One thing that's especially confounding about Zika is that as soon as something about the virus is understood, it comes under question.
Until recently, experts believed Zika was a relatively benign virus spread by mosquitoes. But now that it's been linked to more than 1,000 microcephaly cases, scientists have taken a closer look, recently declaring it "scarier than we initially thought," as one U.S. health official put it this week. The Centers for Disease Control and Prevention (CDC) recently announced it is now absolutely confident that Zika causes microcephaly—a connection that was suspected but not proven. It also appears to be linked to other disorders like Guillain-Barré syndrome and other autoimmune syndromes. The virus, experts now know, can be transmitted through sex.
In a new twist, experts are questioning the idea that mosquitoes are the primary cause of transmission. In a new study, researchers at the University of California, Los Angeles (UCLA), looked at sequences of the Zika virus over decades and found notable changes in the virus over time, suggesting that the virus' ability to mutate is a reason why it is able to trigger different types of disease. They also noticed that the sequenced strains of Zika from mosquitoes do not match all the strains in humans from this outbreak. This suggests that more people than was expected may be getting the virus some other way.
The virus was first discovered in 1947 and has caused some disease in Africa and Asia before notable outbreaks in Micronesia in 2007 and in French Polynesia in 2013. But the current outbreak is by far the worst. To figure out why, the UCLA team partnered with Chinese Academy of Medical Sciences and Peking Union Medical College in Beijing and compared 40 strains of Zika from past outbreaks as well as strains from the current one. The researchers analyzed some strains collected from people, some from monkeys, and some from mosquitoes. When sequences of the viruses were compared, the scientists noted a variety of differences between them.
"The things that change a lot [in the virus] might explain why it causes different disease now," says study author Stephanie Valderramos, a fellow in obstetrics-gynecology at the David Geffen School of Medicine at UCLA .
What's also curious, the researchers note, is that the strains of the virus collected from humans in this outbreak haven't matched the strains seen in mosquitoes. "We haven’t found any human sequences in the mosquito in recent history," says Valderramos. "It could be we haven't been looking hard enough. If we can’t find them, it brings into question whether the mosquito is the primary mode of transmission in the current epidemic."
It's possible, the researchers suggest, that other modes of transmission, like sex, may play a bigger role. Due to cases of sexually transmitted Zika—this week it was revealed it can be spread via anal sex as well as vaginal sex—the CDC released precautions that people who are in areas of active Zika transmission need to practice safe sex, and abstinence may be recommended during pregnancy. There are currently no areas of active Zika transmission in the U.S., but the virus is spreading locally in Puerto Rico.
"What's new and interesting and scary is that this is the first time a virus transmitted by mosquitoes has been shown to spread in any other way,” says Valderramos.
The researchers of the current study say there's much more that needs to be done before scientists will fully understand Zika. The team plans to continue studying the strains involved in the ongoing outbreak as a way to identify possible targets for drug and vaccines. "We need to broaden our thinking about how this disease can be transmitted and how to stop it," says Valderramos.