US health officials said Friday that they are monitoring 279 pregnant women in the United States and Puerto Rico that may be infected with the Zika virus.
Of that number, 157 live in the 50 US states and Washington, while the other 122 live in Puerto Rico, the Centers for Disease Control and Prevention said.
All have “laboratory evidence of possible Zika virus infection,” the CDC said.
The new figure appears to be a dramatic jump from the 110 pregnant women with confirmed Zika cases that the CDC reported as of May 11, but officials say the figures are not comparable, as a new reporting system is being used.
Experts agree that the mosquito-borne Zika virus is behind a surge in cases of the birth defect microcephaly — babies born with abnormally small heads and brains — after their mothers were infected with the virus.
In Brazil, 1,271 babies have been born with unusually small heads and deformed brains since the outbreak of Zika began there last year.
The virus, which usually causes only mild, flu-like symptoms but can cause the rare but serious neurological disorder Guillain-Barre Syndrome, is mainly spread by two species of Aedes mosquito but has also been shown to transmit through sexual contact.
The CDC is now using two separate registries to track pregnant women residing in the United States and all territories, as Puerto Rico is keeping separate records.
“Both of these systems include pregnant women with any laboratory evidence of possible Zika virus infection, with or without symptoms,” said Denise Jamieson, the co-lead on the CDC’s Pregnancy and Birth Defects Team, part of its Zika Virus Response Team.
“We’ve learned a lot in the last four months,” said Margaret Honein, a top official at the CDC’s National Center for Birth Defects and Developmental Disabilities.
Scientists now know of cases of infants born with microcephaly whose test results suggest a Zika infection, but whose mothers did not remember having any symptoms, she said.
The new surveillance systems “cast a broad net to ensure we are monitoring all pregnant women who may be at risk for poor outcome associated with Zika,” Honein said.
The CDC did not release the number of cases of sexually transmitted Zika, saying it cannot definitively separate those cases from mosquito-infection cases.
At least five women contracted Zika without leaving the continental US, but had sexual relations with someone who had, Honein said.
The baseline rate of microcephaly in the United States is normally 6 per 10,000 infants, the CDC said.
There is no vaccine to prevent Zika. Experts have warned the continental United States will likely see an increase in cases during the summer months when mosquitos are more prevalent.
President Barack Obama was due to receive a briefing on the country’s battle against Zika on Friday.
