Zika virus is a mosquito-borne flavivirus primarily transmitted by Aedes aegypti mosquitoes (Hayes, 2009; CDC, 2016 ) and to a lesser extent by Aedes albopictus mosquitoes.
Aedes aegypti and Aedes albopictus mosquitoes, found throughout much of the Region of the Americas, including parts of the United States, also transmit dengue and chikungunya viruses (CDC, 2015). Zika virus infections result from both intrauterine transmission resulting in congenital infection and intrapartum transmission from a viremic mother to her newborn (Besnard et al., 2014; ECDC, 2015). Eventhough Zika virus RNA has been detected in breast milk, Zika virus transmission through breastfeeding has not been confirmed (Besnard et al., 2014).
Humans are the primary host for Zika virus. Approximately 80% of infected persons remain asymptomatic (Hayes, 2009; Duffy et al., 2009). The disease when symptomatic is generally mild and characterized by acute onset of fever, maculopapular rash, arthralgia, or nonpurulent conjunctivitis. Symptoms typically last from several days to 1 week. Severe disease requiring hospitalization is uncommon and fatalities are rare (ECDC, 2015; Duffy et al., 2009). During the current outbreak in Brazil, Zika virus RNA has been identified in specimens (i.e., brain tissue, placenta, and amniotic fluid) from several infants with microcephaly and from fetal losses in women infected with Zika virus during pregnancy (ECDC, 2015; Oliviera et al., 2016; Hennessey et al., 2016). The Brazil Ministry of Health has reported a marked increase from previous years in the number of infants born with microcephaly and intracranial calcifications in 2015, although it is not known how many of these cases are associated with Zika virus infection (ECDC, 2015; Oliviera et al., 2016; Hennessey et al., 2016; Schuler-Faccini et al., 2016; Ventura et al., 2016).
Countries and territories with active Zika virus transmission (source: CDC Jan 26, 2016)
Zika Virus Testing Considerations and Classification
The diagnosis of Zika virus infection is made through molecular and serologic testing (Hayes, 2009). This includes reverse transcription-polymerase chain reaction (RT-PCR) for viral RNA, and immunoglobulin (Ig) M ELISA and plaque reduction neutralization test (PRNT) for Zika virus antibodies. Because it is currently not known which type of testing most reliably establishes the diagnosis of congenital infection, the US CDC has recommended that both molecular and serologic testing of infants who are being evaluated for evidence of a congenital Zika virus infection. No commercial tests for Zika virus are currently available.
Zika virus RT-PCR testing should be performed on serum specimens collected from the umbilical cord or directly from the infant within 2 days of birth (Lanciotti et al., 2008). In addition, cerebrospinal fluid (CSF) obtained for other studies, and frozen and fixed placenta obtained at delivery, should also be tested by RT-PCR.
IgM ELISA for Zika virus and dengue virus should be performed on infant serum, infant CSF, and maternal serum.However, results of these assays can be falsely positive because of cross-reacting antibodies (Hennessey et al., 2016; Lanciotti et al., 2008). PRNT can be performed to measure virus-specific neutralizing antibodies and to discriminate between cross-reacting antibodies from closely related flaviviruses (e.g., dengue or yellow fever viruses). Finally, immunohistochemical staining to detect Zika virus antigen on fixed placenta and umbilical cord tissues can be considered.
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 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
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