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A unit of blood donated this week by a resident of Eastern Ontario has tested positive for West Nile virus, Canadian Blood Services said yesterday. It's the first positive test for the mosquito-borne disease this year.

"The donor and officials from the Ottawa and Eastern Ontario public health units have been notified of the test results," the agency said in a statement.

"The unit of donated blood has been withdrawn from inventory and was not shipped to a hospital for transfusion. The blood donor will not be eligible to donate again for 56 days."

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The blood donation was made in Ottawa.

As of Wednesday, blood donations in Ottawa and Eastern Ontario will be subject to single-unit testing, meaning that each unit of blood is tested for the virus. Ordinarily, mini-pool testing takes place, with six units pooled for testing at one time.

"Single-unit testing is believed to be able to identify extremely low levels of virus in blood donations from people who have just become infected," the agency said. After a two-week period, the region will go back to mini-pool testing if there are no new positive donors reported.

Every blood donation has been tested for West Nile virus since the summer of 2003. More than 1.9 million blood donations have been checked between July 1, 2003, and July 24 of this year. Fourteen positives were detected in 2003, but there were no positives in 2004.

"We want donors and recipients to continue to feel confident in their blood system," Graham Sher, CEO of Canadian Blood Services, said in a release. "A greater risk than West Nile virus is that Canadians will not continue to donate and there will be a blood shortage."

Earlier this week, Saskatchewan officials said they were investigating the province's first possible human case of West Nile virus this year. Preliminary tests on a Regina woman in her forties who was showing mild symptoms of the disease came back positive. But further testing is needed.

Saskatchewan had five human cases of the virus last year, but that was down sharply from 2003, when more than 900 people tested positive and six people died.

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