The Zika virus outbreak in Central and South America has been given the rare designation of a "public health emergency of international concern." The World Health Organization (WHO) in Geneva took that step on Feb. 1 in part because of what is not yet known about the illness, rather than what has been firmly established by medical researchers.
The unsettling fact is that there is more that we don't know about this virus and its impacts than what we do understand.
"With Zika, we’re nowhere near understanding what the big picture is."
The WHO's designation -- which is reserved for the most severe international health crises -- was driven by a spike in devastating health outcomes that are strongly suspected to be linked to the virus, including a potentially deadly birth defect known as microcephaly and a neurological disorder in adults known as Guillain Barré Syndrome.
“The evidence is growing, and it’s getting strong,” said WHO director-general Margaret Chan during a press conference in Geneva on Monday, regarding the ties between the virus and the debilitating outcomes.
“There is urgent need to do a lot more work,” Chan said.
Here are the most important questions facing public health officials as they wrestle with mounting an effective response to the Zika outbreak.
Is Zika causing the microcephaly and Guillain Barré Syndrome?
Establishing causation is by far the most urgent question facing the global health community in relation to the Zika outbreak. In Brazil, health officials have reported a surge in cases of microcephaly, a birth abnormality in which babies are born with small heads and poor brain development. This can be a fatal condition.
Since October 2015, the number of microcephaly cases in Brazil — which usually hovers around 250 per year — has soared to at least 4,180, as of late January. Many of these cases have been reported in northeastern Brazil.
However, questions have been raised regarding the monitoring and reporting of microcephaly cases in Brazil, with some alleging that this condition is being overestimated.
Chan said the link between Zika virus infection and microcephaly, as well as neurological disorders, is “strongly suspected though not yet scientifically proven.”
“It’s very tricky to prove causation," said Andrew Read, an entomologist and senior scholar at Penn State University, in an interview with Mashable.
Answering the critical causation question is a high priority for medical authorities, since it's possible that risk factors other than Zika are behind the microcephaly upswing as well as the worrisome occurrence of Guillain Barré Syndrome, which is immune disorder that can also affect adults and paralyze a victim.
Scientists do not yet know, for example, exactly how the virus is being passed on from a mother to her unborn child, and because of the lack of solid case numbers and controlled experiments looking at pregnant women with Zika who have different birth outcomes, it's impossible to determine a level of risk for expectant mothers who contract Zika.
This lack of knowledge makes getting the outbreak under control while limiting public anxiety extremely difficult.
How large is the ongoing Zika outbreak?
Scientists do not know how many people have been infected with the Zika virus in Central and South America, partly because only about 20% of those who contract the illness experience any symptoms, and also because accurate means of testing for Zika are not widely available.
Plus, the symptoms of Zika -- fever, rash, joint pain and conjunctivitis (red eyes) -- tend to be mild, leading many to go untested.
The lack of hard numbers on the size of the outbreak means that researchers don't yet know how significant a health threat this truly is.
"With Zika, we’re nowhere near understanding what the big picture is,” Read said.
How important is sexual transmission relative to mosquito transmission?
The news on Tuesday that a traveler returned to the U.S. and infected a non-traveler with Zika through sexual contact raised disturbing new questions about this outbreak. Up until Tuesday, there had been one well-documented case of sexual transmission, which occurred in 2008.
Interviewed shortly before the Centers for Disease Control and Prevention (CDC) and Dallas County health authorities announced the sexually transmitted case in Texas, Read said that a sexually transmitted element to this outbreak would be "pretty scary."
The reason for this is that if an individual is able to spread the virus sexually prior to experiencing Zika-related symptoms, then the virus could spread much faster. “You don’t want transmission to happen before symptoms,” Read said.
“Let’s hope that the sexual transmission is a very rare event.”
The Dallas case in turn raises other important questions, such as how long the infectious period lasts in the semen of male Zika patients.
While the major cause of Zika's spread is the Aedes aegypti mosquito, which is an aggressive daytime biter that can be found from the U.S. southward into South America, it's not clear how much more effective mosquitoes are at spreading the virus compared to human sexual contact.
How infectious is Zika?
This is most likely the largest and most widespread outbreak on record since the Zika virus was discovered in the Zika Forest of Uganda in 1947. Because prior outbreaks were relatively small and isolated, it's not known how many cases a single case of the virus can create, nor is it known how efficiently mosquitoes transmit the virus.
A related question about the virus' infectiousness concerns whether it is growing more or less virulent over time as it spreads and evolves in human populations.
How do environmental conditions affect the mosquito transmission of Zika?
The Aedes mosquito has a large range, but the effectiveness of the mosquito in spreading Zika is likely to differ depending on environmental conditions. For example, milder temperatures may enhance the mosquito's breeding cycle and efficiency at spreading the virus, as do wetter conditions. The climate cycle known as El Niño has resulted in flooding in parts of Brazil and Paraguay, though a direct link between El Niño and Zika's spread has not been established.
The Southeast U.S., which is considered to be the area of the U.S. that is most likely to see active transmission of the virus via Aedes mosquitoes, has had a wetter-than-average winter so far this year, which is also related in part to El Niño.
How do we stop this outbreak?
The best approach right now, experts say, is a combination of mosquito control measures, dramatically scaled up medical research and vaccine development.
For mosquito control, everything is on the table from standard insecticides to new and untested options, such as the release of genetically modified mosquitoes that would decimate the population of female Aedes aegypti mosquitoes. (It is the females that are responsible for biting people and spreading the virus.)
However, humans have a long history of trying and failing to control mosquitoes, including this particular species. So the more rapid a vaccine can be developed, the better off we may be.
That research is just getting started, and although Zika is a new priority for agencies like the National Institutes of Health, researchers are not starting from scratch, since the virus is closely related to yellow fever and dengue.
Have something to add to this story? Share it in the comments.
