Proviral load and expression of avian leukosis viruses of subgroup C in long-term persistently infected heterologous hosts (ducks)

Proviral DNA load and expression of avian leukosis viruses of subgroup C (A LV-C) in ducks infected in mid embryogenesis were studied using quantitativ e PCR, RT-PCR, in situ hybridization employing ALV-specific riboprobe ducks was selected for the study and compared to control reviremic animals in or der to obtain information about persisting retroviruses in different duck t issues. A widespread distribution of proviruses in the tested tissues was f ound, but the proviral load was significantly lower in non-reviremic in com parison to reviremic animals. The only exception were brain and blood cells , in which no significant difference in the quantity of integrated provirus es was found between both categories of ducks, thus indicating an exception al position of the brain and blood cells among all tested tissues. Contrary to reviremic, the proviruses were not transcribed in non-reviremic ducks, with the exception of brain and thymus. In the majority of non-reviremic du cks viral RNA was revealed in the brain, but no infectious virus could be r ecovered from this tissue. The opposite situation was observed in the thymu s, where infectious virus was recovered but viral RNA remained below the de tection limit of the assay. As revealed by in situ analysis, infected cells were either disseminated or focally distributed in tissues. From the long- term follow up of ALV-C in intraembryonally infected ducks we conclude that this model is suitable for the study of retrovirus persistence accompained both by the presence and absence of reviremias. The possible consequences of transmission and long-term persistence of retroviruses in the heterologo us host for retroviral evolution are discussed.