Q. What risk does WNV illness during pregnancy present to a fetus?
A. Based on the limited number of cases studied so far, it is not yet possible to determine what percentage of WNV infections during pregnancy result in infection of the fetus or medical problems in newborns.

In 2002, one case of transplacental (mother-to-child) transmission of WNV was reported to CDC. In this case, the infant was born with WNV infection and severe medical problems. It is unclear, however, whether WNV infection caused these problems or whether they were due to other causes (see MMWR Dec 20, 2002).

After the report of this case, CDC and state and local health departments formed a registry to follow birth outcomes among women with WNV illness in pregnancy. Three additional pregnancies in which the expectant mother became infected with WNV were detected and evaluated in 2002; none of these 3 resulted in fetal infection. In one additional case it remains unclear whether the fetus was infected; appropriate testing was not done.

In 2003, the registry identified 74 women who acquired WNV illness while pregnant. Preliminary findings regarding outcomes of these pregnancies were first presented at the Fifth Annual National West Nile Virus Conference in Denver CO on February 2, 2004.

As of May 10, 2004, 62 of these women had delivered live infants, 2 had had elective abortions, 5 miscarried in the first trimester and 5 had not yet delivered.

In 2004, CDC is continuing to gather clinical and laboratory information on outcomes of pregnancies of women with WNV illness during pregnancy. Pregnant women who think they may have become infected with WNV should contact their private health care providers. Clinicians who are aware of WNV infections of pregnant women are encouraged to report such cases by calling their state or local health departments, or by contacting CDC, telephone 970-221-6400. For more information see the section on Clinical Guidance.

Because of ongoing concerns that mother-to-child WNV transmission can occur with possible adverse health effects, pregnant women should take precautions to reduce their risk for WNV and other mosquito-borne infections by avoiding mosquitoes, using protective clothing, and using repellents containing DEET. Repellents with DEET are safe for pregnant women, and there are other options as well such as a soybean oil based repellent that provides good, though quite limited, protection, as judged by a study published in the new England Journal of Medicine. (See Using Repellent Safely.)

Pregnant women who become ill should see their health care provider, and those who have an illness consistent with acute WNV infection should undergo appropriate diagnostic testing.

Additional clinical information on intrauterine WNV can be found in these recent publications:

• Hayes EB and O'Leary DR. West Nile virus infection: a pediatric perspective. Pediatrics. 5 May 2004; 113(5): 1375-1381.
• Alpert SG, Fergerson J, Noel LP. Intrauterine West Nile virus: ocular and systemic findings. Am J Ophthalmol. 2003 Oct;136(4):733-5.
• Chapa et al. West Nile Virus Encephalitis During Pregnancy. Obstetrics and Gynocology. 2003 Aug; 102(2):229-231.
  

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