Zika virus was reported in May 2015 in South America and since then has spread throughout the Americas, including the U.S. The CDC and Pan American Health Organization (PAHO) web sites maintain and update the list of areas where Zika virus transmission has been identified, including a list of U.S. states and territories where active mosquito-borne Zika virus transmission is found (for current locations, see Travel Restrictions).
The virus spreads to humans primarily through infected Aedes species mosquitoes (Ae. aegypti and Ae. albopictus), from mother to her fetus during pregnancy, and through sexual contact, although Zika virus transmission may also occur through blood transfusion and through laboratory exposure. Zika virus disease is defined as having at least one of the following signs or symptoms: acute onset of fever, rash, arthralgia, conjunctivitis and laboratory confirmation of Zika virus infection. It appears that only about one in five infected individuals will exhibit these symptoms and most of these will have mild symptoms. Once a person is infected, the incubation period for the virus is approximately 3–14 days. This time frame is suggested based on limited experience from Zika virus cases as well as extrapolation from data on other flaviviruses. Possible Zika virus exposure is defined as travel to or residence in an area of active Zika virus transmission (http://www.cdc.gov/zika/geo/index.html), or sex without a condom with a partner who traveled to or lived in an area of active transmission. Anyone who lives or travels to an area where Zika virus is found and has not already been infected with Zika virus is at risk of contracting Zika virus. Once a person has been infected, he or she is likely to be protected from future infections, although this is based on limited data and experience from other flaviviruses and has not been confirmed.
It is not known if pregnant women are at greater risk of Zika virus transmission than non-pregnant individuals. However, there is demonstrated causation between Zika virus infection during pregnancy and adverse pregnancy outcomes such as pregnancy loss, microcephaly, and other brain and eye abnormalities. Transmission of Zika virus to the fetus has been documented in all trimesters; Zika virus RNA has been detected in fetal tissue from early missed abortions, amniotic fluid, term neonates, and the placenta. However, much is not yet known about Zika virus in pregnancy. Uncertainties include the incidence of Zika virus infection among pregnant women in areas of Zika virus transmission, the rate of vertical transmission, and the rate with which infected fetuses manifest complications such as microcephaly or demise. However, one study utilizing modeling based on the Zika virus outbreak in French Polynesia (Cauchemez 2016) suggested microcephaly would occur in 1%-13% of babies born to mothers infected in the first trimester, and a recent cohort study from Brazil (Brasil 2016) found abnormal outcomes including stillbirth, growth restriction, and microcephaly and other sonographic abnormalities in 29% of fetuses of Zika virus-infected mothers in all trimesters.
There currently is no vaccine or treatment for this infection.