Abstract
Although lymphoid leukosis (LL) was first described in 1868 (1), the economic significance of the disease has only recently been recognized. Mortality associated with LL tumors, the most common visible manifestation of avian leukosis virus (ALV) infection, is low and occurs mostly in older hens (2,3,4,5,6). Therefore, it was generally believed that the economic damage caused to the poultry industry by ALV infection was small, even though there was some suspicion, based on industrial observations that ALV infection impaired production traits (7). Higher mortality from ALV was observed only rarely, in flocks with a prevalence of congenitally infected chickens (8,9). The situation changed after it was shown that non-neoplastic effects of ALV infection result in serious economic losses. Indications of such effects appeared in 1978 (10) and 1979 (11), followed by assessment of the size of such effects in White Leghorns in 1980 (12). Subsequently, several more studies, discussed in detail below, further elucidated the changes in performance associated with ALV infection.
Published Version
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