Katja Linher

Marek’s disease (MD) is a common lymphoproliferative disease of chickens. The causative agent, a herpesvirus, has been successfully attenuated, leading to the development of vaccines that have dramatically reduced losses associated with MD. However, vaccine breaks are on the rise due to an increase in the pathogenicity of the virus, MDV. The fact that vaccine efficacy is greatly improved by host genetic resistance, coupled with the identification of markers associated with resistance to MD, is attracting the attention of poultry producers. In this study, substrains of chickens developed from strain 7 for contrasting growth hormone (GH) and growth hormone receptor (GHR) genotypes were used to investigate the effects of the growth hormone axis on the early stages of MD infection in lymphoid tissues. A quantitative PCR assay was developed to measure viremia in three-week-old chicks challenged with the virulent RB1-B strain of MDV. Analysis of DNA extracted from the thymus, bursa, and spleen 6 days post-infection demonstrated an average of 0 to 5 viral genome copies per host cell equivalent (ca. 1x10⁵ molecules), depending on the chicken and the tissue sample. Statistical analysis revealed that the thymus had a higher virus to cell equivalent ratio in both genotypes under investigation. In addition, a significant correlation was established between the viral titers in the thymus and the titers in the bursa and the spleen, but not between the bursa and spleen. This indicates the progression of MDV infection from the bursa to the thymus to the spleen, which is in agreement with the established pathogenesis of the virus. A trend for lower viremia exists in the GHR-|GH-/- genotype, while the mean viral titer was found to be twice as high in chicks of the GHR+|GH+/+ genotype. Analysis of the data revealed that the GHR-|GH-/- genotype is associated with a higher antibody response. Based on the number of viral genome copies per cell equivalent, the RB1-B strain of MDV appears to have entered the latent stage of infection at 6 days post-infection. This is in agreement with previously reported findings that MDV enters latency between 6 and 8 days post-infection, depending on the viral strain and host genetic resistance. Further experiments with a larger sample size will add statistical significance to the current results. In addition, a new experiment will be performed to measure viremia in chicks of the two different genotypes over a time course, starting at day 0 post-infection and ending at 15 days post-infection.

References

Calnek, B.W., and R.L. Witter, 1997. Marek's disease. In B.W. Calnek and R.L. Witter (eds.). Diseases of Poultry, 10th ed. Iowa State University Press, Ames, IA, pp. 369-413.

Witter, R.L., 1998. Control Strategies for Marek’s Disease: A Perspective for the Future. Poultry Science 77: 1197-1203.

Clementi, M., S. Menzo, P. Bagnarelli, A. Manzin, A. Valenza, and P.E. Varaldo, 1993. Quantitative PCR and RT-PCR in Virology. PCR Methods and Applications 2: 191-196