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Am Fam Physician. 2004;70(70):204-206

The Centers for Disease Control and Prevention (CDC) has released interim guidelines for the evaluation of infants born to mothers with West Nile virus infection. “Interim Guidelines for the Evaluation of Infants Born to Mothers Infected with West Nile Virus During Pregnancy” appears in the February 27, 2004, issue of Morbidity and Mortality Weekly Report and is available online at

West Nile virus is a single-stranded RNA flavivirus with antigenic similarities to Japanese encephalitis and St. Louis encephalitis viruses. Flavivirus infection during pregnancy has been associated rarely with spontaneous abortion and neonatal illness but has not been known to cause birth defects in humans. In 2002, a woman who had West Nile virus encephalitis during the 27th week of her pregnancy delivered a full-term infant with chorioretinitis, cystic destruction of cerebral tissue, and laboratory evidence of congenitally acquired West Nile virus infection. Although this case demonstrated intrauterine West Nile virus infection in an infant with congenital abnormalities, it did not prove a causal relation between the virus infection and these abnormalities.

Three other instances of maternal virus infection were investigated in 2002. In all three cases, the infants were born at full term with normal appearance and negative laboratory tests for West Nile virus infection; cranial imaging studies and ophthal-mologic examinations were not performed. The CDC is gathering data on pregnancy outcomes for approximately 70 women who had West Nile virus infection during pregnancy in 2003.

The CDC convened a meeting of specialists in the evaluation of congenital infections and has released the following interim recommendations:

  • No specific treatment for West Nile virus infection exists, and the consequences of infection during pregnancy have not been well defined. For these reasons, screening of asymptomatic pregnant women for West Nile virus infection is not recommended.

  • Pregnant women who have meningitis, encephalitis, acute flaccid paralysis, or unexplained fever in an area of ongoing West Nile virus transmission should have serum (and cerebrospinal fluid, if clinically indicated) tested for antibody to West Nile virus. If serologic or other laboratory tests indicate recent infection, these infections should be reported to the local or state health department, and the women should be followed to determine the outcomes of their pregnancies.

  • If West Nile virus infection is diagnosed during pregnancy, a detailed ultrasound examination of the fetus to evaluate for structural abnormalities should be considered no sooner than two to four weeks after onset of illness in the mother, unless earlier examination is otherwise indicated. Amniotic fluid, chorionic villi, or fetal serum can be tested for evidence of West Nile virus infection. However, the sensitivity, specificity, and predictive value of tests that might be used to evaluate fetal infection are not known, and the clinical consequences of fetal infection have not been determined. In cases of miscarriage or induced abortion, testing of all products of conception (e.g., the placenta and umbilical cord) for evidence of infection is advised to document the effects of West Nile virus infection on pregnancy outcome.

  • When an infant is born to a mother who was known or was suspected to have West Nile virus infection during pregnancy, clinical evaluation is recommended. Further evaluation should be considered if any clinical abnormality is identified or if laboratory testing indicates that an infant might have congenital West Nile virus infection.

  • Pregnant women who live in areas with West Nile virus-infected mosquitoes should apply insect repellent to skin and clothes when exposed to mosquitoes and wear clothing that will help protect against mosquito bites. In addition, whenever possible, pregnant women should avoid being outdoors during peak mosquito-feeding times (i.e., usually dawn and dusk).

Guidance on diagnosis of West Nile virus can be obtained from local or state health departments and from the CDC online at resources/fact_sheet_clinician.htm. Recommended clinical evaluation steps are available online at

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