Mother-to-child transmission of the virus that causes AIDS is one of the major ways HIV is spread. But it's been unclear how untreated, pregnant women pass the deadly virus on to their babies, or why some newborns are infected and others are not. A new study suggests that infection of newborns by HIV-positive mothers occurs during labor, which might be avoided by caesarian section.
Transmission of HIV to babies from their untreated, infected mothers is an enormous problem worldwide. It is estimated up to a half million newborns contract the virus this way each year.
Because a lot of blood is involved in the birthing process itself, it's always been assumed that babies become infected as they pass through the birth canal. But that doesn't explain why only some newborns become HIV-positive.
Scientists also say the infants don't become infected as fetuses because the placenta, or sack, that nourishes them also provides a barrier against maternal disease-causing organisms.
So, Steve Meshnick of the University of North Carolina in Chapel Hill and colleagues reasoned the infants must somehow become infected during labor.
"And so what we were proposing was that there might be very small tears in that barrier during the process of birth that may allow the mother's and baby's blood to mix, and that that may be the route by which the virus goes from the mother to the baby," Mr. Meshnick said.
Researchers call these microtransfusions.
To find out whether these tiny tears in the placenta might be responsible for HIV infection in newborns, an international team of researchers studied 149 HIV-positive, pregnant women in Malawi during 2003.
They measured the amount of a placental protein, known as PLAP, in the umbilical cord blood of each woman. The protein is normally too large to cross the placenta on its own.
Professor Meshnick says investigators found the more PLAP they detected in the umbilical cord blood, the more likely it was the newborns had contracted HIV from their mothers.
"So, there was an association between the risk of transmission and the amount of this enzyme in the umbilical cord, suggesting that this was the route that this virus went," Mr. Meshnick said.
Professor Meshnick says the microtransfusions of blood occur at the start of labor when tears begin to form in the placenta.
He says the findings support an observation about the transmission of HIV from mother to child.
"If mothers get elective caesarian sections, meaning they get caesarian sections before they go into labor, the baby doesn't get infected," Mr. Meshnick said. "But if they get emergency c-sections, or caesarian sections after the mother has been into labor, they do tend to get infected. So, our data provides an explanation for it."
Mr. Meshnick says the data might also explain why preventive measures, such as sanitizing the birth canal to prevent HIV transmission, may not work.
The study on microtransfusions was published in the open access journal, Public Library of Science Medicine.