Exposure, Testing & Risks with Zika Virus
Testing for Zika virus infection is recommended for certain clinical scenarios. Read more about testing and the risks of infection around the time of conception.
Limitations of Zika Testing
No test is 100% accurate. A test result can sometimes be negative in the setting of true infection, and the results could be falsely reassuring.
- If the IgM test is performed too early after infection when the antibody levels are not yet high enough, the results could be negative even though a person is infected.
- If the IgM test is performed after IgM levels have declined, the results could be negative. Similarly, if the nucleic acid testing on serum is performed after the virus is no longer in the blood, Zika could still be present in other bodily fluids (e.g., semen). In those situations, the blood test would be negative but the person could still be infected and able to infect others.
- It is also possible for IgM test results to be positive in the absence of current infection because of, for example, extended IgM persistence after a previous infection has resolved or due to cross-reactivity of the IgM test with another virus.
Risks of Infection Before Conception and Fertility Treatment
Because Zika virus infection is a cause of microcephaly and other severe fetal brain defects, pregnancy planning in the context of the risk of Zika is important. There is limited information available about the risk for adverse pregnancy outcomes around periconceptional Zika virus infection (defined as infection during 8 weeks before conception or 6 weeks before last menstrual period). To date, there have been no published reports of adverse pregnancy outcomes after periconceptional Zika virus infection. Infections with other viruses (e.g., cytomegalovirus, rubella, parvovirus) around the time of conception have been associated with congenital infection and adverse pregnancy outcomes, although the exact timing of infection relative to conception was sometimes uncertain.
Pregnancy after Infection has Cleared
There is no evidence that a fetus conceived after the virus has cleared the woman’s body would be at risk for fetal Zika infection. Current evidence suggests that Zika virus infection prior to pregnancy would not pose a risk of birth defects to a future pregnancy. From what we know about similar infections, once a person has been infected with Zika virus, he or she is likely to be protected from a future Zika infection. Currently, we do not have a test to tell if someone is protected against Zika virus.
No instances of Zika virus transmission during fertility treatment have been documented, but transmission through donated gametes or embryos is theoretically possible. Zika virus has been detected in semen, and sexual transmission has occurred. The US Food and Drug Administration has issued guidance pdf icon[PDF – 10 pages]external icon providing recommendations to reduce the potential transmission risk of Zika virus through donated tissues, including donated sperm, oocytes, and embryos from anonymous donors. Fertility treatment for sexually intimate couples using their own gametes and embryos should follow the testing and timing recommendations for couples attempting conception, although recommendations might need to be adjusted depending on individual circumstances.
Persistence of Zika in Semen
A review of the literature to date can be found in CDC’s guidance for preconception counseling and prevention of sexual transmission of Zika virus.
- Evidence of sexual transmission: Briefly, among the currently available reports of sexual transmission of Zika virus, the longest period from symptom onset in the index case to potential sexual transmission to a partner was between 32–41 days.
- Potentially infectious Zika virus in semen: The longest reported period after symptom onset at which potentially infectious virus has been detected in semen by culture or cytopathic effect was 69 days. No other studies reported potentially infectious Zika virus in semen specimens obtained more than 40 days after symptom onset.
- Detection of Zika virus RNA in semen: A large number of publications have also reported on detection of Zika virus RNA in semen, which might not indicate presence of infectious virus at the time of sampling or correlate with the potential for sexual transmission of infectious virus. In the largest cohort study to date, Zika virus RNA shedding declined during the 3 months after symptom onset. At >90 days after illness onset, semen of ≤7% of participants had detectable Zika virus RNA. The estimated mean time to clearance of Zika virus RNA from semen was 54 days. Similar findings have been observed in smaller cohort studies.
Limited data suggest the incidence of Zika virus RNA shedding in semen and its persistence after infection are likely similar for symptomatic and asymptomatic men infected with Zika virus.