Many countries in the Americas now have local transmission of multiple arboviruses that can cause febrile illness with rash, myalgia, or arthralgia. Therefore, laboratory testing has become even more important to confirm the etiology of these diseases. Zika, chikungunya, and dengue virus infections should all be considered for patients with acute fever, rash, myalgia, or arthralgia who have traveled within the previous 2 weeks to an area with ongoing transmission or are living in an area with ongoing transmission (CDC Memorandum, 2016).
The Food and Drug Administration (FDA) has issued an Emergency Use Authorization (EUA) for two diagnostic tools for Zika virus, the Zika MAC-ELISA and Trioplex Real-Time RT-PCR Assay, which are being distributed to qualified laboratories that are certified to perform high-complexity tests in the United States (CDC, 2016a). If a patient is suspected of having a Zika, chikungunya, or dengue infection, a serum specimen must be collected. Other specimens may also be collected for testing in addition to the serum specimen including CSF, urine, amniotic fluid and tissues (CDC Memorandum, 2016).
For symptomatic persons with Zika virus infection, Zika virus RNA can sometimes be detected early in the course of illness. Real-time reverse transcription-polymerase chain reaction (rRT-PCR) testing should be performed on serum collected during the first two weeks after symptom onset. rRT-PCR should also be conducted on urine samples collected less than 14 days after symptom onset. Urine should always be collected with a patient-matched serum specimen. A positive rRT-PCR result on any sample confirms Zika virus infection and no additional testing is indicated. A negative rRT-PCR result does not exclude Zika virus infection and serum should be analyzed by IgM antibody (serological) testing (CDC, 2016, August 9).
For asymptomatic pregnant women who have traveled to areas with active ZIKV transmission, rRT-PCR testing is recommended on serum and urine within 2 weeks of the date of last possible exposure. rRT-PCR testing is also indicated for pregnant women who present for care ≥ 2 weeks after exposure and have been found to be IgM positive. In areas with active ZIKV transmission, asymptomatic pregnant women should undergo IgM testing as part of routine obstetric care in the 1st and 2nd trimester. Reflex rRT-PCR testing is included as a subsequent test for women who are IgM positive (CDC, 2016, August 9).
Zika virus-specific IgM and neutralizing antibodies typically develop toward the end of the first week of illness. IgM levels are variable, but generally are positive starting near day four post onset of symptoms and continuing for 12 weeks Therefore, if rRT-PCR is negative on serum and urine, serum IgM antibody testing for Zika, dengue, and chikungunya virus infections should be performed. In addition, serum samples collected >=14 days after symptom onset, with no earlier samples collected, should be tested for anti-Zika virus, anti-dengue virus, and anti-chikungunya virus IgM antibodies (CDC, 2016, August 9).
The Zika IgM Antibody Capture Enzyme-Linked Immunosorbent Assay (Zika MAC-ELISA) is used for the qualitative detection of Zika virus IgM antibodies in serum or cerebrospinal fluid; however, due to cross-reaction with other flaviviruses and possible nonspecific reactivity, results may be difficult to interpret. Consequently, presumed positive, equivocal, or inconclusive tests must be forwarded for confirmation by plaque-reduction neutralization testing (PRNT). PRNT is performed by CDC or a CDC-designated confirmatory testing laboratory to confirm presumed positive, equivocal, or inconclusive IgM results (CDC, 2016, August 9).
Laboratory testing for congenital Zika virus infection is recommended for infants born to mothers with laboratory evidence of Zika virus infection during pregnancy, and for infants who have abnormal clinical findings suggestive of congenital Zika virus syndrome and a maternal epidemiologic link suggesting possible transmission, regardless of maternal Zika virus test results (CDC, March 16, 2017).
For infants born to mothers with risk factors for maternal Zika virus infection (travel to or residence in an area of Zika virus transmission or sex with a partner with travel to or residence in such an area) for whom maternal testing was not performed before delivery, assessment of the infant, including comprehensive physical exam and careful measurement of head circumference should be performed. Maternal diagnostic testing should be performed and testing of the placenta for Zika virus PCR should be considered. If an infant appears clinically well, further evaluation and infant testing can be deferred until maternal test results are available. However, if there is concern about infant follow-up, infant testing should be performed before hospital discharge (CDC, March 16, 2017).