Important note: Information in this article was accurate in 2005. The state of the art may have changed since the publication date.
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Scripps Howard News Service (11.22.05) - Tuesday, December 13, 2005
Lee Bowman
Without treatment, 25-35 percent of babies born to HIV- positive mothers become infected, resulting in an estimated 500,000 HIV-positive newborns worldwide each year. With HIV treatment, MTCT is sharply reduced, though in many countries these treatments are either not available or not acceptable.
"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, epidemiology professor at University of North Carolina-Chapel Hill's School of Public Health and the study's senior investigator.
In the study of 149 pregnant HIV-positive women in Malawi, Meshnick and an international team sought to assess whether MTCT is caused by a breakdown of the placental barrier. Each mother received a single-dose antiretroviral drug to diminish MTCT risk. Researchers used placental alkaline phosphatase (PLAP) 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," said Meshnick. "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."
Researchers found a "very strong correlation" between PLAP in the cord blood and HIV infection in infants. For each tenfold PLAP increase, risk of MTCT nearly tripled. Researchers suggested MTCT occurred during labor, in the period of contractions, rather than when the infants passed through the birth canal. Sanitization of the birth canal would not be as helpful compared to ARV treatment if this were the case.
"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," said Meshnick. "It has been known for a long time that HIV- infected women who undergo caesarian 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. What we saw was consistent with this. It looks like direct mother-to-child blood-mixing occurring during labor leads to infection," he said.
The full report, "Maternal-Fetal Microtransfusions and HIV-1 Mother-to-Child Transmission in Malawi," was published online in the Public Library of Science Medicine (2005;doi:10.1371/journal.pmed.0030010).
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