Further studies on the eradication and epizootiology of lymphoid leukosis virus infection in a commercial strain of chickens.

The feasibility of eradicating exogenous lymphoid leukosis virus (LLV) from a commercial egg-layer grandparent strain of chickens was examined by selecting replacement chicks from hens expected to have a reduced likelihood of transmitting LLV, followed by testing of progeny chicks and removal of those showing evidence of infection, together with their early contacts. Chicks from unselected hens were hatched and reared separately for comparison. The selected chicks came from hens negative for LLV in vaginal swabs and for LLV and/or group-specific antigen (gsa) in egg albumen, but at hatching 2.5% were infected by LLV in cloacal swab tests compared with 4.1% of unselected chicks, an insignificant difference. The infected chicks from the selected group, and their cage mates, were culled, leaving 85 hens of which seven were identified at different times as being infected and were removed. At 70 weeks 75 hens remained free of LLV infection. Of 369 eggs laid by the selected hens, 0.3% contained LLV in the albumen, and none contained gsa in albumen or LLV in corresponding embryos. Chicks hatched from the selected group were free of LLV infection. LLV infection was allowed to spread naturally in the unselected hens and 85.4% were infected at 70 weeks. Of these hens, 60.0% produced eggs with LLV in albumen, 41.2% with gsa in albumen, and 32.9% with LLV-infected embryos. Of 509 eggs from these hens, 25.1% had LLV in albumen, 30.5% had gsa in albumen, and 10.4% had LLV-infected embryos. In confirmation of other studies, shedding of LLV to embryos was associated with presence of LLV in vaginal swabs and serum. Analysis of associations within eggs between LLV and gsa in the albumen, and LLV in the embryo, provided strong evidence that virus in the oviduct was the cause of embryo infection; no firm evidence was obtained for the occurrence of embryo infection by a non-oviduct route. Infection of cocks in the selected and unselected groups was similar to that in hens in the respective groups. Some cocks used to inseminate unselected hens had infected semen, but this did not influence the frequency of embryo infection. In a comparison of eight tests on hens or eggs from the unselected group for selection of embryos with decreased probability of being infected with LLV, the procedures most likely to be used under practical conditions, namely classification of hens according to LLV in vaginal swabs or gs-antigens in egg albumen, ranked third and fifth respectively, and did not differ from the highest ranking procedure. Age at first egg was significantly earlier, and hen-housed egg production tended to be better, in selected hens compared with unselected hens, but environmental differences other than infective status may have been responsible. In comparisons within the unselected group, hen-housed and hen-day egg production were significantly poorer in viraemic tolerant hens than in immune hens or uninfected hens.