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If you’ve heard about Zika virus and are wondering what in the world it is, you are likely not alone.

A virus few knew about before late last year, Zika has exploded onto the world stage. The reason: Health officials in Brazil have theorized that it is responsible for the country’s alarming rise in women delivering babies with a tragic birth defect known as microcephaly. Babies born with the condition have abnormally small heads and underdeveloped brains. In this case, their life expectancy and development prospects are unclear.

Most years, Brazil would report somewhere around 150 cases of microcephaly. Since last October, more than 4,100 babies have been reported to have been born with the condition.


The country experienced a major outbreak of Zika virus starting in May, with infections estimated in the hundreds of thousands. Health officials have speculated that, for some women, infection in pregnancy has led to an increased rate of miscarriages and an epidemic of babies with microcephaly.

So what exactly is Zika virus? And does it threaten other parts of the world? Read on.


Q: What is Zika?

A: It’s an arbovirus, a broad classification, which means it is a virus that is transmitted by an arthropod — a mosquito, a tick, or a flea. More specifically, it is from a family of viruses called flaviviruses. You likely have heard of some other viruses in that disease-causing family: West Nile virus, dengue virus, and yellow fever virus.

Zika virus, which can be transmitted by mosquitoes, has exploded onto the world stage in recent weeks. Alex Hogan/STAT

Q: Why is it called Zika?

A: The virus was named after the region where it was found — in the Zika Forest of Uganda — in 1947.

Q: What is the infection like? Are there any treatments or vaccines?

A: Zika typically causes a mild infection. Flu-like symptoms — fatigue, aches, and pains — are most commonly reported, along with a skin rash. It often goes undiagnosed; people infected may not seek medical care. When they do, it can be confused for a mild case of dengue fever. Some tests can’t distinguish between the two.

There is no specific treatment. There is no vaccine to prevent infection and a vaccine will be difficult to develop.

It is suspected that Zika infection may trigger lifelong immunity to the virus, says Ann Powers, an arbovirus expert with the Centers for Disease Control and Prevention’s National Center for Emerging Zoonotic and Infectious Diseases. If that turns out to be true, people who have been infected could not be reinfected.

Q: Don’t the experts know whether it triggers lifelong immunity?

Surprisingly little is known about Zika virus. It hasn’t been studied much in the past because it wasn’t thought to be a significant cause of human disease.

It started to garner more scientific attention, though, in 2007 when there was an unexpected Zika outbreak in Yap, an island that is part of Micronesia. Then, in 2013, there was an outbreak in French Polynesia. The virus was on the move. Since Brazil reported its first cases last May, a series of South and Central American countries have diagnosed Zika infections.

Q: Is microcephaly the only serious suspected complication of Zika infection?

A: No. There is also a suspicion it is linked to an increase in cases of Guillain-Barré syndrome, a condition in which progressive paralysis sets in. Patients generally recover fully, but there are rare fatal cases. GBS has been linked to other infections, including influenza.

Q: How does it spread?

A: Aedes mosquitos transmit Zika virus. Mosquitoes become infected by taking a blood meal from an infected person and then they pass the virus as they bite other people. There is also evidence that Zika virus can be transmitted through sex.

Q: Do Aedes mosquitos circulate in the United States?

A: They do.

The southern United States has Aedes aegypti mosquitoes, which can transmit Zika virus. Another type, Aedes albopictus, is hardier and has a range that goes farther north. Lab work suggests they can transmit Zika virus, but it is not clear yet if they are or will be a big player in the spread of the virus.

Q: So, do experts expect Zika virus to circulate in the US?

A: There have already been several dozen cases among people who contracted the virus while traveling outside the country. A patient in the Dallas area is also believed to have contracted Zika through sex with an individual who had previously been infected with the virus in another country.

Experts think it is possible and even likely that parts of the United States, particularly the South, could see clusters of cases. But the experts say such clusters are likely to be small and contained, as was the case in Florida in 2014 with Chikungunya virus, another mosquito-borne virus.

Q: Why?

A: You can thank air conditioning and screens.

Aedes mosquitoes are day-biting bugs that like to live in houses and near people. But it is easier for them to do that where window and door screens are not common and where houses are not closed up to maximize the effect of air conditioning.

Dr. Lyle Petersen, director of the CDC’s division of vector-borne diseases, says the agency thinks Zika virus spread in the United States will play out the way spread of dengue virus has. The CDC has studied eight or nine dengue outbreaks along the Texas-Mexico border. In Texas, the outbreaks are far smaller.

“What we see is there’s plenty of mosquitoes on both sides of the border but we see much less infection on the US side,” said Petersen. “It really does appear that the difference is lifestyle.”

Petersen adds an important caveat, though: “This is a new virus and we don’t know what it’s exactly going to do.”

 Q: Why would infection with Zika virus during pregnancy cause brain development to be stunted in the developing fetus?

A: No one knows, yet.

That said, it is known that infection during pregnancy with some other viruses — rubella and cytomegalovirus or CMV — increases the risk of microcephaly in the infant, says Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston.

Q: What is the evidence for a link between Zika infection and microcephaly?

A: The evidence is largely circumstantial, Weaver says. The sharp rise in the number of babies born with the condition started a few months after the virus arrived. And microcephaly cases have increased in places where lots of Zika cases were diagnosed.

But more persuasive evidence is starting to emerge. Brazilian scientists found traces of Zika virus in amniotic fluid from two women who were carrying fetuses identified as having microcephaly.

And scientists at the CDC have found traces of the virus in brain tissue from two infants with microcephaly who died shortly after birth, as well as in the placenta from two women who miscarried fetuses with the condition.

Q: If the link is real, why haven’t microcephaly cases increased in other places where Zika outbreaks have occurred?

A: Health officials in French Polynesia searched the country’s records for microcephaly cases after hearing of what was happening in Brazil, the CDC’s Peterson says. Where they would normally see no cases or maybe one in a year, they saw 12 in the months after their Zika outbreak.

As for the lack of reports from other countries of Latin America and the Caribbean where Zika virus has been spreading, it may just be that women infected during pregnancy haven’t yet started to give birth, Weaver says.

Also, if a condition is rare, it can be hard to see that there’s been an increase if you are looking in a small population. French Polynesia only saw the increase once they went looking for it. But in Brazil, with more than 200 million people, an increase is easier to see.

Says Weaver: “I think the lack of reporting in other countries is not informative at this point.”

This post will be updated as we learn more information about the virus. 

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