Researchers at the University of Rochester Medical Center say that this is the first time a virus has been shown to become part of the human DNA and then get passed to subsequent generations.
This unique mode of congenital infection may be occurring in as many as 1 of every 116 newborns, and the long-term consequences for a child's development and immune system are unknown.
"At this point, we know very little about the implications of this type of infection, but the section of the chromosome into which the virus appears to integrate is important to the maintenance of normal immune function," said Caroline Breese Hall, M.D., professor of Paediatrics and Medicine at the University of Rochester Medical Center, and author of the study.
"With further study, we hope to discern whether this type of infection affects children differently than children infected after birth," Breese added.
HHV-6 causes roseola, an infection that is nearly universal by 3 years of age. The typical roseola syndrome produces several days and up to a week of a high fever and may have variable other symptoms including mild respiratory and gastrointestinal symptoms.
HHV-6 virus is a closely related virus to CMV, and the congenital infection rate of CMV is similar to that of congenital HHV-6 - about 1 percent.
However, the new study shows that a congenital HHV-6 infection differs greatly from a congenital CMV infection in that it is often integrated into the chromosomes of the baby rather than passed through the placenta.
For the study, researchers enrolled 254 kids between July 2003 and April 2007, out of which 43 had congenital HHV-6 infections based on cord blood samples. Of 211 children without congenital infection, 42 were children who acquired an HHV-6 infection during the study.
The researchers found that of the infants who had congenital infections, 86 percent of them (37) had the virus integrated into their chromosomes. Only six of the congenitally infected babies were infected by the mother through the placenta .
Kids who had integrated HHV-6 had higher levels of virus in the body than those who were infected through the placenta.
HHV-6 DNA was found in the hair of one parent of all children with integrated virus with available parental samples (18 mothers and 11 fathers), which means the children acquired the integrated infections through their mother's egg or father's sperm at conception.
The virus's DNA was not found in hair samples of parents of children who were infected after birth.
The study has been published in Paediatrics.