Study: West Nile poses small risk for pregnancy outcomes

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Mar 9, 2006 (CIDRAP News) – Pregnant women who come down with West Nile virus (WNV) infection probably run a fairly small risk of major birth defects in their babies, according to the first analysis of data collected in 16 states.

Seven of 72 children of mothers who had West Nile infection during pregnancy were born with major abnormalities, but the timing of the illnesses indicates that only three of the babies' defects could have been caused by WNV, according to the report in Pediatrics. There was no clear evidence of WNV infection at birth in any of the three.

After the first US case of congenital WNV infection was reported in 2002, the Centers for Disease Control and Prevention (CDC) worked with state health departments to set up a surveillance registry for women who contracted WNV during pregnancy. The Pediatrics report was prepared by a team from the CDC and six state health departments, with the CDC's Edward B. Hayes, MD, as senior author.

For the registry, healthcare providers reported WNV cases in pregnant women to health departments, which reported laboratory-confirmed cases to the CDC. The CDC then asked the healthcare providers to ask patients to participate voluntarily in the registry.

In 2003 and 2004, 77 women from 16 states had WNV illness and agreed to participate, according to the report. Of those, 52 had West Nile fever, while 18 had neuroinvasive disease, 6 had unspecified illness, and 1 had asymptomatic viremia. Roughly equal proportions of the women were believed to have contracted WNV in the first, second, and third trimesters of pregnancy.

Of the 77 women, 71 gave birth to a total of 72 babies; four women had miscarriages and two had abortions. Sixty-seven babies were born at term, while four were premature and the gestational age was unknown for one.

Umbilical cord serum was available for testing from 55 infants, all but one of whom tested negative for anti-WNV IgM. The baby that tested positive at birth grew normally and tested negative at the age of 1 month and again at 8 months, suggesting that the original test was a false-positive.

The rate of miscarriages, preterm delivery, and low-birth-weight babies was no higher in this group than in the general population, the report says. But 7 of the 72 babies had major abnormalities, which is above the general population rate (10.6% versus 5.5%). The defects included one case of polydactyly (an extra toe), two babies with an abnormally small head (microcephaly), one Down syndrome case, one aortic coarctation, one cleft palate, and one case of lissencephaly (lack of normal convolutions of the cerebral cortex).

But the authors concluded that only three of those abnormalities could have been related to the mother's WNV infection, given the timing of the illnesses. These were the polydactyly case (infection in the first trimester) and microcephaly cases (infection in the the second trimester).

The mother of the baby with Down syndrome—which occurs at conception—contracted WNV in her second trimester, the report says. The other three abnormalities are believed to originate in the first trimester, but the women didn't contract WNV until the third trimester.

"Thus, of the 7 infants with major malformations, only 3 had defects that could have been caused by maternal WNV infection based on the timing of the infections and the sensitive developmental period for the specific malformations, and none had any conclusive evidence of WNV etiology," the article states.

Three babies among the 72 might have been infected during gestation even though their blood tests were negative at birth, the report says. One of these had WNV meningitis at the age of 10 days, but recovered and grew normally. The baby with aortic coarctation had a positive blood test at the age of 1 month but was normal at 14 months. The infant with lissencephaly had WNV encephalitis at 17 days and died at 7 weeks.

The authors say the sensitivity of tests for anti-WNV IgM in cord blood is unknown. Therefore, they write, "We cannot rule out the possibility that occult congenital WNV infection might have contributed to abnormalities that are temporally plausible with the timing of maternal WNV infection."

The report also says that eight babies had abnormal growth, though most of them eventually achieved age-appropriate growth. It is unclear whether congenital WNV could account for some of the growth abnormalities. Prospective studies are needed to clarify the issues, the authors conclude.

O'Leary DR, Kuhn S, Kniss KL, et al. Birth outcomes following West Nile virus infection of pregnant women in the United States, 2003-2004. Pediatrics 2006;117(3):537-45 [Abstract]

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