Figure 1.

Transmission routes of ALVs, and differences between exogenous and endogenous ALVs. Adult birds can transmit the virus through the egg, generally considered as vertical transmission; the consequences are different depending on whether it is in the albumen or yolk (congenital transmission) or through the germline. The three different states of the viral genome are shown: RNA in the viral particle, proviral DNA in the somatic cell (characterized by the presence of LTRs flanking the provirus), and endogenous retrovirus (ERV) in the germline (which usually do not express functional proteins) (adapted from [1]).