Thomas Leitner, Francesca Salvatori and Gabriella Scarlatti

Mother-to-child transmission displays a special case of transfer of an infectious agent. The passage of the agent can take several routes into its new host, and it may do so during a long period of time. Here, we report on a case where at least three different HIV-1 variants were transmitted from mother to child. We have characterized the natural history of the child and the evolution of the virus from prepartum to death using phylogenetic and biological analyses. The transmission occurred on at least two different time points, one during early pregnancy of X4 virus and another close to or at delivery of R5 virus. During the course of the infection three subpopulations, established by the individually transmitted maternal variants, competed with each other in the blood compartment. Immediately after birth only a monophyletic R5 population could be detected, but already after one month, the X4 virus emerges and establishes a second subpopulation. Towards the end of the disease progression, at 33 months, subpopulation two completely takes over and becomes rather homogeneous. At 38 months the child dies. Due to the competition between the subpopulations apparent biological characters of the virus shift in the child. It is interesting to speculate whether the competition between distinct subpopulations expanded the genetic possibilities of the virus, rapidly adapting it to become more pathogenic, and apparently aggravated the disease progression.