Progress towards eradication of lymphoid leukosis viruses--a review.

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.