• The World Health Organization has declared the Zika virus an international public health emergency, prompted by growing concern that it could cause birth defects.
  • As many as four million people could be infected by the end of the year.
  • Officials at the Centers for Disease Control and Prevention have urged pregnant women against travel to about two dozen countries, mostly in the Caribbean and Latin America, where the outbreak is growing.
  • A rare case of the Zika virus being transmitted through sex, not a mosquito bite, has been reported in the US.

What is the Zika Virus?

The Zika virus is a mosquito-transmitted infection related to dengue, yellow fever and West Nile virus. Although it was discovered in the Zika forest in Uganda in 1947 and is common in Africa and Asia, it did not begin spreading widely in the Western Hemisphere until last May, when an outbreak occurred in Brazil. Millions of people in tropical regions of the Americas may now have been infected. Yet for most, the infection causes no symptoms and leads to no lasting harm. Scientific concern is focused on women who become infected while pregnant and those who develop a temporary form of paralysis after exposure to the Zika virus.

How is the Zika Virus Spread?

Zika is spread by mosquitoes of the Aedes genus, which can breed in a pool of water as small as a bottle cap and usually bite during the day. The aggressive yellow fever mosquito, Aedes aegypti, has spread most Zika cases, but that mosquito is common in the United States only in Florida, along the Gulf Coast, and in Hawaii – although it has been found as far north as Washington, D.C., in hot weather. The Asian tiger mosquito, Aedes albopictus, is also known to transmit the virus, but it is not clear how efficiently. That mosquito ranges as far north as New York and Chicago in summer. Although the virus is normally spread by mosquitoes, there has been one report of possible spread through blood transfusion and one of possible spread through sex. The virus was found on one occasion in semen.

How Might Zika Cause Brain Damage in Infants?

The possibility that the Zika virus causes microcephaly – unusually small heads and damaged brains – emerged only in October, when doctors in northern Brazil noticed a surge in babies with the condition. It may be that other factors, such as simultaneous infection with other viruses, are contributing to the rise; investigators may even find that Zika virus is not the main cause, although right now circumstantial evidence suggests that it is. It is not known how common microcephaly has become in Brazil’s outbreak. About three million babies are born in Brazil each year. Normally, about 150 cases of microcephaly are reported, and Brazil says it is investigating nearly 4,000 cases. Yet reported cases usually increase when people are alerted to a potential health crisis.

What is Microcephaly?

Babies with microcephaly have unusually small heads. In roughly 15 percent of cases, a small head is just a small head, and there is no effect on the infant, according to Dr. Constantine Stratakis, a pediatric geneticist and a scientific director at the National Institute of Child Health and Human Development. But in the remainder of cases, the infant’s brain may not have developed properly during pregnancy or may have stopped growing in the first years of life. These children may experience a range of problems, like developmental delays, intellectual deficits or hearing loss. Microcephaly may also result if a pregnant woman consumes alcohol, is severely malnourished or has diabetes. If the defect occurs in a child’s first years, it may be a result of a brain injury during labor. There is no treatment for an unusually small head.

“There is no way to fix the problem, just therapies to deal with the downstream consequences,” said Dr. Hannah M. Tully, a neurologist at Seattle Children’s Hospital who specializes in brain malformations.

How Do I Know If I Have Been infected? Is There A Test?

Until recently, Zika was not considered a major threat because its symptoms are relatively mild. Only one of five people infected with the virus develop symptoms, which can include fever, rash, joint pain and red eyes. Those infected usually do not have to be hospitalized. There is no widely available test for Zika infection. Because it is closely related to dengue and yellow fever, it may cross-react with antibody tests for those viruses. To detect Zika, a blood or tissue sample from the first week in the infection must be sent to an advanced laboratory so the virus can be detected through sophisticated molecular testing.


  • About 1 in 5 people infected with Zika virus become ill (i.e., develop Zika).
  • The most common symptoms of Zika are fever, rash, joint pain, or conjunctivitis (red eyes). Other common symptoms include muscle pain and headache. The incubation period (the time from exposure to symptoms) for Zika virus disease is not known, but is likely to be a few days to a week.
  • The illness is usually mild with symptoms lasting for several days to a week.
  • Zika virus usually remains in the blood of an infected person for a few days but it can be found longer in some people.
  • Severe disease requiring hospitalization is uncommon.
  • Deaths are rare.


  • The symptoms of Zika are similar to those of dengue and chikungunya, diseases spread through the same mosquitoes that transmit Zika.
  • See your healthcare provider if you develop the symptoms described above and have visited an area where Zika is found.
    If you have recently traveled, tell your healthcare provider when and where you traveled.
  • Your healthcare provider may order blood tests to look for Zika or other similar viruses like dengue or chikungunya.


  • No vaccine or medications are available to prevent or treat Zika infections.
  • Treat the symptoms:
    • Get plenty of rest.
    • Drink fluids to prevent dehydration.
    • Take medicine such as acetaminophen to relieve fever and pain.
    • Do not take aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), like ibuprofen and naproxen. Aspirin and NSAIDs should be avoided until dengue can be ruled out to reduce the risk of hemorrhage (bleeding). If you are taking medicine for another medical condition, talk to your healthcare provider before taking additional medication.
  • If you have Zika, prevent mosquito bites for the first week of your illness.
    • During the first week of infection, Zika virus can be found in the blood and passed from an infected person to another mosquito through mosquito bites.
    • An infected mosquito can then spread the virus to other people.
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