Where did Zika come from?
Zika is a flavivirus related to the viruses behind yellow fever, dengue, West Nile and Japanese encephalitis. It’s named after Zika Forest in Uganda, where it was first identified in a monkey in 1947. It’s endemic to Africa, and likely spread to Asia 50 years ago, but for decades only sporadic human cases were reported.
The first documented outbreak of Zika occurred in 2007, on Yap Island in the Federated States of Micronesia. The virus then spread to other South Pacific islands, aided by the lack of immunity among people in that region. It was first confirmed in Brazil in May 2015, and has since spread rapidly through the country — as well as to 21 other countries and territories across the Americas.
How does it spread?
Zika is transmitted by infected Aedes mosquitoes like Aedes aegypti and Aedes albopictus, also known as the Asian tiger mosquito. Those two species evolved in Africa and Asia, respectively, but they’re now common in warm, wet climates worldwide. They often live near buildings in urban areas and are usually active during the day, with peak biting periods in early morning and late afternoon.
The virus can also be transmitted from a pregnant mother to her baby during pregnancy or around the time of birth, according to the U.S. Centers for Disease Control and Prevention (CDC), although it’s unclear how frequently that happens. Some evidence suggests it can spread through blood transfusion or sexual transmission, too, but the WHO describes those modes as “very rare.”
What are the symptoms?
Only about one in five people infected with Zika virus will get sick, according to the CDC, and even then the symptoms are usually mild. Common symptoms include a low-grade fever, rash, joint pain, headache and conjunctivitis. The incubation period for Zika is typically two to seven days, which means any symptoms should appear within about a week of being bitten by an infected mosquito.
Hospitalization is rare, and the WHO says no reported deaths have been associated with Zika. But the virus may still be dangerous in less direct ways; it has been associated with Guillain-Barré syndrome, for example, an immune disorder than can cause paralysis. And due to its subtlety, infections are easy to miss or misdiagnose.
After Zika’s 2015 arrival in Brazil, doctors began to notice an uptick in cases of microcephaly, a birth defect characterized by an undersized head and incomplete brain development. Brazil typically has about 150 reports of microcephaly per year, but the country is now investigating nearly 4,000 cases from 2015 — 20 times the normal amount. The connection wasn’t widely reported until October, and since then the prospect of microcephaly has made Zika as dreaded as much deadlier diseases.
This association still isn’t well-understood, and scientists can’t say with certainty whether Zika causes microcephaly. The surge in cases is certainly alarming, but microcephaly can be caused by a wide range of genetic, environmental or infectious factors.
A causal relationship is “strongly suspected,” according to Chan, partly since research shows the virus can cross the placental barrier and enter amniotic fluid. But more research is needed to prove Zika is responsible for the current crisis.
If there is a link to microcephaly, why wasn’t it noticed much earlier, in Africa or Asia? That’s still a bit of a mystery, but it could be because there’s more immunity in those populations than in the Americas, where the virus is a newcomer. Plus, given Zika’s relative mildness, it has faced less scrutiny than some other insect-borne diseases.a