Medical Definition of avian leukosis
: any of a group of diseases (as lymphoid leukosis) of poultry that are caused by strains of a retrovirus (species Avian leukosis virus of the genus Alpharetrovirus), that involve disturbed blood formation, and that are distinguished individually by special manifestations (as paralysis, tumor formation, leukemia, and eye damage)—called also avian leukosis complex
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