Brazil's Health Minister Marcelo Castro said on Monday that the Zika epidemic in his country is worse than believed because in 80 percent of the cases the infected people have no symptoms.

In an interview with Reuters, Castro said Brazil will start mandatory reporting of cases by local governments next week when most states will have labs equipped to test for Zika, the mosquito-borne virus that has quickly spread through Latin America.

Castro said Brazilian researchers are convinced that Zika is the cause of the 3,700 confirmed and suspected cases of newborns with brain defects in Brazil. He said the virus cannot be transmitted from person to person, only by mosquito.

Brazil will follow the U.S. decision last week to prohibit blood donations from people who have been infected with Zika, he said. The disease, detected for the first time in the Americas in Brazil last year, has no vaccine and no known cure.

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