Lymphoid leukosis virus infection: effects on production and mortality and consequences in selection for high egg production.

Lymphoid leukosis (LL) is virus-induced, lymphoblastic malignancy of chickens that can be congenitally transmitted. Mortality from LL is generally low. Effects of LL virus (LLV) on production and mortality were investigated in approximately 2000 Leghorn pullets in each of two consecutive years. The pullets were from nine strains developed in Ottawa, of which three were unselected control strains and six were strains under selection for up to 27 generations for high egg production and a complex of related commercially important traits. The overall frequency of birds shedding LL virus of gs antigen into eggs (LL-S) was significantly lower in the selected strains (3.9%) than in the control strains (18.5%), indicating that LLV may have negative effects on production and cause elimination of LL-S birds by selection. Such significant effects were indeed detected: the LL-S pullets produced to 497 days of age in 1976 and 1977, respectively, 30 and 25 eggs less per hen-housed than the nonshedders. The LL-S birds matured sexually later, produced smaller eggs at a lower rate, and their eggs had a lower specific gravity, indicating thinner shells. Mortality from all causes to 497 days was significantly higher in LL-S birds (+14.8%) in 1976. In 1977 the increase (+5.5%) did not reach statistical significance. In both years the mortality from LL itself remained very low. In another study, eggs from one of the control strains were incubated and hatched from the dams were 291 and 483 days old. The eggs from LL-S dams had 2.4% lower fertility and 12.4% lower hatchability. The effects on hatchability were more pronounced in the older dams. Since the lower production of LL-S birds results in a lower frequency of such birds in strains selected for high egg production, it is suggested that a part of the difference between the performance of the selected and control strains (delta S) is due to reduction in the frequency of LL-S birds (delta L) rather than due to true genetic gain. In this study, the size of delta L relative to delta S was estimated at 4 to 14% for egg production and 3 to 7% for egg weight. The negative effects of LLV infection on egg production, mortality, hatchability, and genetic gains show the desirability of producing chickens free of LLV infection.