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Carefully sealed containers of Zika virus, sent by the Public Health Agency of Canada, are pictured at Brock University in St. Catharines, Ont. Canadian officials have confirmed the country's first sexually-transmitted Zika case.

Canada's first confirmed case of sexually transmitted Zika virus highlights how much remains unknown about the frequency with which the disease spreads and the length of time exposed individuals continue to pose risks, according to a Canadian infectious disease expert.

The Public Health Agency of Canada announced Monday that an individual from Ontario has contracted the Zika virus from a sexual partner who is believed to have contracted the virus while in one of the countries where the disease is actively circulating. Ontario's Ministry of Health and Long-Term Care said it could not provide any details about the patient due to privacy rules.

The confirmation is important because it's the first instance of person-to-person transmission of the Zika virus in Canada. To date, there are 55 confirmed cases of travel-related Zika virus acquired by Canadians visiting areas such as Central and South America. Two of those cases involve pregnant women.

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The Zika virus, which is transmitted primarily by a bite from an infected Aedes species mosquito, has been around since the 1950s in Africa and parts of Asia. It was detected outside of those regions for the first time in 2007, and last May, Brazil had its first confirmed case of the disease. Brazil is continuing to deal with an outbreak of the virus and an explosion in the number of babies born with microcephaly.

Researchers confirmed earlier this month that the Zika virus can lead to a number of birth defects, including microcephaly, which causes babies to be born with abnormally small heads and can result in other developmental problems. Thousands of babies in Brazil have been born with microcephaly linked to the Zika virus in recent months. The World Health Organization says cases of Zika-related microcephaly have also been reported in Cape Verde, Colombia, French Polynesia, Martinique and Panama.

Canada's public-health agency is warning pregnant women or those planning a pregnancy to avoid Zika-affected countries. If women can't avoid travel, they should wait two months after their return to Canada before trying to conceive, the agency says.

Men who have visited a country affected by Zika are being told to use condoms throughout the duration of a partner's pregnancy or to wait six months before trying to conceive. Men are also being told to use condoms every time they have sex for six months after their return from a country affected by Zika.

But Mark Loeb, an infectious disease expert at McMaster University in Hamilton, said that advice could change, depending on the information that emerges as more research is conducted.

"Guidelines are being built on currently available evidence that is potentially a moving target with more information," Dr. Loeb said.

Researchers don't yet know how often the Zika virus is transmitted between sexual partners or whether there is transmission when individuals are asymptomatic. About 80 per cent of those with the virus remain asymptomatic. There are also conflicting reports about the risks to pregnant women, with some estimates suggesting as few as 1 per cent of babies whose mothers are infected with Zika will be born with microcephaly, while others say the number could be as high as 30 per cent, Dr. Loeb said.

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"We don't have the well-defined studies yet," he said.

For instance, studies have shown that the virus can survive in a man's semen for up to 62 days, Dr. Loeb said. But if longer studies are conducted, it could turn out the virus survives even longer than that.

Dr. Loeb said he is designing a study that will follow individuals in Nicaragua and Colombia over a period of time to determine how long the virus remains in a person's body and whether certain health conditions, such as having had dengue fever, have an impact on how long the virus survives.

In February, the World Health Organization declared a public emergency over Zika and its related birth defects. Since last year, a total of 42 countries have experienced an outbreak of the virus for the first time, according to the WHO.

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