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This file photo taken on January 25, 2016 shows an Aedes Aegypti mosquito photographed on human skin in a lab of the International Training and Medical Research Training Center (CIDEIM) in Cali, Colombia.LUIS ROBAYO/AFP / Getty Images

Many countries battling outbreaks of the Zika virus are overwhelmed by efforts to control the mosquito population and spread of the infectious disease. But they also face the enormous challenge of trying to keep the blood supply safe.

According to the U.S. Centers for Disease Control and Prevention, there is a "strong possibility" that Zika can be spread through blood transfusions. The CDC says there are suspected cases of Zika transmitted by blood transfusions in Brazil. During the 2013 outbreak in French Polynesia, about 3 per cent of blood donors tested positive for Zika. Because as many as 80 per cent of people infected with the virus don't develop symptoms, many blood donors may not realize they have it.

There is still no way to reliably screen the blood supply for the Zika virus, although the U.S. Food and Drug Administration has approved an investigational screening tool for blood donations in Puerto Rico.

The risk of infection in Canada is vastly lower compared with countries with local mosquito transmission. As of May 5, the federal government has confirmed 67 cases of Zika in people who travelled to affected regions and one case of the virus transmitted sexually in Canada. The mosquitoes that transmit the virus can't survive well in our climate, so experts predict the biggest source of Zika infection will be from travellers returning from Central and South America.

Still, Canada needs to remain vigilant to keep the blood supply safe, said Dr. Mark Loeb, an infectious-disease expert at McMaster University in Hamilton.

"It's a very important issue," Loeb said. "That the risk might be low … for transfusion, but the consequences are extraordinarily high to pregnant women."

To prevent contamination of the blood supply, Canadian Blood Services has imposed a three-week deferral for anyone travelling to a country where Zika is transmitted locally. Dr. Margaret Fearon, director of medical microbiology with Canadian Blood Services, says about 1 per cent of donors have been deferred as a result. Fearon said the peak time for Zika-related travel among Canadians is most likely the winter months, but that the deferral program could present a "challenging time" during the busy summer season when blood donations are often badly needed.

The vast majority of cases of Zika are transmitted by a bite from an infected Aedes species mosquito. While the virus has been around for decades in Africa and parts of Asia, it has more recently spread to other regions. Last May, Brazil had its first confirmed case of the disease and now the country is battling an outbreak of the virus and a surge in the number of babies born with microcephaly, or abnormally small heads, and other developmental problems. The World Health Organization says that as of May 4, 57 countries are dealing with mosquito-borne transmission of the virus.

Dr. Michael Busch, co-director of the Blood Systems Research Institute, in San Francisco, said contamination of the blood supply will be a "very serious problem" this summer in countries that have local mosquito transmission of the virus.

Busch said that countries dealing with Zika outbreaks are doing their best to safeguard blood given to pregnant women. But many of those countries are also dealing with a host of other conditions that make it difficult to control spread of the virus, such as having populations that live in close quarters.

"It's tragic what's happening. Most of those countries, they don't have the resources. No one can really stop the mosquitoes," he said. "The whole socioeconomic ecological context there is very challenging."

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