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CFR Backgrounder: The Zika Virus - February 22, 2016

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The Zika virus, a mosquito-borne illness, has been linked to a dramatic rise in birth defects in Brazil and was reported to be spreading across the Americas in early 2016, prompting the World Health Organization (WHO) to declare a Public Health Emergency of International Concern. By February 2016, mosquitoes carrying the virus had been detected in more than two dozen countries, and the WHO projected that as many as four million people could be infected by the end of the year. Health officials say there are strong indications that the Zika virus is behind the twentyfold increase in cases of microcephaly, a condition in which infants are born with unusually small heads and brains that usually results in developmental disabilities. The U.S. Centers for Disease Control and Prevention (CDC) has said pregnant women, or women who may become pregnant, should consider postponing travel to the nearly thirty countries where the Zika virus has been transmitted. Some governments, including those of Colombia, Ecuador, and El Salvador have advised women against becoming pregnant in the near future.

What is the Zika virus?

The Zika virus is a mosquito-borne illness carried by Aedes aegypti mosquitoes. Most people who are infected do not become ill, but an estimated 20 percent experience symptoms including rash, fever, joint pain, red eyes, muscle pain, and headaches. The incubation period—the time between exposure to exhibiting symptoms—is unknown, but, according to the CDC, it is likely between a few days and a week. In most cases symptoms are mild and last up to a week.

The virus was first discovered in 1947 in the Zika forest in central Uganda, but until 2007, there had only been fourteen documented cases in humans. Experts say the disease likely did not spread among humans in Uganda because the Aedes africanus mosquitoes that transmit the virus there are poorly adapted to human environments, and therefore preferred to prey on monkeys. Researchers found evidence of infections elsewhere in Africa, as well as in Asia, but local populations there appear to have developed resistance to the virus, preventing large-scale outbreaks.

In 2007 officials confirmed forty-nine cases of Zika on the island of Yap, in the Federated States of Micronesia, in the western Pacific. In a 2013–2014 outbreak, nearly four hundred cases were confirmed in French Polynesia, more than five thousand miles southeast of Yap. Researchers say the virus likely arrived in Brazil in 2014, either during the World Cup or a canoe race that brought teams from several Polynesian islands to Rio de Janeiro.

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