Age related resistance to avian leukosis virus. III. Infectious virus, neutralising antibody and tumours in chickens inoculated at various ages

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Abstract
Viraemia and neutralising antibodies were determined in chickens of six age‐groups following inoculation with leukosis virus of subgroups A and B at the age of 1 day, and 2, 4, 6, 8 and 10 weeks respectively. The birds were kept in a filtered air positive pressure (FAPP) house. A seventh age‐group, accommodated in a separate FAPP‐house, was used as an untreated control. Serum samples, received at biweekly intervals between 1–17 weeks post‐inoculation, from birds of the groups inoculated at 4, 6, 8 and 10 weeks of age, showed at 1 week post‐inoculation a transient viraemia followed by neutralising antibodies at the later sampling times. Neutralising antibody to subgroup A virus was detected in nearly all birds tested; this was not so for antibody to subgroup B. In all four groups the average titre of the former antibody was higher than that of the latter. Midway through the laying period birds of each group inoculated with leukosis virus, and some of the uninoculated controls, were challenged by infection with either subgroup A or B virus. At termination of the experiment survivors from each group were tested for the presence of leukosis virus. The virus recovery was performed with plasma samples, white blood cell preparations and expiant cultures of various organs. The plasma samples were all negative; the great majority of blood cell specimens received from birds inoculated early with leukosis virus were positive, whereas the majority of the preparations from the birds inoculated later remained negative. The organ expiants from the two youngest age groups were mostly leukosis virus‐positive, from the birds inoculated at 4 weeks of age the spleen and kidney expiants contained leukosis virus whereas in the groups inoculated at 6, 8 and 10 weeks of age only the spleen expiants of birds challenged with subgroup A virus In a subsidiary experiment, started 4 months after the challenge infection, four birds from each group (two challenged with leukosis virus of subgroup A and two with subgroup B) were accommodated in isolators. The birds were challenged again, this time with Rous sarcoma virus (RSV) of the homologous subgroup used for the previous challenge. The tests for virus just prior to the challenge showed leukosis virus only in the white blood cell preparations from the birds in the three youngest age groups; the birds from the older groups were virus‐negative. The serological tests after challenge showed neutralising antibodies to both subgroups in birds of nearly all groups. Tumour formation at the site of injection was mainly observed in the chickens challenged with RSV of subgroup B. The virological and serological results as well as the tumour response show that the immune system of birds between 0–4 weeks of age is insufficiently developed to cope with a controlled exposure with leukosis virus, whereas in birds of 4–10 weeks of age an adequate immunological response has developed. The significance of the presence of leukosis virus in sera, plasma, white blood cell preparations and organ expiant cultures is mentioned. In programmes for the control of lymphoid leukosis in reproductive stock the use of information on virus and neutralising antibodies is recommended.