Zika virus is a mosquito borne illness that is found in South and Central America. The most common symptoms include fever, rash, joint pain, and conjunctivitis (red eyes). The virus is spread by mosquitos primarily in the Aedes aegypti and Aedes albopictus species which also carry other tropical diseases such as Chikungunya and Dengue. These mosquitos bite humans primarily in the daytime. It is estimated that 80% of people infected with the Zika virus are asymptomatic. In most people with symptoms, the illness is self-limited and resolves in 5-7 days. Disease requiring hospitalization is rare.
Recently, there have been reports in Brazil of an increased rate of microcephaly and other poor pregnancy outcomes in babies from women who were infected with the Zika virus while pregnant. However, further studies are needed to understand the relationship between these outcomes and infection. In the meantime, the Centers for Disease Control and Prevention (CDC) have issued special travel precautions for pregnant women and women trying to get pregnant.
So who should be tested for Zika virus and what testing is available?
Initially, the CDC advised that a pregnant woman should only be tested if she has symptoms of Zika virus within the first week of being in an endemic area. If the mother is positive, then the infant should be tested for congenital infection.
However, very recently, the CDC updated the guidelines to include asymptomatic pregnant women who live in or have traveled to endemic areas. The update recommends that serologic testing be offered to pregnant women can be offered testing within 2-12 weeks after they return from travel. For asymptomatic pregnant women who live in endemic areas, testing is recommended at the initiation of prenatal care with follow-up testing mid-second trimester.
For infants that have microcephaly or intracranial calcifications detected prenatally or at birth with a mother who was potentially infected with Zika virus during pregnancy, the infant should be tested. If infants have positive or inconclusive test findings, the case should be reported to the State or local health department for follow-up. If the infant tests negative, other possible etiologies for the microcephaly should be investigated.
For infants without microcephaly or intracranial calcifications with a mother who was potentially infected with Zika virus during pregnancy, subsequent evaluation depends on the mother's results. If the mother test's negative, no further testing is required. If the mother received positive or intermediate results, then the infant should be tested. If the infant test's negative, then no further testing is required. If the infant test's positive then further clinical evaluation (including comprehensive physical exam, cranial ultrasound and ophthalmologic evaluation) should be performed and the infant should be followed for long term sequelae.
No commercial tests are yet available for Zika virus. Testing is performed at the CDC and some local health departments. The tests currently performed include RT-PCR, ELISA for IgM and a plaque reduction neutralization test (PRNT).
Infants who are being evaluated should have RT-PCR performed on serum (from infant or umbilical cord) within 2 days of birth. CSF if available should also be tested by RT-PCR. ELISA for IgM should be performed on infant serum and CSF.
Mothers being evaluated should have serum tested using ELISA. RT-PCR can be performed during the first week of viral infection. Amniocentesis should be offered to pregnant women who test positive or indeterminate and RT-PCR should be performed on the amniotic fluid.
Note that false positives can occur in the ELISA assay due to cross reactivity with other related flaviviruses such as dengue or yellow fever. PRNT can be used to distinguish false positives from true positive results. If neutralizing antibody titers are ≥ 4-fold greater than dengue virus neutralizing antibody titers, then Zika virus is considered positive. Immunohistochemistry can also be performed on fixed placenta or umbilical cord tissue. If any of any of the tests are positive, the infant is considered congenitally infected.
Currently, there is no anti-viral treatment or vaccination for Zika virus. Treatment is supportive. The best defense against Zika is preventing maternal infection by avoiding mosquito bites. It is important to note that, when used as instructed, insect repellants containing DEET, picardin, and IR3535 are safe for pregnant women.