Pathogenesis of pf Marek's disease in old chickens: lesion regression as the basis for age related resistance

Chickens of various age levels, free from prior infection, were simultaneously exposed to Marek's disease virus, and the response of each age group was recorded. Four and 20 week old chickens of lines 15x7 and CM (commercial source) had substantial resistance to mortality and gross lesions. In contrast, in line 7, which was tested at 1 day, 2, 4, 8, 12 and 16 week age levels, 4 week old chickens were fully susceptible to clinical Marek's disease (MD), although resistance was demonstrated at 8 week and older age levels. Genetically resistant chickens of line 6 maintained their resistance at all age levels tested. Pathogenesis of MD was compared in 12 week old and 1 day old chickens of line 15x7. Within the 1 day old group, 23% of the chickens died because of MD, whereas there were no deaths in the 12 week old group. Both groups developed viremia although duration, incidence, and levels of virus in the 1 day old group were higher than in the 12 week old group. Although initially the 12 week old group responded by producing higher levels of antibody, the long term incidence of agar gel precipitin, immunofluorescent, and virus neutralization antibody in the two groups was similar. Gross and microscopic lesions of MD developed in both groups, but lesions regressed in the 12 week old group and persisted in the 1 day old group. It is concluded that age resistance to MD was expressed through lesion regression.