[Methods for the control of avian tumour virus diseases (author's transl)].

Methods developed in the Netherlands for the control of avian tumour virus diseases, Marek's disease and lymphoid leukosis, are discussed. Marek's disease is successfully kept under control by vaccination of all day-old chickens raised for egg production. Broilers are not vaccinated. During 1975 over 23 million chicks were vaccinated with cell-associated Marek's disease virus CVI 988 vaccines and about 3 million doses of HVT vaccine were administered, mostly in lyophilised form. The mechanism of the immunity conferred by the MD vaccines is discussed. Lymphoid leukosis was successfully controlled in three inbred lines of White Leghorn chickens and in a commercial grand parent flock of White Plymouth Rocks. The control method that can be applied to flocks in the field is based on three elements: From an infected flock hens are selected in whose eggs no lymphoid leukosis viruses can be detected in pooled extracts of groups of embryos. Only eggs from hens that demonstrably do not shed virus congenitally to their embryos are used for the production of progeny. The offspring are reared in isolation until two months of age at which time the age related resistance against tumour formation appears to be sufficiently developed. The chickens are subsequently inoculated intramuscularly with lymphoid leukosis viruses of subgroups A and B and transferred to a conventional chicken house. The inoculated birds become persistently viremic, resist intramuscular challenge infections or exposure by contact, but produce virus-negative eggs during the laying period. For lymphoid leukosis, congenital infection is considered the usual mode of virus transmission. Horizontal virus transmission becomes important when virus-free chickens, reared in isolation for two months, are exposed in field conditions without previous "vaccination". During the egg-laying period of these flocks, virus shedding was observed in a similar percentage as in flocks with congenitally infected birds.