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Much is still unknown about how the virus and infant brain damage are related, how the disease spreads and how to stop it. Here's what scientists have confirmed, and what they suspect

Aedes aegypti mosquitoes in a laboratory at the University of El Salvador.

Aedes aegypti mosquitoes in a laboratory at the University of El Salvador.



Zika is a flavivirus, related to dengue, that was first identified in rhesus monkeys in Uganda in 1947 and in humans five years later. It's communicated mainly by mosquitoes. The World Health Organization declared a recent outbreak in the Americas to be a global health emergency.

Earlier this year, Brazilian health officials reported a spike in microcephaly – babies born with unusually small heads, sometimes leading to severe brain damage – that they suggested might be linked to Zika. In March, the WHO acknowledged a scientific consensus that the disease can cause microcephaly and Guillain-Barré syndrome; the U.S. Centers for Disease Control and Prevention followed suit in April.

But there is still a crucial unsolved mystery: Why would the virus cause microcephaly in some fetuses, but not others? In Brazil, for instance, there is a cluster of congenital Zika syndrome in the country's northeast, where the fetal brain defects were first discovered last October, but an expected surge in cases elsewhere in the country never took place – which mean there may be other social or environmental factors at work. Brazil's Health Ministry has launched an investigation to find out more. "We started to think that in this central area maybe more than Zika is causing this intensity and severity" in the brain defects, Fatima Marinho, co-ordinator of epidemiological analysis and information at the ministry, told The Globe.

We asked an expert to clear up some of our Zika questions


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Microcephaly: Microcephaly is, in itself, not uncommon already. It can be caused by other types of infections, such as rubella and syphillis, or by maternal malnutrition. It's still unclear whether babies born with the condition are necessarily more brain-damaged than those who aren't. In one case profiled by The Globe, baby Ester Sophia was born with a piece of her brain missing, but didn't have a smaller head than usual.

Guillain-Barré syndrome: Also worrisome is Zika's potential connection to Guillain-Barré syndrome, a rare autoimmune disease that can cause pain and weakness in the muscles. Complications from Guillain-Barré can sometimes be fatal.

Possible effects on adults: Some adult deaths have been linked to the virus. The CDC, for instance, reported a link between the virus and the April death of a man in his 70s in Puerto Rico who died from severe thrombocytopenia, a bleeding disorder caused by abnormally low blood platelets, which are needed for blood clotting.

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  • 1947: Scientists researching yellow fever in Uganda’s Zika Forest isolate the virus in samples taken from a rhesus monkey.
  • 1948: Virus recovered from Aedes africanus mosquito in the Zika forest.
  • 1952: First human cases detected in Uganda and Tanzania.
  • 1960s-1980s: Zika detected in mosquitoes and monkeys in band of countries stretching across equatorial Africa.
  • 1969-1983: Zika is found in equatorial Asia, including India, Indonesia, Malaysia and Pakistan.
  • 2007: Zika spreads from Africa and Asia to cause the first large outbreak in the Pacific island of Yap.
  • 2012: Two distinct lineages of the virus, African and Asian, are identified by researchers.
  • 2013-2014: Zika outbreaks in French Polynesia, Easter Island, the Cook Islands and New Caledonia. Retrospective analysis shows possible link to birth defects and severe neurological complications in babies in French Polynesia.
  • March 2, 2015: Brazil reports an illness characterized by skin rash in northeastern states.
  • Apr 29, 2015: Brazilian samples test positive for Zika.
  • July 17, 2015: Brazil reports detection of neurological disorders in newborns associated with history of infection.
  • Oct 30, 2015: Brazil reports unusual increase in cases of microcephaly among newborns.
  • Nov 11, 2015: Brazil declares a national public health emergency.
  • November, 2015-January, 2016: Multiple countries in Latin America report Zika outbreaks.
  • Feb 1, 2016: World Health Organization declares a public health emergency of international concern.
  • March 31, 2016: The World Health Organization says there is a strong scientific consensus that Zika can cause microcephaly and Guillain-Barré syndrome, though conclusive proof may take months or years.


Between 2007 and July, 2016, mosquito-borne Zika transmission has been reported in 67 countries and territories, the WHO says.

Countries with locally transmitted cases of the Zika virus
Drag and zoom to navigate the map

Of the affected countries, Brazil has been hit hardest: Authorities in the country registered 91,387 likely cases from February until April 2.

As of July, there have been 168 Canadian Zika cases, Health Canada says. One was by sexual transmission, and one was from mother to child.

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People read zika virus flyers from an information campaign by the Chilean Health Ministry at the departures area of Santiago's international airport, Chile January 28, 2016.

People read zika virus flyers from an information campaign by the Chilean Health Ministry at the departures area of Santiago’s international airport, Chile January 28, 2016.


Can Zika be transmitted sexually?

Yes. The WHO says sexual transmission is relatively common. Canada confirmed its first case of sexually transmitted Zika in April.

Can it be spread through blood transfusions?

The U.S. CDC says that's a "strong possibility." Blood agencies are being careful: In February, Canadian Blood Services instituted a 21-day waiting period for prospective donors returning from international journeys outside Europe or the continental United States.

Is Aedes aegypti actually the main Zika carrier?

Maybe, though maybe not. Brazil's government pointed the finger at Aedes aegypti as the main carrier of the virus, but this is now disputed: Brazilian researchers have found the virus in the far more common Culex quinquefasciatus too.

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A city worker fumigates a public school against mosquitoes in Tegucigalpa, Honduras.

A city worker fumigates a public school against mosquitoes in Tegucigalpa, Honduras.


Is Zika treatable?

Not yet, though there are research projects to develop one, such as Sanofi SA's partnership with the U.S. Army.

Is there an effective test to see if someone has Zika?

Not yet. Short of sending infected blood or tissue for advanced molecular tests, there's no easy way to see if someone has Zika. The infected person might not even notice they're sick.

Can safer sex prevent transmission?

The WHO, CDC and Health Canada urge the use of condoms, dental dams or abstinence in order to prevent spreading Zika. But there's much researchers don't know about how long the virus remains in a person's semen or vaginal fluid once infected. There is also no clear evidence that Zika can be transmitted by kissing.

Are genetically modified mosquitoes our best defence against Zika virus?


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The Zika outbreak comes at an especially bad time for the hardest-hit country, Brazil: It is playing host to the Olympic Games in Rio de Janeiro this month, not to mention grappling with a recent political and economic crisis.

Earlier this year, health experts urged the WHO to consider recommending that the Olympic venues be moved, but the health agency said that doing that wouldn't make a significantly difference to Zika's spread. Several major athletes (including Canadian tennis star Milos Raonic) and some Olympic support staff opted not to travel to Brazil over fears of Zika infection.

Others have seen Zika prevention at the Games as a business opportunity. S. C. Johnson & Son Inc., makers of the insect repellent Off!, is an Olympic sponsor, and is donating free products at the Games. Greenlid Envirosciences, a Toronto-based small business that makes biodegradable mosquito traps, has donated about 100 traps to the Canadian Olympic Foundation.

This billboard in Brazil is designed to trick and kill mosquitoes


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With reports from Stephanie Nolen, Ivan Semeniuk, Wency Leung, Selena Ross, Susan Krashinsky, Alex Migdal, Evan Annett, Reuters and Associated Press