Note: This post was updated on February 5. As more information comes to light about Zika virus, there will be further updates.
A once obscure disease is now making headlines. First identified in Africa in 1947, Zika virus came to the Americas for the first time in early 2015, in Brazil. That was the beginning of a large ongoing outbreak that quickly spread north and has now reached more than 20 countries. And a virus once thought to be not too serious has taken an alarming turn. This past fall, doctors in Brazil noticed a spike in the number of babies with a serious congenital brain deformity called microcephaly — when an infant is born with an underdeveloped brain and skull. Researchers believe the birth defect could be linked to cases of Zika contracted during pregnancy. Earlier this month, the Centers for Disease Control and Prevention issued recommendations for pregnant women or women thinking about getting pregnant to avoid travel to countries where Zika virus is circulating. Here's what we know about Zika — and what we're trying to find out.
What is it? Zika virus infection can cause Zika fever — an illness often accompanied by rash, fever, joint pain and conjunctivitis. It is spread by mosquitoes of the Aedesgenus, but primarily A. aegypti, the same mosquito species that carries the dengue and chikungunya viruses. Zika is usually a mild disease. Only 1 in 5 people infected experience symptoms. Those who do get sick report fever, rash, joint pain and conjunctivitis, and sometimes headaches, muscle pain and vomiting. The illness lasts about a week. Cases severe enough to require hospitalization are rare, and no deaths have been reported. But public health concerns have been raised because of the hypothesis that the virus may cause microcephaly if a pregnant woman becomes infected. The virus may also be linked to Guillain-Barre syndrome in a small proportion of cases. Health authorities are investigating the exact relationship between Zika virus and these neurological conditions. "We are still trying to understand the full spectrum of illness," says Dr. Susan Hills of the Center for Disease Control's Division of Vector-Borne Diseases.
Where is it? After it was first identified in 1947 in Uganda, Zika virus sporadically infected people in Africa and parts of South and Southeast Asia. Then, in 2007 Zika virus was spotted for the first time outside Africa and Asia on Yap Island in the western Pacific Ocean, where it caused a large outbreak. The virus subsequently spread to other island nations in the Pacific, including French Polynesia, where it caused another widespread outbreak in 2014. Eventually, it landed in the Americas, in Brazil, in 2015. Since then, the virus has been leap-frogging north. The pattern of spread has been similar to chikungunya, another mosquito-borne disease that's recently fanned out across South and Central America.
How is it spread?Zika virus is spread by several different species of Aedesmosquitoes, but primarily by A. aegypti. A mosquito that bites an infected person can pick up the virus and after an incubation period of about 10 days, that mosquito can spread the virus when it bites other people. There have been sporadic reports that Zika virus can be spread in other ways. Authorities worry that it might be spread via blood transfusions, during childbirth and possibly during sexual intercourse via male semen, though there is not enough evidence to properly evaluate these risks. The link to congenital microcephaly means pregnant mothers may be transmitting the virus to their fetuses, but researchers are still looking for definitive evidence of this. Mosquitoes, however, are by far the major avenue of spread and the focus of prevention strategies.
Is an outbreak in North America likely?So far, several dozen cases of Zika among continental U.S. residents have been reported, all in travelers returning from countries with Zika outbreaks with one exception — in Texas, a person who'd contracted the Zika virus in another country transmitted it to another person through sexual contact. There are Aedes mosquitoes in parts of the United States, and small clusters of Zika may occur as they have with diseases like dengue, but the likelihood of a large-scale outbreak like Brazil's is low, according to most experts. A. aegypti, the mosquito most likely to spread Zika virus, is found mostly in southeast states along the Gulf Coast. Most people in those states have air-conditioning and adequate window and door screens, measures that keep mosquitoes from biting people.
How is the disease diagnosed in humans?Because Zika fever is so similar to diseases like dengue or chikungunya, lab tests are necessary to verify Zika infection. But that process can be tricky. First, there is no commercial test for Zika virus so blood or tissue samples must be sent to a public health laboratory with specialized testing capacity. Second, the virus is difficult to detect with existing lab tests. Within the first week, it is possible to detect evidence of circulating virus using a molecular testing technique called PCR. After a week, though, the virus is usually no longer in the body. But at this point, samples can be tested for Zika virus antibodies, though the test is not very precise and may detect dengue virus antibodies. Further virus-specific tests need to be done in order to distinguish between the two infections. The CDC is developing rapid laboratory tests that are specific to Zika virus, but it is unclear how long that will take.
Treatment and cure? There is no treatment for Zika virus infection. Painkillers and fever reducers help manage the symptoms until the infection clears, usually in about a week. President Obama has called for increased efforts to find treatments and develop a vaccine. Most estimates are that such treatment is at least three to five years off, although there are reports of a vaccine that could go into trials this year.
How do we stop it?Keep the mosquito population in check by eliminating pools of standing water near dwellings, promote the use of window screens and bed nets, and prevent mosquito bites by wearing long sleeves and pants and using a repellent containing DEET, picaridin or oil of lemon eucalyptus.
Our sources: Dr. Susan Hills of the Center for Disease Control's Division of Vector-Borne Diseases, Nikos Vasilakis of the University of Texas Medical Branch, Brian Foy of Colorado State University.
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