Jose Wesley, who was born with microcephaly and screams uncontrollably for long stretches, is attended to on Jan. 30 in Bonito, Pernambuco state, Brazil. (Felipe Dana/AP)

Brazilian officials pushed back Thursday at claims that the country’s export controls are preventing international researchers from obtaining badly needed samples of the virus.

Brazil’s Health Ministry said two-thirds of the Zika samples collected in the country during recent fieldwork performed with a team from the U.S. Centers for Disease Control and Prevention would be sent to the United States, after export of the samples was approved by a medical ethics review board.

The comments came a day after frustrated scientists and international health officials speaking anonymously to the Associated Press said Brazil had not been sending Zika samples abroad because of the country’s tight controls on the export of genetic and biological materials.

According to the AP, scientists at laboratories in the United States and Europe have mostly had to rely upon older Zika samples from patients infected during an outbreak in the South Pacific in 2013.

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As the virus spreads rapidly across the Americas, Brazil has linked Zika to an apparent surge in the number of babies born here with abnormally small heads, the condition known as microcephaly.

Yet the relationship between Zika and microcephaly is not confirmed and is an urgent matter of inquiry for scientists at some of the world’s top medical labs.

Brazil’s policy is well known among scientists who have worked there. Non-Brazilian scientists have been able to collaborate on research after building trust, said one U.S. scientist who is working on Zika-related research with partners in Brazil. The scientist spoke on the condition of anonymity for fear of jeopardizing ongoing research.

Some materials are allowed to leave the country, but biological materials, such as tissue samples, are not allowed out without “a tremendous amount of paperwork,” he said.

“A lot of people are frustrated” by the lack of biological materials that are needed to conduct critical studies, he said. “But the laws of a country have to be respected.”

President Obama has spoken to Brazil’s president, and scientists hope that “an exception can be made because of Brazil’s pandemic to allow biological materials to be freely shared,” the scientist said.

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Meanwhile on Thursday, government researchers at a facility in northeastern Brazil — where the country has registered the biggest spike in microcephaly reports — said that they had identified indications that 12 infants born with the birth defect had Zika. “It is a major breakthrough,” said Lindomar Pena, a virologist and researcher on the government task force studying Zika in Brazil’s Pernambuco state.

Zika has so far been identified in only a handful of the more than 4,000 suspected cases of the rare congenital condition microcephaly reported in Brazil in recent months — in part, scientists say, because the virus lives in the body only for a few days and is extremely difficult to detect months later.

Luciana D’Angelo, executive secretary for health surveillance for the state government, said the test results “strengthen the hypothesis that there is a relationship between Zika and microcephaly.”

This week, the World Health Organization declared a global public health emergency over Zika and its suspected complications in newborns, but neither it nor the CDC has scientifically confirmed a link between the virus and microcephaly.

So far, Brazilian experts have scrutinized 1,113 of the reports of microcephaly received since last fall and found 404 cases in which the condition may have been caused by Zika. The other cases either were linked to something else, such as a genetic factor, or did not turn out to be microcephaly at all.

The Pernambuco researchers tested 12 babies with microcephaly and, in their spinal fluid, found antibodies that the human body makes to fight viruses such as Zika, said Pena, who led the study.

Sun reported from Washington.

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