UW News

January 7, 2003

UW study of risk of herpes transmission from mother to infant at birth shows surprising results, confirms old assumption

For over 30 years, physicians have assumed that any expectant mother with genital herpes lesions at delivery must deliver her baby by Caesarian section to minimize chances of transmission of the disease to the infant, although there was no clinical or research information supporting that practice. A study by University of Washington physicians Dr. Zane Brown, professor of obstetrics and gynecology; Dr. Lawrence Corey, professor of medicine and pediatrics; and Dr. Anna Wald, UW associate professor of medicine and epidemiology and their colleagues confirms that Caesarian section does prevent transmission of HSV (herpes simplex virus).

The study, done in the center of a community with a high rate of neonatal herpes transmission and published in the Jan. 8 edition of the Journal of the American Medical Association (JAMA), clarifies the risk factors for women transmitting HSV to their infants during birth.

“HSV is still a severe problem for those babies who are exposed to it,” Wald said. “Reducing the chances of acquiring HSV in the final trimester of pregnancy is the best way to prevent transmission of the disease to infants.”

Antibody testing to detect HSV for both the mother and any partner should be used more frequently in early pregnancy, the authors advise. Additionally, they suggest that women be counseled against unprotected sex in the final three months of pregnancy, particularly if their partners test positive for HSV.

The 18 year-long cohort study analyzed 48,390 deliveries at University of Washington Medical Center and Madigan Army Medical Center where HSV blood tests were routinely performed as a part of deliveries. Twenty-three percent of the women tested negative for any herpes simplex virus, 49 percent had only HSV-1 antibodies, 11 percent had only HSV-2 antibodies, and 17 percent had both HSV-1 and HSV-2 antibodies.

HSV-2 is commonly known as genital herpes. HSV-1 is associated with cold sores.

The authors note that some surprising information arose from their analyses of the cases in which HSV was transmitted to infants at birth. First of all, while women in all HSV categories were at risk of transmitting HSV to their infants, the highest risk was among women whose blood showed no HSV antibodies. This high rate, 1 in 1900 cases, reflects the high efficiency with which the viruses can be transmitted by women who have had HSV-1 and/or HSV-2 for only a short time. Women who acquire HSV in the final three months of pregnancy through sexual activity are more likely to be shedding HSV. Meanwhile, the research shows that they do not show HSV antibodies in the blood, and their infants have not received protective HSV antibodies from their mothers, making them more vulnerable to the HSV being actively shed in the mother’s birth canal.

Secondly, women with previous HSV-2 infections are at a reduced risk, two births in 5,761, for transmitting HSV-2 to their infants. This reflects the relatively inefficient transmission of HSV-2 when the infant has specific antibodies for the virus received from the mother.

Third, the transmission rate of HSV is highly influenced by how delivery is managed. That includes recognition of lesions, taking appropriate steps to prevent infant exposure and maintaining the infant’s skin integrity during labor, such as avoiding the use of fetal scalp electrodes.

Neonatal transmission of herpes occurred less frequently among women who had genital lesions than among women who were experiencing shedding of the virus into the birth canal but had no lesions. This happened because active lesions were interpreted as a condition requiring Caesarian section.

“Perhaps the most clinically important observation from our study is the finding that caesarian delivery protects against neonatal transmission of HSV,” Brown said. “This is the first demonstration of this effect, despite it being the standard obstetrical practice in the United States for 30 years.”

The researchers concluded that it is important to develop a strategy to reduce transmission of HSV from mother to child at birth.

Wald adds that the best solution would be to find a vaccine that would protect women from HSV in the first place. The National Institutes of Health are starting a trial for a vaccine that appears to be effective among women who test negatively for HSV-1 and HSV-2. The UW is one of the participating sites. Women without a history of oral or genital herpes are encouraged to call the UW Virology Research Clinic at (206) 720-4340 for details on participation in the research.