Hens from a commercial source were selected because they were infected with lymphoid leukosis virus (LLV). LLV was detected in vaginal swabs from 17 viremic hens and from 27 of 44 hens that were not viremic. All hens that were positive on the vaginal swab test (VST) produced one or more eggs with virus in albumen or in embryos, whereas in comparable tests, virus was detected only in eggs from 5 of 17 hens that were negative on VST. Congenital transmission of LLV was erratic and neither the VST nor tests for virus in egg albumen prior to incubating eggs identified all hens that transmitted infection. For example, 14 hens negative on VST produced 50 eggs negative for virus in albumen and yet one of the embryos from these eggs was infected. Eggs from other hens had infectious virus in albumen and about half of the embryos from these were infected. Tests for virus in cloacal swabs from one-day-old chicks were as sensitive as tests on embryos for detecting congenital transmission. Titers of LLV in the meconium of congenitally infected chicks were as high as 10(7) infectious units per ml. The cloacal swab test should be a valuable adjunct to the VST and tests on egg albumen in programs designed to eradicate lymphoid leukosis from chickens.

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More than 20 years have now elapsed since technology was developed for producing chickens free of infection with exogenous lymphoid leukosis virus (LLV). However, it is only in recent years that commercial poultry breeders have initiated programmes to reduce the prevalence of infection in their stocks. This review considers advances that make large scale eradication feasible, even though methods for detecting infection and thus breaking the cycle of virus transmission are not completely effective. Congenital transmission of LLV occurs before eggs are incubated. Although chickens infected in this manner shed virus throughout their lives in faeces, saliva and remnants of cornified cells from skin, horizontal spread is slow. Nevertheless, horizontal transmission is important since it frequently results in persistent low level infections that are difficult to detect. Since horizontally infected dams are often erratic in congenitally transmitting virus, the prevalence of hens in a flock that have the potential for congenital transmission may be markedly higher than the actual rate of such transmission. Naturally infected chickens may have generalised infections, but even in these there are localised sites within certain organs that are prone to production of complete virus particles. While LLVs are usually considered avirulent at the cellular level, myocardial cells from some adult chickens may contain intracytoplasmic viral matrix inclusion bodies that are accompanied by swelling of these cells. Myocardial lesions may be one of many factors that contribute to reduced egg and meat production associated with subclinical LLV infections. While the enzyme-linked immunosorbent assay for group specific viral antigen in egg albumen appears to be the most efficient method for detecting dams that congenitally transmit virus, no combination of test procedures has proven 100% effective in identifying infected chickens. Rearing newly-hatched chickens in small isolated groups for 6 to 8 weeks would allow infection to spread among those in direct contact and this should facilitate identification of infected groups. Management practices applied by the poultry industry to maintain flocks free of mycoplasmas and other pathogens should be adequate for the control of LLV.

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SummaryAnimal production efficiency, and product volume and quality can be greatly increased by reducing disease losses. Genetic variation, a prerequisite for successful selection, has been found in animals and poultry exposed to a variety of viral, bacterial and parasitic infections. Breeding for disease resistance can play a significant role alone or in combination with other control measures including disease eradication, vaccination and medication.Feasibility of simultaneously improving resistance to specific diseases and production traits has been demonstrated. However, selection for specific resistance to all diseases of animals and poultry is impossible. Development of general disease resistance through indirect selection primarily on immune response traits may be the best long‐term strategy but its applicability is presently limited by insufficient understanding of resistance mechanisms. Another hindrance may be negative genetic correlations among various immune response functions: phagocytosis, cell mediated and humoral immunity. To better assess the feasibility of increasing general disease resistance by indirect selection we must obtain estimates of heritability for immune response, disease resistance, and economic production traits, as well as genetic correlations among these traits.The present level of disease resistance in farm animals resulted from natural selection and from correlated responses to selection for production traits while the influence of artificial selection for resistance was minimal. Future research should be directed towards developing and applying breeding techniques that will increase resistance to diseases without compromising production efficiency and product quailty. This will require cooperation of immunogeneticists, veterinarians and animal and poultry breeders. Significant progress in the improvement of resistance to diseases may result from the application of new techniques of molecular genetics and cell manipulation.