That's a half million newborn worldwide every year facing chronic illness and premature death on their first day out of the safety of the womb.
"The question has always been how does the virus get from the mothers to the babies?" said principal investigator Dr. Steven Meshnick, professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health. "We have known very little about it."
Now, Meshnick and colleagues think they have discovered a major reason why that occurs. A new study they conducted with 149 pregnant HIV-infected women in the African nation of Malawi showed that tiny amounts of virus-laden blood leak from women's placentas to babies during labor. All of the subjects and their babies had been given single-dose anti-retroviral drugs to minimize HIV transmission.
"This work shows strongly for the first time that what we call placental microtransfusions during birth are responsible for a large part of the transmission of HIV from mother to baby," the scientist said. "To our knowledge, there have never been any data like this before."
A report on the research appears today (Nov. 21) in the latest issue of the journal PloS Medicine. PloS is an acronym for Public Library of Science.
Besides Meshnick, UNC epidemiology authors are Dr. Jesse Kwiek, a postdoctoral fellow; graduate student Alisa Alker; and assistant professor Dr. William Miller, also assistant professor of medicine at the UNC School of Medicine. Other authors are Drs. Victor Mwapasa, Eyob Tadesse and Malcolm Molyneux of the University of Malawi, Dr. Danny A. Milner, a Harvard University pathologist, and Dr. Stephen Rogerson, associate professor of medicine at the University of Melbourne.
The study involved measuring -- in umbilical cord blood samples -- a protein known as placental alkaline phosphatase as an indicator of maternal-fetal microtransfusion.
"This enzyme, made in the placenta, is very big -- usually too big to pass through the natural barrier that protects babies from disease-causing organisms mothers sometimes carry before birth," Meshnick said. "We figured that if we found it in blood from the umbilical cord, which links the placenta to the baby, that would be an indication of mixing, that something tore in the placenta and leaked the mothers' virus-contaminated blood to the infant."
Researchers then compared what they found in cord blood to whether or not infants were infected with HIV, he said.
"We looked for a statistical relationship between the presence of the enzyme and the risk of infection in the babies," Meshnick said. "We found a very strong correlation between the enzyme and the risk of infection."
It appears that transmission of the virus occurs during labor -- when contractions occur -- rather than during passage of the baby through the birth canal, he said.
"It has been known for a long time that HIV-infected women who undergo caesarean section before they go into labor do not transmit the virus, whereas those who undergo emergency C-sections after they go into labor do transmit it," Meshnick said. "What we saw was consistent with this. It looks like direct mother-to-child blood mixing occurring during labor leads to infection."
The new findings indicate that intervening just before birth by such suggested methods as sanitizing the birth canal may not work, he said. A better alternative might be to have pregnant women start taking anti-retrovirals before going into labor.