This story is part of a partnership between MedPage Today and VICE News.
The Centers for Disease Control and Prevention says it is monitoring 279 pregnant women in the United States and its territories who have tested positive for Zika virus, which has been shown to cause serious birth defects including microcephaly, in which babies are born with abnormally small heads.
The agency announced today that as of May 12, 157 pregnant women had Zika virus in the United States and an additional 122 women had the virus in US territories. For the first time, the CDC is including asymptomatic women because of recent reports that women have tested positive for the virus after delivering babies with Zika-linked birth defects even though they didn't recall having any Zika symptoms.
"Reporting the total number of pregnant women with any laboratory evidence of possible Zika virus infection ... will provide a more comprehensive picture of the effects of Zika in US states and territories," the CDC said in a statement.
Only about one in five people who catch the Zika virus will get sick, according to the CDC. The virus is spread via bites from infected mosquitos. Its symptoms, which include a fever, rash and conjunctivitis (pink eye), last about a week and usually don't require hospitalization or result in death.
The total number of pregnant women being monitored nearly doubled from 113 to 279 overnight as a result of the reporting change, not rapid virus spread, officials stressed.
"This information will help healthcare providers as they counsel pregnant women affected by Zika and is essential for planning at the federal, state, and local levels for clinical, public health, and other services needed to support pregnant women and families affected by Zika," the agency said in a statement.
After months of suspicion and research, the CDC and the World Health Organization last month declared that they had enough information to definitively link Zika virus to microcephaly and other fetal brain abnormalities. The virus has also been linked to a rare autoimmune disorder called Guillain-Barré syndrome, which affects the nervous system and can cause paralysis.
On average, 6 out of every 10,000 babies born in the U.S. have microcephaly, but it's too early to say how much the risk of developing it goes up when pregnant mothers are exposed to Zika virus.
The World Health Organization has also confirmed today that the strain of Zika virus that has been circulating the United States and originated in Asia is now circulating in Cabo Verde, an archipelago nation off the coast of Africa. There were 7,557 cases as of May 8.
"The findings are of concern because it is further proof that the outbreak is spreading beyond South America and is on the doorstep of Africa," Dr, Matshidiso Moeti, WHO Regional Director for Africa, said in a statement. "This information will help African countries to re-evaluate their level of risk and adapt and increase their levels of preparedness."
The WHO has recommended that African countries increase surveillance and raise awareness about potential complications and how citizens, especially pregnant women, can protect themselves. It will support these countries in their preparedness efforts, building on systems strengthened during the Ebola outbreak in West Africa.
Zika was first discovered in 1947 when a monkey in Uganda became infected with the virus, but it wasn't reported in humans for another 20 years. The virus remained in Africa and Asia as part of small outbreaks until 2007. It arrived in the Americas for the first time when it hit Brazil in May 2015, and in November officials noticed that it was linked with an uptick of microcephaly cases in the country, prompting public health investigations around the world. As of May 18, the WHO reported that mosquito transmission of Zika virus was ongoing in 60 countries and territories.