Zika virus: What doctors know about how it potentially spreads

Click to play video: 'U.S. confirms case of Zika virus caught through sexual transmission'
U.S. confirms case of Zika virus caught through sexual transmission
The U.S. has confirmed a case of the Zika virus spreading through sex — not through a mosquito bite as in most cases. As Aarti Pole reports, that has raised concerns Zika could spread more quickly from human to human than first thought – Feb 3, 2016

You can catch Zika virus by getting bitten by an infected mosquito but now global scientists are studying transmission through contaminated blood, sexual intercourse and even from expectant mom to baby.

While the mosquito-borne virus was discovered in the 1940s, its epidemic spread through Latin America and the Caribbean is forcing the medical community to better understand the once-rare disease.

“This is a work in progress…there’s an enormous push to understand this virus and to answer some of the burning questions we have,” Dr. Isaac Bogoch, a tropical infectious disease specialist at the University of Toronto and St. Michael’s Hospital, said.

Unraveling the mystery behind a disease once it hits global proportions is commonplace, according to Dr. Andrew Simor, division head of infectious diseases at Sunnybrook Health Sciences Centre.

READ MORE: 4 cases of Zika virus in Canada, but threat ‘very low,’ chief public health officer says

It occurred with SARS in Canada and China, MERS coronavirus in the Middle East, and Ebola in West Africa most recently, he said.

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“This is exactly the pattern when there’s an explosion of a new, emerging infectious disease. We’re trying to catch up to better understand how the new infectious disease arose, what caused it, how it spreads, how long it stays in the body, what the consequences are and what the risk factors are. This is all new information that is critically important for us to understand the disease,” Simor told Global News.

READ MORE: 5 things Canadians need to know about Zika virus

Here’s what global scientists are quickly piecing together about Zika virus and how humans could get infected.


Like Dengue, West Nile and Yellow fever, Zika virus is a mosquito-borne tropical disease, meaning they transmit the disease to humans.

In short, mosquitoes draw blood from the infected animal then re-inject the blood – with the virus in tow – into others at their next meal.

READ MORE: Should Canadians worry about Zika virus?

After that, the incubation period for the virus ranges from three to 12 days. The illness lasts for up to a week, Tetro says.

From mother to baby

Health officials in El Salvador, Brazil, Jamaica, Ecuador, Honduras and Colombia told residents to stave off pregnancy until as far ahead as 2018 as doctors better understand if the infection tampers with brain development in infants.

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So far, it’s been linked to a 20-fold increase in a rare defect called microcephaly in babies, in which the newborns are born with irregularly small heads and underdeveloped brains.

READ MORE: What pregnant women need to know about Zika virus and travel

But experts keep stressing that the link isn’t backed up by evidence yet.

“We need more evidence to confirm that association and then we need to identify what the mechanism is,” Simor said. He hypothesized that Zika in the blood of a pregnant woman crosses into the placenta and affects the fetus during a critical time in neurological development and growth in the central nervous system.

Sexual intercourse

This week, U.S. health officials confirmed the country’s first case of Zika virus transmitted through sexual intercourse. In Texas, a patient was infected after having sex with a traveller who had just returned from a country battling the mosquito-borne virus.

The incident marked the third reported case of Zika virus transmission through sexual intercourse.

READ MORE: U.S. officials confirm first case of Zika virus spread through sex

“This is not something we can ignore. We need to pay attention to this and learn from this because it’s bringing up more questions,” Bogoch warned.

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So far, the transmission has been from infected male partners. In a 2013 outbreak in French Polynesia, a 44-year-old Tahitian man tested positive for Zika virus in his semen and urine even though his blood didn’t. In 2008, a Colorado man who had travelled to Senegal passed on the disease to his wife even though she hadn’t travelled or been exposed to infected mosquitoes.

Scientists need to quickly learn how long Zika stays in semen, and if men present with symptoms or not while shedding the virus.

“Public health agencies are waking up to this,” Simor said. In England, for example, health officials are urging the public to use condoms for one month if they’ve just returned from a Zika-affected country.

Similar recommendations poured in during the Ebola outbreak once scientists learned that the deadly virus could be passed on in semen. The World Health Organization said that men who survived the virus needed to abstain from sex for three months.

Blood transfusions

On Wednesday, Canadian Blood Services said Canadians returning home from outside of the country, continental United States and Europe won’t be permitted to donate blood for three weeks.

The same precautions kicked in in the United States with a 28-day waiting period.

WATCH: Brazilian doctors taking new blood transfusion precautions

Click to play video: 'Doctors taking precautions after 2 Zika cases contracted through blood transfusion'
Doctors taking precautions after 2 Zika cases contracted through blood transfusion

“It’s everyone’s interpretation of the limited data,” Bogoch said of the discrepancies in wait times between the two nations.

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READ MORE: Don’t donate blood for 3 weeks after returning home from travels

Twenty-one days is plenty of time, according to Bogoch. If a Canadian was bitten by an infected mosquito on the airport tarmac on his arrival home, he’d face a three-to-seven day incubation period, followed by an illness of about three days to a week at most. Then, the immune system would create antibodies forcing the virus to dissipate from the bloodstream.

Zika isn’t like HIV or Hepatitis B or C. In those instances, the virus lingers in the blood, Bogoch said.

Local officials in Brazil said Thursday that two people contracted Zika through contaminated blood. Both transfusions occurred during the first four months of 2015 – one patient was a liver-transplant patient and the other was suffering from a gunshot wound.

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