There are many things we don’t know about Zika virus, the mosquito-borne disease that may infect three to four million people before the epidemic dies out, according to the World Health Organization.
Experts suspect Zika may be responsible for thousands of babies born with birth defects and an increase in neurological disorders, but it’s going to be a while before we have clear answers on if, and how, the virus actually causes these problems.
On Monday WHO declared that clusters of these birth defects in French Polynesia and Brazil, as well as their possible link to Zika virus, are a public health emergency of international concern. The designation, which the international health agency has only ever applied to three other disease outbreaks, will marshal global resources to help research the virus and its possible connection to the birth defect microcephaly and neurological disorders like Guillain-Barre syndrome. The PHEIC declaration will also help expedite research on creating fast diagnostic tools, vaccines and cures.
Complex, long-term studies are underway to assess the virus, WHO officials said. Until then, here's what we know about Zika and its potential causal link to these serious conditions.
1. What is Zika virus?
Zika virus is a disease that can produce mild symptoms such as fever, rash, conjunctivitis and joint pain. The disease is so mild, in fact, that researchers estimate about four out of five people infected with Zika virus won’t even know they have it -- and those who do feel symptoms will only experience them for two to seven days.
Zika is a type of flavivirus, which means it is closely related to other diseases like chikungunya, dengue, and yellow fever. It's transmitted by the Aedes aegypti mosquito, which also carries the other flaviviruses.
The virus is named after the Zika forest in Uganda, where it was first isolated from a rhesus monkey in 1947.
Handfuls of cases have popped up across Africa and parts of Asia, but researchers considered it an obscure illness until 2013, when it caused a widespread outbreak in French Polynesia. Eleven percent of the population sought care for symptoms. Then in 2015, the disease began to spread rapidly throughout Brazil, where it’s estimated that more than one million people may have contracted the virus.
2. How do you get Zika virus?
Zika virus is a mosquito-borne disease, which means that if a mosquito bites a person infected with Zika virus and then bites a second person, that second person could contract the disease.
The fact that Zika virus appears to coincide with Brazil’s sharp increase in babies born with microcephaly suggests that pregnant mothers can also pass the virus on to their fetuses.
There’s also some early lab evidence to support this: Traces of the virus were found in the amniotic fluid samples of two pregnant women from Paraiba, Brazil, whose fetuses had been diagnosed with microcephaly via ultrasound, the Pan American Health Organization reported in December. Traces of Zika virus were also found in the blood of a microcephalic baby who died soon after birth.
Some preliminary studies also suggest Zika virus could be transmitted sexually. One report from 2011 described how a husband who had traveled through Senegal, where he caught Zika virus, went home to Colorado. There he appears to have sexually transmitted the virus to his wife, who hadn’t traveled with him, reports The New York Times. Then in 2013, scientists found traces of the virus in the semen of a man from French Polynesia, despite the fact that there were no more traces of the virus in his blood.
The U.S.'s first locally transmitted case of Zika virus appears to confirm this research. Dallas health officials announced Feb. 2 that a resident contracted the disease by having sex with a person who had recently traveled to Venezuela. CDC officials later confirmed the mode of transmission.
Dr. Anne Schuchat, principal deputy director of the U.S. Centers for Disease Control and Prevention, emphasized that mosquitos are the main mode of transmission for Zika virus in a press conference Jan. 28.
"The science is clear to date that Zika virus is primarily transmitted to people through the bite of an infected mosquito,” she said. Schuchat also pointed out that the virus is in the bloodstream very briefly. Most people who get it will clear it from their bloodstream in about a week.
3. Why is everyone so scared of Zika virus?
During the 2013 outbreak in French Polynesia, health officials there noted the outbreak coincided with a 20-fold increase in Guillain-Barre syndrome, a neurological disorder in which a body’s immune system attacks its own nerves. It can result in tingling in the extremities, muscle weakness and temporary paralysis. Most people will make a full recovery, but the syndrome can cause death in some rare cases.
But it wasn’t until 2015, during Brazil’s outbreak, that scientists linked the outbreak to a severe birth defect called microcephaly. Microcephaly is when a baby is born with an abnormally small head, possibly because its brain didn't grow properly in the womb. The birth defect can result in lifelong complications like intellectual disability, developmental disorders and seizures.
From 2010 to 2014, Brazil only saw an average of 156 cases of microcephaly per year. But from Oct. 2015 to Jan. 2016, Brazil has recorded over 4,000 reported cases, though they have been able to investigate and discard 462 initial diagnoses according to an update last month.
4. Is there a way to protect myself against Zika virus?
Unfortunately, there is no vaccine or cure for Zika virus.
The CDC expects there will be a few local transmissions of Zika virus in the U.S., while the WHO expects Zika virus to touch every country in the Americas except Canada and Chile, as those places aren't good environments for the kind of mosquito that transmits the disease.
However, it’s important to remember that a Zika outbreak in the U.S. is likely to be limited and short-lived, and that this isn't an issue most Americans should worry about. Generally, if you’re not in any of the Latin American or Caribbean destinations with ongoing Zika virus transmission, you’re not in any danger of getting bitten by Zika virus-carrying mosquitos.
If you’re a pregnant woman contemplating travel to these destination, the CDC advises you to postpone your trip. Barring that, all American travelers should follow the strictest mosquito bite prevention protocol while in these areas. They include wearing long sleeves and pants; treating clothing, gear and tents with permethrin; and using repellants with active ingredients like DEET, picaridin and IR3535.
Scientists suspect that once you’ve been infected with Zika virus, it does confer immunity to future infections. However, Schuchat pointed out, they don’t know how long this immunity lasts.
"This is a fairly new virus in terms of what we understand of it, and so we try to keep an open mind,” she said.
5. Do we know for sure that Zika virus causes microcephaly?
It sure seems like it, but global health experts are reserving judgement on this until scientists gather more evidence.
What we know for sure is that Brazil notified WHO about an alarming increase of babies born with microcephaly in Oct. 2015. They then later connected it to the country’s ongoing and widespread outbreak of Zika virus, which began spreading dramatically in May of 2015.
So far in preliminary tests, scientists have found some correlational evidence for the link between Zika virus and stunted brain development in babies. The CDC published research last week that suggests an association between the virus and the birth defect: among a cohort of 35 babies born with microcephaly in Brazil between Aug. to Oct. 2015, all of their mothers had either lived in or visited a Zika virus-affected area during pregnancy. Twenty-five of the babies had severe microcephaly, which is when the head circumference is more than three standard deviations below average for their age, and 17 had at least one neurological abnormality.
Not all researchers think that Brazil's initial count of over 4,000 reported microcephalic infants are cases of true microcephaly, however, suggesting that the numbers may be spiking because more people are aware of the condition and looking for it.
Schuchat concedes this may be true. In the press conference, she said microcephaly had a “looseness” about it. The condition can also be caused by chromosomal abnormalities, drug and alcohol abuse, severe malnutrition, and infections like German measles, chicken pox, toxoplasmosis and cytomegalovirus. Initial microcephaly diagnoses need to be double-checked, she said, and researchers have to make sure the condition wasn’t caused by any other environmental factor.
While a causal link between Zika and the birth defect is still unclear, the initial evidence was enough to prompt governments in Brazil, El Salvador, Colombia, Jamaica and Puerto Rico to advise women against getting pregnant.
6. Does a pregnant woman need to have symptoms for her fetus to run the risk of microcephaly?
Zika virus itself is such a mild experience that only one in five people ever feel any symptoms. But we don’t yet know whether a pregnant woman needs to experience symptoms for her fetus to be at risk of microcephaly, Schuchat said.
In the face of this ambiguity, the CDC advises pregnant women who have just traveled from a Zika virus-affected country to tell their OB/GYN about their travels and monitor their fetus’ progress with serial ultrasounds for signs of microcephaly.
The jury (of scientists) is still out on this issue, but French Polynesia reported an unusual increase in central nervous system abnormalities in fetuses and infants during 2014 and 2015, around the same time the virus was spreading through the islands. While none of the mothers had experienced Zika symptoms, four tested positive for signs of flavivirus, the genus of viruses that includes Zika, dengue, chikungunya and yellow fever. According to the European Centre for Disease Prevention and Control, this suggests “a possible asymptomatic [Zika virus] infection.”
While scientists suspect that such injury to a fetus’ brain occurs in the first trimester, there isn’t currently enough information to judge whether or not contracting Zika virus is more or less risky at different stages of pregnancy, said Schuchat.
"We do believe that the injury of microcephaly begins in the first trimester, but we don't believe we have sufficient knowledge now to know what, if any, effects infection in the second and third trimester would have,” she said.
Schuchat also said that while more research needs to be conducted, scientists don’t believe getting Zika virus will have consequences for future pregnancies.
"We believe this is a time-limited infection in women and men and children -- that people have symptoms up to a week, and so we don't think this is one where you have months or years of chronic viral infection that could cause harm to [a future] baby,” she said.
7. What should pregnant women who have traveled from a Zika virus-affected country do now?
If you experienced any Zika virus symptoms during your travels or for up to two weeks after you get back, immediately alert your doctor, who will then decide whether or not to submit your blood sample for a Zika virus test. These can take up to two weeks to turn around and are conducted by a small number of specialized labs across the U.S.
If you’re symptomless, tell your OB/GYN about your travel and schedule an ultrasound as soon as possible to detect any brain or skull abnormalities that could reveal microcephaly. Consider scheduling a series of ultrasounds to closely monitor the development of your fetus, advises the CDC, and if your doctor finds evidence of possible microcephaly, consider getting an amniocentesis (which samples the amniotic fluid in the baby’s environment) to test for the presence of Zika. This procedure can only be performed after 15 weeks of pregnancy and comes with a small risk of miscarriage.
8. Does Zika virus also cause Guillain-Barré syndrome?
Guillain-Barré syndrome is a rare neurological disorder that can be caused by many different kinds of infections. The CDC notes that two-thirds of those who develop GBS do so after being sick with diarrhea or a respiratory illness. Bacterial and viral infections can also trigger GBS, and in the very rarest of cases, it can also develop after a person receives a vaccination.
Because of the sharp increase of Guillain-Barré syndrome in French Polynesia after its 2013 Zika outbreak, as well as the uptick in cases in Brazil, scientists believe Zika virus could be another illness that triggers the disorder.
But just like the pregnancy connection, more research needs to be conducted to understand the link between Zika virus and this rare disorder.
9. What should I do if I’ve traveled to a Zika virus-affected area and think I have the illness?
Go to your doctor if you have any of these four symptoms: a rash, joint pain, fever or conjunctivitis. Let them know about where you went, and when. Even if it isn’t Zika virus, it could be one of many other mosquito-borne diseases that need medical attention, like malaria or dengue fever.
To bring down your symptoms, take over the counter drugs like acetaminophen or paracetamol (Tylenol, etc.), but do not take aspirin, ibuprofen or any other nonsteroidal anti-inflammatory drug. Rest and drink plenty of liquids.
Perhaps most importantly, take extra care to prevent mosquito bites, as the insects may be able to transfer the virus from you to an uninfected person.
This story was updated to include mention of the U.S.'s first locally transmitted case of Zika virus.
CORRECTION: A previous version of this story swapped the percentage of people with Zika virus who are likely to be asymptomatic with those who are not. Only about one in 5 people with Zika virus will experience symptoms.
Read more Zika virus coverage:
Hawaii Baby Contracts The First Case Of Zika Virus In The U.S.
Pregnant Women Shouldn't Travel To Countries With Zika Virus, CDC Says