US health officials have concluded that infection with the Zika virus in pregnant women is a cause of the birth defect microcephaly and other severe brain abnormalities in babies.
"It is now clear, the CDC has concluded, that the Zika virus does cause microcephaly," Tom Frieden, director of the US Centers for Disease Control and Prevention, said in a conference call with reporters on Wednesday. "There isn't any doubt that Zika causes microcephaly."
US and world health officials have been saying for some time that mounting scientific evidence points to the mosquito-born virus as the likely cause of the alarming rise in microcephaly in Zika-hit areas of Brazil. It had not been declared as the definitive cause until now.
Never before in history has a bite from a mosquito been seen as the cause of birth defects, Frieden said.
The announcement comes at a critical time for the Obama Administration, which has been trying to get Congress to come up with funding to fight the Zika virus, which is already affecting Puerto Rico and is expected to hit parts of the United States with the coming of mosquito-friendly warmer weather.
The administration has requested about $1.9 billion for Zika prevention and fighting efforts, but the CDC has been forced to divert funds intended for Ebola while awaiting new funding from Congress.
The removal of any lingering doubt about the cause of the birth defects may help spur more intensive efforts to develop diagnostics and vaccines specific to Zika, which is closely related to dengue and other mosquito-borne viruses.
The CDC said it believes that microcephaly, characterised by unusually small head size that can lead to developmental problems in babies, is likely part of a range of serious birth defects being caused by Zika. It also said the microcephaly cases it has reviewed in Brazil are a particularly severe form with devastating brain abnormalities.
Brazil has confirmed more than 1,100 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. It is investigating more than 3,800 additional suspected cases of microcephaly.
Among answers being sought are what percentage of babies born to Zika-infected mothers are likely to suffer birth defects, or whether infected pregnant mothers who did not have symptoms of the virus pose a danger to their babies. Researchers also want to discover the full range of brain and developmental issues that may crop up later in life for infected babies.
CDC officials said they expect to start seeing cases of microcephaly in Colombia soon, based on when reports of infections began in that South American country.
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