Age‐related resistance to Avian Leukosis virus I. Influence of age at exposure on mortality and congenital transmission

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Abstract
Groups of White Leghorn chickens were inoculated at 1 day and at 2, 4, 6 and 8 weeks of age respectively with a mixture of leukosis viruses of subgroups A and B. The five infected groups were kept in a filtered air positive pressure house. A sixth group was accommodated separately in a similar house as a control. All birds which died or were removed were subjected to pathohistological examination; diagnosis of lymphoid leukosis was made upon either gross lesions plus microscopical lesions or microscopical lesions only. The incidence of lymphoid leukosis in the infected groups appeared inversely proportional to age of infection, i.e. the mortality due to lymphoid leukosis decreased from 54.3% in the group infected at 1‐day‐old to 7.4% in the group infected at 8 weeks of age. Prevalence of leukosis in the latter group may be attributed to a small number of chicks already infected vertically with the virus. Congenital transmission of leukosis virus was demonstrated in embyros in the groups infected at 1‐day‐old, 2, 4 and 6 weeks of age. In the latter group congenital transmission was extremely low; from 214 pooled embryo extracts (1007 embryos) only 2 (0.9%) contained leukosis virus. In the group infected at 8 weeks of age no virus was detected in the embryos. Congenital transmission of leukosis virus appeared to be related to age of infection, i.e. early infection went parallel with a high rate of transmission. The pattern of congenital transmission was erratic and the number of hens shedding leukosis virus continually was small. From the results in this trial it is concluded that both lymphoid leukosis and congenital transmission occur rarely if chickens (born free of leukosis virus) are kept free from infection during the first 6–8 weeks of life and subsequently are injected with a relatively high dose of leukosis virus.