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Abstract
Madam:-The infection of hens with viruses of the avian leukosis virus (ALV) groups are an economic problem to layer and broiler poultry breeders in most areas of the world. This is because the infection is associated with a delay in sexual maturity, reduced egg production, smaller and thinner eggs, reduced body weight and an increase in mortality from causes other than lymphoid leukosis. (Citation1) These viruses are retroviruses and as such have the ability to integrate their viral genome into that of the host cell and cause tumours in various avian tissues. The virus can be vertically inherited when it is integrated into the germ line; this is termed an endogenous virus (e.v.). A more common mode of infection is however horizontal. While the core nucleocapsid protein or group specific (gs) antigen of the virus is common to all ALVs, the antigenic properties of the outer viral envelope are used to divide them up into five subgroups (A-E). It is worth noting that the subgroup E viruses are all endogenous viruses. They have not been associated with any disease and are not considered to be of economic significance. (Citation2)(Citation3)(Citation9)