence the virus replication and its features in vivo. However, there is no evidence that meq alteration and REV insertions are related to the strains'' virulence.     

Indications of immunodepression in chickens infected with various strains of Marek's disease virus

The effect of infection by various strains of Marek's disease virus (MDV) on the immune function of 3-week-old chickens was examined. MDV strains of low (CU-2, RB-7) and high (RB-3, MD-5, and MD-11) pathogenicity were compared with prototype JM-10 strain of moderate pathogenicity. Mortality, whole body weight, relative weights of lymphoid organs, histopathology, humoral antibody responses to thymus-dependent and -independent antigens, and in vitro lymphocyte responses to mitogen stimulation were investigated at 1, 2, and 3 weeks postinfection. MDV strains of high pathogenicity significantly depressed responses at 3 weeks postinfection, seeming to indicate the ability of these viruses to induce severe immunodepression. However, the fact that the moderately pathogenic and even some of the low-pathogenicity strains induced immunodepression suggests that other viral mechanisms are also important in its determination.     

Kinetics of phytohemagglutinin response in chickens infected with various strains of Marek's disease virus

Phytohemagglutinin (PHA) response of whole blood lymphocytes from white leghorn inbred line 7(2) chickens infected with various strains of Marek's disease virus (MDV) was monitored sequentially for 6 weeks postinfection. A significant difference between JM and GA strains was shown. A two-phase depression in the PHA response was observed in chickens infected with the JM strain. Early depression occurred 1 week postinfection and was followed by recovery a week later. The second depression occurred at 4 weeks postinfection and lasted until the end of the experiment. The GA strain-infected group, on the other hand, began to show depression 4 weeks postinfection, and most chickens died within a short time thereafter. PHA response of chickens infected with strain Md11/75C, attenuated in cell culture from highly virulent strain Md11, was almost the same as that of control chickens.     

Marek's disease vaccination of chickens

Interpretive summaries of interesting articles on veterinary pharmacology are provided by the Pharmacology Chapter of the Australian College of Veterinary Scientsts     

Mortality of one-week-old chickens during naturally occurring Marek's disease virus infection

Marek's disease (MD) is a disease of chickens that occurs worldwide and has serious economic consequences. MD can present as one of several forms, with the most commonly occurring forms being the lymphoproliferative diseases. Under experimental conditions, an early mortality syndrome has been recognized following infection by some but not all strains of MD virus (MDV). This is the first report of a confirmed case of mortality due to naturally occurring MDV infection in 1-week-old, nonvaccinated, chickens. Necrotizing lesions were observed in the bursa of Fabricius, lung, duodenum, jejunum, and proventriculus, and large intranuclear inclusion bodies were a striking feature in tissues with lesions in all birds. Immunohistochemical staining for the pp38 protein of MDV revealed abundant pp38 antigen in the affected tissues, confirming the presence of MDV within the lesions. PCR yielded an amplicon with 97% homology to the meq gene of MDV. No evidence of co-infection by either of the immunosuppressive agents chicken anemia virus and infectious bursal disease virus was detected.