Genetic Susceptibility to and Presence of Endogenous Avian Leukosis Viruses Impose No Significant Impact on Survival Days of Chickens Challenged with Very Virulent Plus Marek’s Disease Virus

Chicks of distinct genotypes at the tumor virus B locus (TVB) in combination with presence or absence of endogenous avian leukosis virus ev21 gene in their genomes were examined for survival day patterns after challenge with very virulent plus Marek’s disease virus (vv+MDV) in three consecutive trials under controlled conditions. The distinct genotypic groups of the challenged birds were TVB*S1/S3 & ev21+/-, TVB*S3/S3 & ev21 +/-, TVB*S1/S3 & ev21-/-, and TVB*S3/S3 & ev21-/with a frequency of approximately one quarter each. Chickens with the genotype TVB*S1/*S3 are genetically susceptible to the subgroup E avian leukosis viruses; and those with the TVB*S3/*S3 genotype are resistant. Marek’s disease (MD) was diagnosed in all birds within 8 weeks post-challenge with a partially attenuated vv+MDV strain (648A passage 40), despite the fact that all chicks challenged were homozygous for MHCB*21 haplotype, which is known to be relatively resistant to MD. No significant difference in survival day patterns was detected between the chicken genotype groups (P> 0.05). The survival day pattern difference was only detected between the challenged and the unchallenged control groups in all three trials (P< 0.001). The findings from this study suggest the genetic nature in resistance or susceptibility to subgroup E avian leukosis viruses in combination with the presence or absence of the endogenous avian leukosis virus ev21 gene in the chicken genome are not capable of altering MD incidence nor distorting survival day patterns in chickens like those White Leghorns used in this study under the specific challenge conditions. If any influence of avian leukosis virus on MD exists, it may be heavily dependent on genetics of chickens and challenge conditions including varied virulence of MDV.