The Zika virus may cause birth defects — and experts think it's coming to the US

The Zika virus — a rare tropical disease that's causing a panic in Brazil because it may cause babies to be born with abnormally small heads — has made its way to Puerto Rico. And experts fear the mainland United States could be next.

Until last year, the Zika virus, which is spread by mosquitoes, was mainly confined to Africa and Asia. But in 2015 the disease made the leap to the Western Hemisphere, affecting more than a million people in Brazil. It's since spread to Colombia, Venezuela, Mexico, and 10 other countries.

On December 31, the first confirmed case in the US was reported in Puerto Rico. This case suggested local transmission — that a mosquito in Puerto Rico was carrying the virus — since the patient hadn't been traveling.

Meanwhile, travelers returning to other parts of the US have also been diagnosed with the virus, but so far there have been no other reported cases of local transmission on the mainland.

The puzzling spread of Zika is part of an uptick of mosquito-borne illnesses (including West Nile, dengue, and chikungunya) in areas that have never experienced them before.

CDC

"It’s spreading really fast," said Scott Weaver, the director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston. "I think [the Zika virus] is going to be knocking on the doorstep in places like Florida and Texas probably in the spring or summer."

The overwhelming majority of people who come into contact with Zika — through bites from infected mosquitoes — seem to experience either a flu-like illness or no symptoms at all.

But there's reason to pay attention to this virus: Researchers say that Zika may cause microcephaly, a congenital condition that's associated with a small head and incomplete brain development in newborns.

Why the Zika virus is so freaky: It may lead to babies with abnormally small heads

Until very recently, if you asked a health researcher about the Zika virus you would probably get one of two answers.

First: that they've never heard about it. Though Zika was discovered in 1947 (in the Zika forest in Uganda), it hasn't bothered humans much in all these years. Until 2007, there had been fewer than 20 documented cases, followed by mostly local outbreaks in the South Pacific and French Polynesia.

Second: Until recently, most researchers didn't think it was that dangerous. In most people, Zika only seemed to produce mild symptoms — rash, headaches, pain in the bones, and fever — that usually show up between three and 12 days after a mosquito bite. These symptoms go away within a week, and one in four people don’t even develop any illness after being infected with the virus.

Though Brazil usually sees several hundred cases of the birth defect microcephaly each year, in 2015 officials documented more than 3,000 cases

But over the past year, public health officials have started to wonder whether the virus is more harmful than they previously thought.

Consider Brazil. Health officials there estimate that some 1.5 million people have been infected with Zika last year alone, making it ground zero for the largest outbreak in the Americas. (Researchers speculate that the disease may have arrived with World Cup travelers in 2014.)

And here's where things get worrisome. Though Brazil usually sees several hundred cases of microcephaly each year, in 2015 officials documented at least 3,500 cases — that's nearly 20 times more than the usual number.

The uptick in microcephaly started appearing a few months after the virus was identified, which is why Brazilian health officials believe there may be a terrible link between Zika and the birth defect. When they studied the amniotic fluid in pregnant women carrying babies with microcephaly, they found Zika.

More recently, scientists at the CDC provided the strongest evidence yet of a potential link between Zika and microcephaly: They found Zika virus in the brains of two babies with the condition who died within 24 hours of being born.

There’s also a chance that Zika causes Guillain-Barré syndrome, a very rare neurological syndrome that attacks the muscles and can cause paralysis. Zika has also been linked to more than 40 deaths. With such a large outbreak and so many cases to study, researchers think they might be seeing that the virus is actually more dangerous than they previously imagined.

That said, there's a lot the scientific community still has to learn about the virus. Because it was previously so rare, it wasn't very well studied. And it's possible that something else is causing the rise in microcephaly in Brazil, even another virus.

For now, the World Health Organization and its regional outfit the Pan American Health Organization (PAHO) are warning doctors and patients to be on the lookout for Zika, and the US Centers for Disease Control and Prevention recommends that women who are pregnant avoid mosquitoes while traveling in Brazil and Latin America.

Devastating mosquito-borne illnesses are popping up in new places — including the United States

Elife Sciences.org

The spread of Zika is part of an unnerving trend: Several mosquito-borne tropical illnesses have lately been spreading into regions of the world that have never experienced them.

A viral disease called chikungunya — which had never appeared in the Western Hemisphere until 2013 — has lately affected Central and South America, even making an appearance in Florida last year. (Its name comes from the Makonde language of Tanzania, where it was discovered in 1952, and it means "that which bends up," referring to the contorted physique of a person afflicted by the virus.)

Dengue fever, known as "breakbone fever," has also seen new outbreaks in Puerto Rico, Florida, Gulf Coast states, and Hawaii — all places that hadn't usually been affected. In 2015, Brazil reported nearly 1.6 million dengue cases, a big increase from 569,000 in 2014.

Elife Sciences.org

Zika, dengue, and chikungunya are all spread by a type of mosquito called Aedes (in particular the Aedes aegypti and Aedes albopictus mosquitoes). For reasons researchers don't understand, these mosquitoes have been more effective at bringing diseases to new places lately, affecting fresh populations that don't yet have the antibodies to fight off the viruses.

Heidi Brown, a professor of epidemiology at the University of Arizona, explained there are at least three factors that help these illnesses spread: the number of mosquitoes out there, the number that are biting humans infected with the virus, and the number that are surviving long enough to infect other humans.

"The survival of the mosquito is driven a lot by temperature," she added. Mosquitoes thrive in warm and moist environments. "So people go to the idea of global warming — that climate change and changes in precipitation patterns and temperature are helping mosquitoes survive in different areas." In other words, warming is helping expand the range of places that are habitable to mosquitoes.

There are other factors that may be driving the trend, too: People are traveling more than ever, bringing diseases to new locales. More and more people live in crowded cities, where it's easy for viruses to jump from person to person and for mosquitoes to find large concentrations of humans to feast on.

Officials are predicting that Zika is likely to follow the same pattern that dengue fever in the United States has — reaching Puerto Rico first, followed by outbreaks in Florida, Gulf Coast states, and maybe Hawaii. (No one will speculate as to whether the disease will affect babies here.)

Whatever happens, you'll probably be hearing a lot more about Zika and these other viruses very soon. "As temperate areas in the South America or in the US become a little warmer," added Weaver, "Aedes [mosquitoes] will be able to increase their distribution. That will increase areas at risk for dengue, Zika and chikungunya."

Weaver recommends pregnant women avoid traveling to places where the Zika virus is circulating. "Even though the link is not proven, it's strong enough that I would not want to risk doing that," he says. "For normal, healthy adults, it's not a big risk."

Will you join us?

Our biggest supporters are our readers — and we’re so grateful to everyone who has made a contribution during our September campaign. We’re less than 1,000 contributions away from reaching our goal for the month, which in turn will allow us to say yes more often when our incredible journalists come to us with questions they want to answer and projects they want to pursue. Will you make a contribution before the month ends and support our policy coverage through 2024 and beyond?

One-Time Monthly Annual

$5/month

$10/month

$25/month

$50/month

Other

$

Yes, I'll give $5/month

Yes, I'll give $5/month

We accept credit card, Apple Pay, and Google Pay. You can also contribute via