Age related resistance to avian leukosis virus. II. Influence of age at inoculation on mortality and congenital transmission

Six age groups of specified pathogen free White Leghorn chickens housed in the same filtered air, positive pressure building, were inoculated at 1 day, 2, 4, 6, 8 and 10 weeks of age respectively, with a mixture of leukosis viruses of subgroups A and B. Birds which died during the experiment were examined for gross and microscopical lesions. The incidence of lymphoid leukosis (LL) in the various groups was inversely proportional to the age of first virus exposure, Le., mortality was 62.5% in the group inoculated at 1-day-old and decreased to zero in the group inoculated at 10 weeks of age. In the age group inoculated at 8 weeks, only one chicken succumbed to the disease. Congenital transmission of leukosis virus (LV) was detected only in chickens inoculated immediately after hatching or at 2 weeks of age. In the latter group, all leukosis virus positive embryos were derived from one hen only. A seventh age group, housed in a separate filtered air, positive pressure building, served as uninoculated controls. Challenge infection was performed on the same day with chickens of all groups at ages varying from 59 to 71 weeks. Testing of pooled embryo extracts collected between 1 and 15 weeks post challenge did not change the virus shedding pattern. Even the chickens of the control group which were challenged (for the first time) at 71 weeks of age remained negative for congenital transmission. The results of this experiment show that the chickens inoculated with LV at 4 weeks of age or older had a low incidence of LL (decreasing to zero when the chickens were inoculated at 10 weeks of age) but did not congenitally shed virus.