Lymphoid Leukosis Virus Infection: Effects on Production and Mortality and Consequences in Selection for High Egg Production

Abstract

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.

Evaluation of a commercial avian leukosis Elisa kit for detecting hens which shed leukosis virus to their progeny

“…The benefits expected from such programmes are the prevention of losses from leukosis and non-neoplastic disorders and improved performance (Gavora et al, 1980;. This method of disease prevention has become feasible as a result of two developments (1) recognition that hens that produce congenitally infected chicks have LLV in vaginal swabs or in their egg albumen, and (2) improvements in the techniques available for large scale testing for LLV (Spencer et al, 1977;Clark et al, 1981 ;Fadly et al, 1981 ;Payne et al, 1982).…”

Hatchability and mortality following infection of chick embryos with subgroup a rous sarcoma virus

“…Under field conditions LLV-A is the most common agent; it induces lymphoid leukosis at a low mortality rate (5%), and also reduces the overall performance of the flock because of reduced hen-housed egg production, delayed sexual maturity, poor yolk index and, above all, a significantly higher percentage of non-specific deaths (Spencer et al, 1978(Spencer et al, , 1979Gavora et al, 1980;Garwood et al, 1981;Payne et al, 1982;Romero et al, 1983). These research reports have aroused considerable interest amongst commercial poultry breeders in eradicating LLV-A from chicken flocks in order to improve their performance.…”

Performance of meat‐type chickens test‐positive and ‐negative for lymphoid Leukosis virus infection1

“…It has been shown previously (Spencer et al, 1979) that there is good agreement between the tests for the gs antigen and LLV. The fact that gs antigen is shed into egg albumen with a high degree of consistency is documented by the good agreement between tests on eggs collected from the same hens around 250 and 470 days of age (Gavora et al, 1980). Cloacal swabs were obtained from the 42-day-old crosses of the primary breeding stocks hatched in 1979 and they were used for detection of LLV by the phenotypic mixing test (Okazaki et al, 1975).…”

“…Recently, however, Gavora et al (1980) reported that egg production was significantly reduced (25 to 30 eggs per hen housed) in Leghorn hens shedding LL virus (LLV) into eggs. This reduction resulted from later sexual maturity, higher mortality from causes other than LL, and lower egg production rate.…”