On Jan. 15, the U.S. Centers for Disease Control issued an unusual advisory. Pregnant women were told to postpone travel, if possible, to 14 countries in Central and South America and the Caribbean, including Mexico, Panama and Brazil. That same day, the Public Health Agency of Canada put out a similar notice. This follows the arrival of a mosquito-borne virus in the Western hemisphere, called Zika. New evidence suggests that, for reasons that aren’t understood, infected pregnant women are at higher risk of having babies with the birth defect microcephaly, which causes abnormally small heads and underdeveloped brains. “This is really unprecedented. A travel advisory of this kind has never happened before,” says Dr. Kamran Khan of St. Michael’s Hospital in Toronto, who studies globalization and the spread of infectious diseases. In our interconnected world, we’re becoming more vulnerable to such events—whether it’s the spread of SARS, Ebola, or now Zika. “The world has become much smaller,” Khan says. “Clinicians [and patients] have to think about diseases that occur not just in our backyard, but anywhere in the world.”
Until very recently, Zika wasn’t considered much of a threat. First detected in the Zika Forest of Uganda in the 1940s, it generally causes only mild symptoms, if any, like joint pain or a rash. (Eighty per cent of people who are infected don’t even realize it, Khan says.) The virus has circulated in parts of Africa and Asia for some time, but suddenly in May 2015, it appeared in Brazil—some have suggested that it came over with World Cup tourists, which hasn’t been confirmed—and that was like throwing a match on a powder keg. One of the world’s most populous countries, Brazil has 200 million people with little or no developed immunity to this virus, Khan notes, and plenty of Aedis aegypti mosquitoes, the species responsible for transmission. Once a person is infected, he can pass the virus along to an uninfected mosquito that bites him, Khan explains. In that way, the virus quickly spreads. “Humans introduce the virus into new geographies, and infect mosquitoes who then propagate and perpetuate transmission.” (The virus’s incubation period is thought to be about 10 days, followed by five days of symptoms, says Khan.)
A possible link to microcephaly in infants caught everyone off-guard. Brazil has seen a twenty-fold increase in infants born with microcephaly compared with the previous year. “When you get a [huge population] like Brazil, and you start to have a large number of cases, rare events become more readily visible,” Khan says. The Centers for Disease Control recently found evidence of the virus in the placentas of two women who gave birth to babies with microcephaly, and in the brains of the two newborns who died—not conclusive proof the virus is the cause, but another piece of evidence that it may be so. Last week, the first case of a U.S. baby born with microcephaly after Zika infection was reported, in Hawaii. The child’s mother had been in Brazil early in her pregnancy. “When we’re dealing with emerging diseases,” Khan says, “we’re learning as we go.” The Pan American Health Organization (PAHO), a branch of the World Health Organization, has asked that member countries report any increases in neurological problems or birth defects, based on increasing evidence that Zika infection could be to blame.
As of Jan. 18, says the PAHO, Zika is circulating in 18 countries and territories in Latin America and the Caribbean. (Barbados recently reported its first cases.) The spread will continue. Khan is author of a recent paper, published in The Lancet, that looks at the virus’s possible trajectory. He and his team mapped the final destinations of international travellers leaving airports in Brazil, from September 2014 to August 2015. Of 9.9 million travellers, 65 per cent were going to the Americas, 27 per cent to Europe and five per cent to Asia, with the greatest number going to the U.S. Members of the team from Oxford University mapped the global geography of the mosquito that transmits Zika, then modelled climate conditions necessary for the virus’s spread between mosquitoes and humans. More than 60 per cent of the populations of the U.S., Argentina and Italy live in areas where seasonal transmission is possible, they say, whereas the U.S. has 22.7 million people living in areas where it could be transmitted year-round. There is no vaccine against Zika, and no antiviral therapy available. Doctors’ best advice to those in affected areas is to cover up with clothing and mosquito repellent. Khan emphasizes that any travellers returning from Zika-infected areas should avoid getting bitten by mosquitoes once they’re home, too, for at least 10 days—on the off chance they are Zika carriers, and might spread it to local bugs and then more people.
In Canada, there’s little risk of that for now, as the Aedis aegypti mosquito doesn’t live here. Its hardier cousin, Aedes albopictus, has been spotted as far north as Chicago and New York (it can also transmit the virus, but perhaps not as efficiently). Even so, our warming climate means these disease-carrying mosquitoes are moving farther north, and will probably one day live in parts of Canada.
More immediately pressing is the situation in Brazil, where families are coping with the outbreak. Thousands of tourists will be flocking there over coming months. On Feb. 5, Rio’s famous Carnival begins; and Brazil plays host to the Summer Olympics in August. Khan is worried that some tourists could unsuspectingly carry the virus home with them. “I’m not as concerned about the Olympics,” he says. “By August, the epidemic might look very different than it does today.” Any epidemic is a quickly moving target. With a warming climate and an interconnected world, the only certainty is that, after Zika, more diseases will follow.