RIO DE JANEIRO – The number of Brazilian babies born with abnormally small heads – a condition known as microcephaly – as a result of prenatal infection with the Zika virus rose 7 percent last week, the Health Ministry said.

The number of babies born with microcephaly this year in an epidemic blamed on Zika jumped from 2,782 as of Dec. 22 to 2,975 in Tuesday’s report.

There were 147 reported cases of microcephaly during 2014, when the virus had not spread across the country.

The Health Ministry is investigating the deaths of 40 babies with suspected microcephaly in recent weeks to determine if it was caused by Zika, a virus transmitted by the Aedes aegypti mosquito whose presence was detected for the first time this year in some Latin American countries.

According to the latest bulletin, based on figures as of Dec. 26, the number of cities with Zika-linked microcephaly cases jumped from 618 a year ago to 656 now.

Suspected cases of microcephaly have been registered in 20 of Brazil’s 27 states.

Although Zika was considered a less serious disease than dengue fever and Chikungunya, which are also transmitted by Aedes aegypti, the realization that it can cause malformations in fetuses and even death prompted Brazilian officials to declare a health emergency last month.

Microcephaly is an irreversible malformation of the brain usually associated with mental, visual and auditory problems.

Since there are no vaccines to prevent or treat any of the three viruses transmitted by the mosquito, the Health Ministry has focused on eliminating Aedes aegypti breeding areas.