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Zika Virus Tests

Zika virus is a single-stranded RNA virus in the genus Flavivirus and is closely related to dengue, West Nile, Japanese encephalitis, and yellow fever viruses. Zika and dengue virus have similar geographic distribution and transmission cycles. Most infections are spread by mosquitoes such as Aedes aegypti, Aedes albopictus and Aedes hensilli. These same mosquitoes are also vectors for dengue fever and chikungunya virus.

Zika virus breeds within the mosquito vector and becomes infectious after a 15 day development period. It remains for the remainder of its 50 day lifespan. The virus is spread to the next susceptible host when the mosquito takes a blood meal. As with other flaviviruses, replication of Zika virus is thought to occur in human dendritic cells located near the site of the bite from the infected mosquito. The virus then spreads to the lymph nodes and bloodstream.

Zika virus can also be transmitted sexually, maternofetally and via blood. Zika RNA has been detected in blood and semen for six months after the onset of symptoms. Zika virus demonstrates trophism for neural progenitor tissue, resulting in neuronal degeneration and cell death. A fetus is at greatest risk of developing microcephaly if infection occurs during the first trimester of pregnancy. The overall risk of microcephaly is estimated to range between 1 and 13%.

Many people do not develop symptoms after infection with Zika virus. Those who do,  exhibit symptoms similar to other flavivirus infections that include fever, skin rashes, conjunctivitis, muscle and joint pain, malaise, and headache. These symptoms are usually mild and last for 2-7 days. Rarely, some individuals develop Guillain-Barré syndrome, which can cause temporary paralysis.

The definitive diagnosis of Zika virus infection can only be made on the basis of laboratory testing. Current testing methods for assessment of acute infection rely on serologic testing for presence of IgM to Zika virus and detection of Zika virus RNA in serum and urine. Zika virus IgM may not be detectable until 4 days after onset of symptoms and remain detectable up to 12 weeks later.

Neutralizing IgG antibodies to Zika virus develop shortly after IgM antibodies. Neutralizing antibodies usually persist for many years after flavivirus infections and may confer prolonged immunity. Neutralizing antibody titer against a flavivirus to which a person was previously exposed or vaccinated against might be higher than the titer against a virus causing a recent infection.

The CDC Zika IgM Antibody Capture Enzyme-Linked Immunosorbent AssayZika is peformed in qualified public health and reference laboratories in the United States for the qualitative detection of Zika virus IgM antibodies in serum or cerebrospinal fluid collected from persons meeting the clinical and epidemiologic criteria for suspected Zika virus disease. False positive results may occur due to crossreactivity with other flavivirus infections or vaccinations.

If either the Zika or dengue virus IgM antibody testing yields positive, equivocal, or inconclusive results, plaque reduction neutralization test (PRNT) against Zika and dengue viruses should be performed. PRNT is a confirmatory test that measures virus-specific neutralizing antibody titers to identify the infecting virus and rule out false positive ELISA results. Without PRNT it is not possible to determine if a positive IgM ELISA result against Zika virus indicates recent infection or a false positive result. A PRNT titer of 10 or higher against Zika virus, together with negative PRNT titers against other flaviviruses confirms the presence of a recent Zika virus infection. A PRNT titer of 10 or higher for both Zika and another flavivirus indicates a recent flavivirus infection but cannot determine the specific virus. A negative PRNT against Zika virus in a specimen that is collected more than 7 days after illness onset rules out Zika virus infection. For specimens collected less than 7 days after onset of symptoms, the combination of a negative rRT-PCR and a PRNT titer less than 10 suggests that Zika virus infection did not occur. In the absence of rRT-PCR testing, a PRNT titer less than10 by itself may reflect specimen collection before development of detectable neutralizing antibodies and does not rule out infection.

Nucleic acid detection by reverse transcriptase-polymerase chain reaction targeting the non-structural protein 5 genomic region is the definitive test for Zika virus. RT-PCR, or NAT testing for Zika virus, is extremely sensitive and specific. Due to the short period of viremia, serum and urine samples must be collected within 14 days of the onset of symptoms. If possible, urine should always be collected with a patient-matched serum specimen. If the specimen can be collected within 14 days of illness onset, the use of PCR will nullify the cross-reactivity issues noted with serologic testing for Zika.

A positive PCR result is considered confirmatory and no additional testing is required. If molecular testing for Zika is negative, this does not rule out the possibility of Zika virus infection, and serological testing for Zika IgM is recommended. RT-PCR can be performed on serum, plasma, or urine.

CDC issued new recommendations for Zika virus testing in July 2017 due to overall declines in prevalence of Zika disease in the Americas. Diagnostic testing is currently recommended for assessment of infection in individuals who are symptomatic, have a significant travel history to, or residence in, a Zika infested area.

All pregnant women should be asked about Zika infection at each prenatal visit, regardless of symptoms. This includes individuals with recent travel or residence in a Zika-affected area within eight weeks before conception or individuals with sexual contact with a person who traveled to or lives in an area with risk of Zika.

For asymptomatic pregnant women with recent possible, but not ongoing exposure (such as travel), routine testing for Zika is no longer recommended, due to the risk of increased false-positive test results in light of declining Zika prevalence.

For pregnant, symptomatic women with ongoing risk of Zika exposure, concurrent serologic and nucleic acid testing (NAT) of serum and urine should be performed. A positive NAT result is confirmatory for maternal Zika infection. For asymptomatic women with ongoing risk of Zika exposure, NAT testing should be offered at the initiation of prenatal care and twice more during the duration of the pregnancy.

Due to persistence of IgM antibodies, Zika infection before pregnancy may confound results of serologic testing performed during pregnancy. Therefore, IgM testing is no longer recommended.

If there is presumptive or confirmed Zika virus infection in the mother, ultrasound studies should be performed every 3 to 4 weeks in order to monitor fetal anatomy and growth. In neonates at risk, both nucleic acid testing of serum and urine and IgM testing on serum should be performed. Testing of cord blood is not recommended.

References

  1. Ioos S, Mallet HP, Leparc Goffart I, et al. Current Zika virus epidemiology and recent epidemics. Med Mal Infect. 2014;44(7):302-307.
  2. Faye O, Diallo D, Diallo M, et al. Quantitative real-time PCR detection of Zika virus and evaluation with field-caught mosquitoes. Virol J. 2013;10:311.
  3. http://www.cdc.gov/zika.
  4. Oduyebo T, Polen KD, Walke HT, et al. Update: interim guidance for health care providers caring for pregnant women with possible Zika virus exposure —United States (including U.S. territories), July 2017. MMWR Morb Mortal Wkly Rep. 2017;66:781-793.
  5. Musso D, Gubler DJ: Zika virus. Clin Microbiol 2016;29(3):487-524
  6. Baud D, Gubler DJ, Schaub B, et al: An update on Zika virus infection. Lancet 2017, June 21.

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