Clue found to mother, baby transfer

Researchers think transfer of AIDS occurs during labor, not the actual birth.
SCRIPPS HOWARD NEWS SERVICE
Infants may be getting the AIDS-causing virus from their HIV-positive mothers because of small tears in the placenta during birth, a new study suggests.
Between 25 percent and 35 percent of babies born to untreated HIV-infected women become infected themselves -- an estimated half-million newborns worldwide each year. Doctors have learned that treating pregnant women with anti-HIV drugs reduces the risk sharply, but those drugs are either not available or not accepted by women in many countries.
"The question has always been, how does the virus get from the mothers to the babies? We have known very little about it," said Dr. Steven Meshnick, a professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health and lead investigator for the study.
Exchange of blood
For the new study, published Monday by the journal Public Library of Science-Medicine, an international team of scientists worked with 149 pregnant HIV-infected women in the West African nation of Malawi to show that tiny amounts of virus-laden blood led from the women's placentas to their babies during labor. All of the women had been given a single dose of an anti-retroviral drug 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," Dr. Meshnick said.
The researchers used a large protein called placental alkaline phosphatase as a marker for maternal-fetal blood exchange. "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 that mothers carry before birth," Meshnick said.
"We figured 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."
Strong correlation
The researchers then compared what they'd found in the cord blood with the actual infection rate for infants, and found a "very strong correlation" for the enzyme and HIV. For every tenfold increase of enzyme activity in the cord blood, the risk of mother-to-child HIV transmission nearly tripled.
So, according to the researchers, the transmission of the virus seems to occur during labor, when contractions occur, rather than during passage of the baby through the birth canal.
Thus, some prevention techniques that involved trying to sanitize the birth canal probably aren't helpful compared with having women start taking the drugs for some interval before they go into labor.
"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 underwent emergency C-sections after they go into labor do transmit it," Dr. Meshnick said. "What we saw was consistent with this. It looks like direct mother-to-child blood-mixing occurring during labor leads to infection."