An age-related resistance of chickens to Marek's disease: some preliminary observations

Chickens of 2 strains, 20 to 22 weeks of age and free of prior infection, were substantially more resistant than l-day-old chicks to mortality and tumor induction caused by exposure to Marek's disease virus. These older chickens were refractory to inoculation with up to 10(4) chick tumor-inducing doses of Marek's disease virus and, following contact infection, lesion frequencies were 8 to 30% of corresponding responses in chick controls. Resistance in contact exposed chickens was observed as early as the 8th week and, although variable, did not increase appreciably through the 20th week. Median latent periods to death were similar regardless of age at exposure. Older birds were fully susceptible to infection with Marek's disease virus and were only slightly resistant to development of microscopic lesions, thus suggesting that this resistance might be 'mediated through enhancement of lesion regression. Age-related resistance was confirmed to be unrelated to previously described tumor resistance caused by prior infection or genetic constitution: