It is now clear the CDC has concluded that Zika does cause Microcephaly. This confirmation is based on a thorough review of the best scientific evidence conducted by CDC and other experts in maternal and fetal health and mosquito-borne diseases. We continue to do everything possible to protect pregnant women and we're undertaking further studies to determine the broader range of birth defects beyond Microcephaly that could occur from Zika infection in pregnancy. We believe that Microcephaly is likely to be a part of a range of birth defects that may affect women at a particular time or at any time in pregnancy.
There is still a lot that we don't know, but there is no longer any doubt that Zika causes Microcephaly. We're undertaking further studies to look at the spectrum of disorders that the virus may cause, and as with much of scientific research, there is no single piece of evidence that provides conclusive proof of this connection, rather, mounting evidence from many studies and careful review of causal criteria was needed to determine that Zika causes Microcephaly and other birth defects.
Dr. Tom Frieden, Director of the Centers for Disease Control and Prevention
Press Briefing Transcript, April 13, 2016
In humans, Zika virus infection is characterized by mild fever (37.8°C–38.5°C); arthralgia, notably of small joints of hands and feet; myalgia, headache; retro-orbital pain; non-purulent conjunctivitis; and cutaneous maculopapular rash. Zika virus infection is believed to be asymptomatic or mildly symptomatic in most cases. Other rarely observed symptoms include digestive problems (abdominal pain, diarrhea, constipation), mucous membrane ulcerations (aphthae), and pruritus (Gourinat et al., 2015).
Maculopapular rash on patient infected with Zika virus, Colorado, USA. Source: Foy et al., 2011.
An estimated 80% of persons infected with Zika virus are asymptomatic (Petersen et al., 2016). Clinical illness is usually mild, with symptoms lasting for 2 to 7 days. Severe disease requiring hospitalization is uncommon and case fatality is low (CDC, 2016a).
Infection may go unrecognized or be misdiagnosed as Dengue, Chikungunya, or other viral infections with fever and rash. An association with Guillain-Barré syndrome (GBS) and other autoimmune neurologic complications was suspected during the 2013–2014 outbreak in French Polynesia and remains under investigation (ECDC, 2015). A possible association with microcephaly in infants was noted in Brazil in September of 2015, although public health officials remained uncertain, largely because for more than 50 years, no flavivirus has ever been shown definitively to cause birth defects in humans, and no reports of adverse pregnancy or birth outcomes were noted during previous outbreaks of Zika virus disease in the Pacific Islands (NEJM, 2016).
On April 13, 2016, the Centers for Disease Control and Prevention (CDC) announced a definitive link between Zika infection and congenital birth defects, particularly microcephaly (CDC Newsroom, 2016). According to the CDC briefing, there is no longer any doubt that ZIKV causes microcephaly and other birth defects. Work by the Brazilian Ministry of Health and scientists have demonstrated Zika virus genomic DNA in the blood and tissues of a baby with microcephaly, and detection in the amniotic fluid of two pregnant women whose fetus was diagnosed through ultrasonography with microcephaly (Hotez & Askoy, 2016).
Microcephaly occurs when a baby’s brain fails to develop during pregnancy. It is a lifelong condition and, depending on the severity, microcephaly has been linked to seizures, developmental delays, vision and hearing problems, movement and balance deficits, and cognitive difficulties. It can sometimes be seen on ultrasound at the end of the second trimester or the beginning of the third trimester. Otherwise, a diagnosis is made after the birth of the child.
In November (2015), health authorities in French Polynesia reported an unusual increase of central nervous system malformations in fetuses and infants that seemed to coincide with the Zika outbreak there. At the end of January (2016), came news reports of the first child born in the United States with microcephaly possibly linked to Zika. The child’s mother had lived in Brazil during her pregnancy before moving to Oahu, Hawaii (Collins, 2016).