Zika virus infections have been documented through intrauterine transmissions resulting in congenital Zika virus infection and intrapartum transmission from a viremic mother to her newborn (Vogel, 2015).
Zika virus RNA by RT-PCR testing of amniotic fluid can be performed to detect intrauterine Zika virus infection. However, the sensitivity and specificity of this approach for diagnosing congenital infection are unknown. It is reasonable to offer amniocentesis to women at 15 weeks or greater gestational age with a history of travel to an area with Zika virus transmission and either positive/inconclusive laboratory testing for Zika virus infection or relevant ultrasound findings (fetal microcephaly or intracranial calcifications).
Amniocentesis is associated with an overall 0.1% risk of pregnancy loss when performed at less than 24 weeks of gestation (Vogel, 2015). Amniocentesis performed at 15 weeks or later gestational age is associated with lower rates of complications than those performed at earlier gestational ages. Amniocentesis performed at 14 weeks or less of gestational age is not recommended. Healthcare providers should discuss the risks and benefits of amniocentesis with their patients. It is unknown whether a positive amniotic fluid RT-PCR result is predictive of a subsequent fetal abnormality and, if so, what proportion of infants born after infection will have abnormalities. A positive RT-PCR result on amniotic fluid should be considered suggestive of intrauterine Zika virus infection and potentially useful to pregnant women and their healthcare providers.
Serial ultrasound examinations (every three to four weeks) are appropriate to monitor fetal growth and anatomy in pregnant women with laboratory evidence of Zika virus infection in serum or amniotic fluid (Vogel, 2015). Data on fetal ultrasound findings in infected pregnant women is limited. One small study reported variable fetal ultrasound findings that may include lower-than-expected head circumference for the time of gestation (e.g., more than two standard deviations below the mean), focal brain abnormalities in areas such as the cerebellum and intraocular and brain calcifications.
For live births, Zika virus laboratory testing is recommended for:
- Infants with microcephaly or intracranial calcifications were born to women who traveled to or resided in an area with Zika virus transmission while pregnant.
- Infants born to mothers with positive or inconclusive test results for Zika virus infection.
Because the diagnosis of Zika virus infection is made through molecular and serologic testing, including RT-PCR for viral RNA and IgM and plaque reduction neutralization test (PRNT) for Zika virus antibodies, it is currently unknown which type of testing most reliably establishes the diagnosis of congenital Zika virus infection. The CDC recommends molecular and serologic testing of infants being evaluated for evidence of a congenital Zika virus infection.
The following Zika virus testing on infants per CDC interim guidelines includes (Vogel, 2015):
- Test infant serum for Zika virus RNA per RT-PCR, Zika virus IgM and neutralizing antibodies, and dengue virus IgM and neutralizing antibodies. If possible, the initial sample should be collected either from the umbilical cord or directly from the infant within 2 days of birth.
- If cerebrospinal fluid (CSF) is obtained for other studies, test for Zika virus RNA by RT-PCR, Zika virus IgM and neutralizing antibodies, dengue virus IgM and neutralizing antibodies.
- The mother's serum should be tested for Zika virus IgM and neutralizing antibodies, as well as dengue virus IgM and neutralizing antibodies, if not already performed during pregnancy.
- Testing frozen and fixed placental and cord tissue for Zika virus RNA by RT-PCR.
- Testing of cord serum for Zika and dengue virus IgM and neutralizing antibodies.
The results of these assays can be falsely positive because of cross-reacting antibodies. Plaque-reduction neutralization testing (PRNT) can measure virus-specific neutralizing antibodies and discriminate between cross-reacting antibodies in primary flavivirus infections (e.g., dengue or yellow fever viruses).
Finally, histopathologic examination of the placenta and umbilical cord tissues with Zika virus immunohistochemical staining to detect Zika virus antigen on fixed tissue and Zika virus RT-PCR on fixed and frozen tissue can be considered (Vogel, 2015).
An infant is considered congenitally infected if Zika virus RNA or viral antigen is identified in any of the samples submitted, including testing of amniotic fluid and testing of the placenta or umbilical cord. In addition, Zika virus IgM antibodies with confirmatory neutralizing antibody titers that are 4-fold or higher than dengue virus neutralizing antibody titers in the infant serum or CSF constitute evidence of a congenital Zika virus infection. If Zika virus neutralizing antibody titers are less than 4-fold higher than dengue results, the results are inconclusive.
Fetal tissue testing is warranted for evaluation of fetal losses in women with a history of travel to an area of Zika virus transmission, together with either symptom consistent with Zika virus infection during or within two weeks of travel or findings of fetal microcephaly. In such cases, Zika virus RT-PCR and immunohistochemical staining should be performed on fetal tissues, including the umbilical cord and placenta.