Zika Questions From Medscape Readers: The CDC Responds

Denise Jamieson, MD, MPH
March 10, 2016
CDC Expert Commentary

Editor's Note:
Medscape works with the Zika team at the Centers for Disease Control and Prevention (CDC) so that we can bring our readers the very latest information on identifying and treating Zika virus. The following are questions and concerns posed by Medscape readers and answered by CDC. You will find all of our Zika-related information in the Zika Virus Resource Center.

What Is the Prevalence of Microcephaly in Regions and Countries Affected by the Zika Virus?
It is difficult to monitor microcephaly in populations because the term is defined and used inconsistently. It may not be possible to diagnose microcephaly until late in the second or early in the third trimester of pregnancy. Sometimes, microcephaly is not diagnosed until after birth.
Birth defect programs in each country may collect prevalence data in different ways. The outbreak of Zika virus infection in Brazil occurred earlier than in many other countries. It is possible that other countries will start to see microcephaly or other adverse pregnancy outcomes later as the Zika virus outbreak evolves, additional pregnant women in areas with Zika virus transmission receive prenatal care and testing, and pregnant women deliver. These factors may affect estimates of prevalence and explain some of the differences in estimates between countries.
What Is the Long-term Risk of Zika Virus for Pregnant Women, Infants, and Children?
Zika virus can pass from a pregnant woman to her fetus during pregnancy. We do not know how often this occurs. We do not know the likelihood of a fetus developing birth defects if the mother is infected with Zika while she is pregnant. Currently, there is no evidence to suggest that Zika virus, after it is cleared from the pregnant woman's blood, poses a risk for birth defects in future pregnancies.
Information on long-term outcomes among infants and children with acute Zika virus disease is limited. Most children infected with Zika virus are asymptomatic or have mild illness, similar to the findings seen in adults with Zika virus infection. (For more on children and Zika virus, see "Zika for Pediatricians: Critical Update."
When Are Pregnant Women Most at Risk for Fetal Effects?
We do not know when pregnant women are most at risk for fetal effects. Pregnant women can be infected with Zika virus at any time of their pregnancy. Zika virus can be passed from a mother to her fetus during pregnancy. CDC is investigating the link between Zika and microcephaly. (For more information, see "Advising Pregnant Women About Zika: The Latest Guidance From CDC."
Does Zika Immunity Result After an Infection?
If Zika behaves like similar infections, once you have been infected with Zika virus, you are likely to be protected from future infections.
Are There Other Possibilities That Might Explain the Increase in Microcephaly Cases in Brazil?
Microcephaly can happen for many reasons, including genetics, maternal infections, and exposure to toxins during pregnancy. Results of recent epidemiologic and laboratory studies performed in Brazil strongly suggest but don't yet prove a link between Zika virus infection during pregnancy and microcephaly.
You may have heard recent media reports suggesting that a pesticide called pyriproxyfen might be linked with microcephaly. These media reports stem from a single publication authored by an Argentine physicians' organization, which claims that the use of pyriproxyfen in drinking water in Brazil is responsible for the country's increase in microcephaly cases. Pyriproxyfen is a registered pesticide in Brazil and other countries and has been used for decades.
A team of scientists from the World Health Organization (WHO) recently reviewed data on the toxicology of pyriproxyfen, one of 12 larvicides that WHO recommends to reduce mosquito populations. It found no evidence that the larvicide affects the course of pregnancy or the development of a fetus. The US Environmental Protection Agency and EU investigators reached a similar conclusion when they carried out a separate review of the product.
CDC is working closely with international partners to study infants with microcephaly to better understand what role various factors, including Zika virus, may play in this birth defect.
Have There Been Any Reports of Airborne Transmission of Zika Virus?
There have been no documented cases of airborne transmission of Zika virus. Zika virus, like other flaviviruses, such as dengue and chikungunya, is spread primarily through the bite of an infected mosquito.
What Is the Zika Virus Incubation Period?
Although the exact incubation period of Zika virus has yet to be determined, evidence from case reports and experience from related flavivirus infections indicate that the incubation period is probably 3 days to 2 weeks.
How Long Does Zika Virus Infectivity Last?
Zika virus usually remains in the blood of an infected person for about a week. We do not know how long Zika virus is present in the semen of men who have been infected. Evidence suggests that Zika virus can be detected in the semen longer than in the blood. One report found the virus in semen at least 2 months after illness, but this was not a test for live virus; therefore, we do not know if the semen was infectious. Another report found live virus in the semen at least 2 weeks after illness onset. In both of these cases, no follow-up testing was done to determine when Zika virus was no longer present in the semen. At this time, we do not know how long after exposure Zika virus can be sexually transmitted from a male partner.