RIO DE JANEIRO (AP) — Two people in southeastern Brazil contracted the Zika virus through blood transfusions, a municipal health official said Thursday, presenting a fresh challenge to efforts to contain the virus on top of the disclosure of a case of sexual transmission in the United States.

The two unrelated cases in Brazil may be the first of people contracting Zika via blood transfusions in the current outbreak, though the U.S. Centers for Disease Control and Prevention, along with other health bodies, have said that Zika could be spread via blood transfusions.

That concern led the U.S. Red Cross to announce that it is asking travelers to Zika outbreak countries to wait at least 28 days before donating blood. Canadian officials said that people who have traveled outside of Canada, the continental United States and Europe won't be able to give blood for 21 days after their return.

Brigina Kemp, a top health official in the Brazilian city of Campinas, told The Associated Press that a gunshot victim and a transplant patient each tested positive for Zika after receiving blood transfusions from different donors.

Kemp said staff at the University of Campinas' hospital first noticed something was wrong in the middle of last year, when Brazil's first cases of Zika were beginning to be reported. Generally so mild that it only causes symptoms in about one out of five cases, Zika began to raise alarm bells after doctors here started to notice a possible link between the virus — spread by the Aedes aegypti mosquito — and the birth defect microcephaly.

The hospital staff noticed abnormal blood work on a young gunshot wound victim who had spent months at the facility. The patient received dozens of blood transfusions from 18 donors between February and May 2015, when he died.

Because the region was in the throes of a dengue outbreak at the time, the staff suspected that disease, which is closely related to Zika, and tested him for it, Kemp said. But the tests came back negative and the blood sample was shelved.

But when an organ transplant patient tested positive for Zika after developing a fever, the hospital's blood bank staff started looking for other possible Zika cases and tests on the gunshot victim's blood samples came back positive.

Transfusions in the two cases were traced to separate donors who had Zika, both of whom reported having suffered symptoms days after they gave blood.

The blood bank then informed Sao Paulo's Adolfo Lutz Institute, which also tested the samples and informed Campinas' health department of the results last month.

The Health Ministry said in an email to The Associated Press, that while the case of the gunshot victim was not yet part of a scientific study, "the case is among multiple investigations under way into the behavior of the virus."

Dante Langhi, president of the Brazilian Association of Hematology and Hemotherapy, told the AP that an academic paper about the transplant case was slated to be published shortly in a specialized medical journal.

Langhi said he had been told that researchers investigating the transplant case had determined that the patient contracted Zika through the transfusion, and not through a bite by the Aedes mosquito that is the virus' main vector.

"The situation must be evaluated and discussed by technical and government authorities," Langhi said.

The Zika virus was first identified in Uganda in 1947 and subsequently spread to parts of Asia. Brazil's first case was recorded in the middle of last year, and the disease quickly spread across country and to more than 20 nations in the region, the Caribbean and beyond, leading the World Health Organization this week to declare an international emergency.

Meanwhile, a Brazilian health workers union called off a strike set to start Thursday because it could affect the country's battle against Zika.

The union's members include workers who go door-to-door in Rio de Janeiro trying to eradicate the mosquito.

The union had threatened to strike if the national health ministry failed to meet demands for better work conditions by Thursday.