Zika virus: 6 things to know about the growing outbreak

The mosquito-borne Zika virus is drawing global attention due to its rapid spread and its possible connection to a rare neurological birth defect.

Earlier today, the head of the World Health Organization said Zika is "spreading explosively" and that an emergency meeting would take place Monday to decide if the virus outbreak should be declared an international health emergency.

Brazil has reported more than 4,000 cases of babies born recently with microcephaly, a brain condition characterized by an abnormally small head that can lead to developmental issues or even death. That number compares with fewer than 150 cases in the country for all of 2014.

And with no vaccine or treatment for Zika, it is likely to keep spreading.

Here's what you need to know about the virus that is putting health officials around the world on alert.

What is Zika?

The Zika virus is a mosquito-borne illness first discovered in 1947 when it was isolated from a monkey found in Uganda's Zika Forest.

The virus has been reported in humans in Asia and Africa since the 1950s, and was first found outside its usual geographic area in 2007, when there was an outbreak in Micronesia, a small cluster of islands in the western Pacific Ocean.

The virus eventually made its way to the Americas and was reported on Easter Island, off the coast of Chile, in 2014. Brazil reported the first case of local Zika transmission in May 2015.

Health officials are particularly concerned about Zika because as a relatively new disease, there has been little to no exposure to the virus by the general population, leading to low immunity.

This helps the virus spread — and quickly.

How is Zika transmitted?

The virus is primarily transmitted through the Aedes aegypti mosquito, a particularly aggressive, white-speckled mosquito that bites during the day. (This is the same mosquito that spreads dengue and chikungunya viruses, making it a triple threat.)

Transmission occurs when an Aedes mosquito feeds on a person infected with Zika, and then spreads the virus by biting an uninfected person.

According to the U.S. Centers for Disease Control and Prevention (CDC), there has been one possible case of transmission through blood transfusion and one possible case of transmission through sexual activity.

The Pan American Health Organization (PAHO), a regional office of WHO, notes Zika can be transmitted through blood, "but this is an infrequent mechanism."

So far there is limited evidence on whether Zika can be transferred from mother to child during pregnancy or at the moment of childbirth. ​But because of the rash of microcephaly cases in Brazil, which spiked after the first confirmed case of Zika, this maternal link is "strongly suspected" and being closely studied.

The Aedes mosquito is prevalent throughout the Americas, with the exception of Canada and Chile, which is another reason for the virus's rapid spread.

What are the symptoms?

Only one in five of those infected with Zika show symptoms, which develop up to one week after being bitten.

Most symptoms are mild, and include fever, rash, headache, muscle and joint pain, and conjunctivitis (red eyes). They can last for two to seven days.

Most symptoms can be easily treated with rest and plenty of fluids. If symptoms worsen, a doctor should be consulted.

There's no vaccine to prevent the Zika virus and no medication to treat it.

There is some evidence of a Zika infection causing serious complications, including microcephaly, with the greatest risk appearing to be associated with the first trimester of pregnancy.

The CDC notes that severe cases of Zika requiring hospitalization are "uncommon" and death is "rare."

There are also links between Zika and Guillain-Barre Syndrome (GBS), with a number of GBS patients in Brazil, French Polynesia and El Salvador also having symptoms consistent with Zika virus infection. GBS can start with tingling and weakness in the body and progress to muscle weakness and paralysis. Most people with GBS recover completely.

Neurological symptoms, such as meningitis, have also been reported in outbreaks of Zika.

Which areas are affected by Zika?

Zika is currently present in 24 countries and territories, 22 of which are in the Americas.

As a response, a number of countries around the world have issued Zika-related travel warnings.

Earlier this month, the Public Health Agency of Canada issued a level-two travel notice, warning Canadian travellers to "practise special precautions" if heading to areas where the virus is circulating. In particular, PHAC warns pregnant women to visit their health-care provider before and after a planned trip to assess the risk.

The CDC issued a similar notice, suggesting pregnant women postpone travel to affected regions.

Officials in some countries, including Brazil, Colombia and El Salvador, have gone so far as to warn women against getting pregnant.

Will it come to Canada?

There have already been travel-related cases of Zika reported in Canada in travellers returning from places where the virus is circulating. But there have been no reports of locally acquired cases, and it's very unlikely there ever will be.

The transmitting vector — the Aedes aegypti and possibly the Aedes albopictus species of mosquito — does not live in Canada.

Climate largely determines where these mosquitoes live, with the species thriving in tropical and sub-tropical regions. Canada's colder conditions mean the species is unlikely to become established here.

And without this mosquito, Zika cannot be transmitted from person-to-person.

Because of the possibility of blood-based transmission, the Canadian Blood Services is going to stop accepting donations from people returning from Zika-hit countries. The U.S. FDA is said to be considering similar measures.

"We are carefully monitoring the Zika virus issue and are currently planning revisions to our travel-deferral policies in order to protect the Canadian blood supply from the threat of this virus. We are working with Health Canada and Héma-Québec to come up with the best strategy," Dr. Dana Devine, chief medical and scientific officer for Canadian Blood Services, said in an email statement.

What's being done to prevent Zika?

Since the primary transmitter of Zika is the Aedes mosquito, efforts are being made to prevent reproduction by eliminating their breeding grounds.

The mosquitoes lay eggs near standing water, so officials are encouraging those in the affected areas to empty out containers where water can pool, such as buckets, flower pots, old tires or pets' water bowls, especially if they're close to a residence. Larvicide can also be added to standing water.

Another way to prevent transmission is to avoid mosquito bites. The use of mosquito repellent, clothing that covers most of the body, mosquito nets at night and screens on windows and doors will all help prevent bites.

In Brazil, officials have vowed to soon mobilize some 220,000 troops to help eradicate the Aedes mosquito through door-to-door visits. Those efforts may be doubled again in August, when the country plays host to the Olympic Games.

Ahead of Monday's emergency WHO meeting, the agency also says it's ramping up its surveillance systems in the affected countries and will "prioritize the development of vaccines and new tools to control mosquito populations."

Tiffany Bateman is a registered nurse and a Fellow in Global Journalism at the Munk School of Global Affairs, University of Toronto.