Zika virus infection is a mosquito-borne viral infection that typically causes no symptoms but can cause fever, rash, joint pain, or infection of the membrane that covers the white of the eye (conjunctivitis, or pinkeye). Zika virus infection in a pregnant woman can cause microcephaly (a serious birth defect) and eye abnormalities in the baby.
- The Zika virus is spread by mosquitoes, but it can also be spread through sexual intercourse, through blood transfusions, and from a pregnant woman to her baby before or during birth.
- Symptoms of Zika virus infection, if they occur, are usually mild.
- Doctors suspect Zika based on a person’s symptoms and recent travel history and confirm the diagnosis based on results of blood or urine tests.
- The best ways to prevent Zika virus infection are to avoid being bitten by mosquitoes and to avoid unprotected sex with a partner who has or may have the infection.
- There is no specific treatment for Zika virus infection, but rest, plenty of fluids, and acetaminophen to relieve fever and pain can help.
The Zika virus, like the viruses that cause dengue, yellow fever, and chikungunya disease, is an arbovirus, spread by a certain species of mosquito, the Aedes mosquito, which breeds in areas of stagnant water. These mosquitoes prefer to bite people and live near people, indoors and outdoors. They bite aggressively during the day indoors and in shady areas outdoors. They are most active during the several hours after sunrise and before sunset. They also bite at night.
In 1947, the Zika virus was first identified in monkeys in the Zika Forest of Uganda. It was relatively unknown until 2007, when the first large-scale outbreaks occurred in the South Pacific islands. In May 2015, local transmission was reported in South America, then in Central America and in the Caribbean, reaching Mexico by late November 2015. Local transmission means that people are bitten by an infected mosquito where they live or work, as opposed to becoming infected while they were traveling.
As of May 2017, cases of locally transmitted Zika virus infection were reported in Miami-Dade County in southeastern Florida and Brownsville, Texas. As of December 2019, no new cases of locally transmitted Zika virus have been reported in the continental United States. Zika virus infection has been reported in travelers returning to the United States after travel to countries where the virus is transmitted locally.
The Centers for Disease Control and Prevention (CDC) issues travel alerts when outbreaks occur. As of December 2019, there are no areas with Zika outbreaks.
Transmission of Zika Virus
During the first week of infection, Zika virus is present in blood. When mosquitoes bite an infected person, they ingest blood containing the virus. The virus replicates in the mosquitoes and, after a few days, when the mosquitoes bite another person, they can transmit the virus to that person.
People who have traveled to areas where Zika virus infection is common may have Zika virus in their blood when they return home. If they live in an area with Aedes mosquitoes, the mosquitoes may bite them, then spread the virus to other people living in the area, resulting in local transmission of the Zika virus.
Although the Zika virus is usually spread by mosquitoes, it may be spread in other ways:
- Through sexual intercourse (vaginal, anal, or oral sex) and the sharing of sexual devices
- From an infected pregnant woman to her baby before or during birth
- Through blood transfusion or organ transplantation
- Through accidental exposure in a laboratory
People with Zika virus infection can transmit the virus through sexual intercourse in many circumstances:
- If the infection never causes any symptoms
- Before their symptoms start
- While they have symptoms
- After their symptoms end
Zika virus remains in semen much longer than it remains in blood and other body fluids. It can be spread by infected men to their sex partners (male or female) through unprotected (no condom) sexual intercourse, including vaginal, anal, and oral sex (see also Zika: Sexual Transmission and Prevention).
The Zika virus also remains in vaginal fluids after it disappears from blood and urine. Infected women can spread the virus to their male partner during sexual intercourse.
Transmission by blood transfusion has been reported in Brazil. However, at present, no cases of transmission by blood transfusion have been confirmed in the United States.
The Zika virus can be spread from mother to child during pregnancy or around the time of birth.
At present, there have been no reports of infants becoming infected with Zika virus through breastfeeding. Even though the genetic material of Zika virus has been found in breast milk, risk of transmission from breast milk is outweighed by the nutritional benefits of breast milk for the infant.
Most people who become infected with the Zika virus have no symptoms, and many do not know they are infected. If symptoms occur, they are usually mild. Infections severe enough to require hospitalization are uncommon. Death due to Zika virus infection is rare.
Did You Know...
Symptoms of Zika virus infection include fever, conjunctivitis (pinkeye), joint and muscle aches, pain behind the eyes, headache, and a red, bumpy rash. Symptoms last 4 to 7 days.
Rarely, Guillain-Barré syndrome develops after a Zika virus infection. Guillain-Barré syndrome is a nerve disorder that causes muscle weakness and a pins-and-needles sensation or loss of sensation that usually lasts for a limited time.
Microcephaly and other abnormalities in babies
Zika virus infection during pregnancy can cause microcephaly in the baby. Microcephaly refers to an abnormally small head. The head is small because the brain does not develop normally and is small.
Babies infected before birth can have many problems besides microcephaly, sometimes including:
- Delays in development (for example, children may have problems with speech and may sit, stand, and walk later than expected)
- Intellectual disability
- Problems with movement and balance
- Feeding problems, such as difficulty swallowing
- Hearing loss
- Vision problem
In the United States, several cases of microcephaly have been linked to the Zika virus. The mothers of these babies were probably infected when they traveled to a country where Zika virus infection is common.
Zika virus infection can cause other abnormalities in the brain and in the eyes (including cataracts). Infected babies may have excess skin on the scalp and, rarely, joints with a limited range of motion (such as clubfoot).
- Blood and urine tests
Doctors suspect Zika virus infection based on symptoms and on the places and dates of travel. However, symptoms of Zika virus infection resemble those of many tropical diseases (such as malaria, dengue, and other mosquito-borne viral infections), and it occurs in the same areas as these infections. Thus, tests are needed to confirm Zika virus infection.
Blood or urine tests can detect the virus if tests are done within a week or two after symptoms begin. These tests may use the polymerase chain reaction (PCR) technique, which increases the amount of the virus’s genetic material and thus makes the virus easier to detect. Tests are also done to check for Zika virus antibody in the blood.
Currently, men who may have been exposed to the Zika virus are not tested to determine whether they are infected and thus at risk of transmitting the virus through sexual intercourse. Instead measures to prevent transmission (such as condoms) are recommended whenever people who may have been exposed to the Zika virus have sexual intercourse (including vaginal, anal, and oral sex).
Testing pregnant women
If pregnant women have traveled to areas where Zika virus infection is common, blood tests for Zika virus are usually done—whether women have symptoms of Zika virus infection or not. Also, if pregnant women may have been exposed to Zika virus, ultrasonography is done to determine whether the fetus is developing normally.
If pregnant women live in areas where Zika virus infection is common, Zika virus infection is a risk throughout pregnancy. If they develop symptoms suggesting Zika virus infection, tests are done during the first week of illness. If they do not have symptoms, doctors usually do tests for Zika virus at the first prenatal visit. If the results are negative, another test is done during the middle of the 2nd trimester to check again for infection. Ultrasonography to check on the fetus’s development is done at 18 to 20 weeks of pregnancy.
If a pregnant woman traveled to or lives in an area where Zika virus infection is common, whether the baby should be tested for Zika virus depends on the following:
- What the mother's Zika virus test results are
- Whether the baby has microcephaly, other brain abnormalities, and/or eye abnormalities
If mothers have positive Zika virus test results, the baby is tested for Zika virus.
If mothers have negative Zika virus test results or were not tested for Zika virus and their baby does not have microcephaly or another abnormality, no testing is needed.
If the baby has microcephaly or another brain abnormality that can be caused by Zika virus infection, the baby is tested for Zika virus, regardless of the mother's test results.
The Centers for Disease Control and Prevention (CDC) has recommended that pregnant women not travel to areas with ongoing Zika virus outbreaks (see also Zika: Pregnant Women). Before travel to areas with risk of Zika virus infection, the CDC recommends pregnant women talk with their doctor about the risks of infection by that virus and about the precautions to be taken to avoid mosquito bites during the trip.
Currently, there is no vaccine to prevent Zika virus infection, although research to develop a vaccine is progressing.
Prevention of transmission of Zika virus by mosquitoes
Prevention of Zika virus infection depends on control of mosquitoes in areas where Zika virus infection is common and on prevention of mosquito bites when traveling to such areas.
To prevent mosquito bites, people should take the following precautions:
- Wear long-sleeved shirts and long pants.
- Stay in places that have air conditioning or that use window and door screens to keep mosquitoes out.
- Sleep under a mosquito bed net in places that are not adequately screened or air-conditioned.
- Use Environmental Protection Agency–registered insect repellents—with ingredients such as DEET (diethyltoluamide) or other approved active ingredients—on exposed skin surfaces.
- Treat clothing and gear with permethrin insecticide (do not apply it directly to the skin).
For children, the following precautions are recommended:
- Do not use insect repellent on infants under 2 months old.
- Do not use products containing oil of lemon eucalyptus (para-menthane-diol) on children under 3 years old.
- For older children, adults should spray repellent on their own hands and then apply it to the children's skin.
- Dress children in clothing that covers their arms and legs, or cover the crib, stroller, or baby carrier with mosquito netting.
- Do not apply insect repellent to the hands, eyes, mouth, or cut or irritated skin of children.
Prevention of sexual transmission of Zika virus
The Zika virus can be sexually transmitted from men to their sex partner (male or female) and from women to men. The CDC has made recommendations about preventing Zika virus from being transmitted during sexual intercourse (CDC: Zika: Sexual Transmission and Prevention).
For pregnant women, precautions to prevent sexual transmission of the Zika virus are particularly important. If their partner (male or female) lives in or has traveled to an area where Zika virus infection is common, the couple should use a preventive measure for the entire duration of the pregnancy. The couple should do one of the following:
- Abstain from sex (vaginal, anal, or oral)
- Consistently and correctly use a barrier birth control method during sex (vaginal, anal, or oral)
This recommendation applies whether the partner has symptoms or not because most Zika virus infections do not cause symptoms, and when symptoms do develop, they are usually mild.
The CDC also provides recommendations when there is no existing pregnancy. The time periods for men and women differ because the Zika virus remains in semen longer than in other body fluids.
Men who have traveled to an area with current or past Zika virus transmission should
- Use condoms or abstain from sex for at least 3 months after he returns, or from the start of symptoms or the date he was diagnosed with Zika.
Women who have traveled to an area with current or past Zika virus transmission should
- Use condoms or abstain from sex for at least 2 months after she returns, or from the start of symptoms or the date she was diagnosed with Zika.
Partners who have both traveled to an area with current or past Zika virus transmission should
- Use condoms or abstain from sex for at least 3 months after the later traveler returns, or from the start of symptoms or the date either was diagnosed with Zika.
Barrier methods include condoms (male and female) and dental dams (used during oral sex). These barriers can reduce the risk of getting the Zika virus during sex. To be effective, condoms and dental dams must be used correctly. They should be placed before sexual activity begins and remain until it ends (see Condoms). They should be used each time, during vaginal, anal, and oral sex.
Not sharing sex devices can also reduce the risk of sexual transmission.
Prevention of transmission of Zika virus through blood transfusion
The risk of Zika virus transmission through blood transfusions is considered extremely low. Nonetheless, the US Food and Drug Administration (FDA) recommends that blood centers in the United States screen donated whole blood and blood components for Zika with a blood screening nucleic acid test (see CDC: Zika and Blood Transfusion).
- Supportive care
There is no specific antiviral drug for Zika virus infection.
Treatment of Zika virus infection is supportive. It includes the following:
- Fluids to prevent dehydration
- Acetaminophen to relieve fever and pain
People who may have Zika virus infection should not take aspirin and or other nonsteroidal anti-inflammatory drugs (NSAIDs) until dengue has been ruled out. If people have dengue and take aspirin or another NSAID, the risk of excessive bleeding (hemorrhage) is increased. Dengue weakens blood vessels, making them more likely to break or leak, and NSAIDs make blood less likely to clot. Also, because of the risk of Reye syndrome, children (aged 18 years and under) should not be given aspirin.
If the Zika virus is detected in pregnant women, doctors may recommend that ultrasonography be done every 3 to 4 weeks to monitor how the fetus is developing. Doctors may refer women to a maternal-fetal medicine or infectious disease specialist with expertise in pregnancy management.
- CDC: Zika Virus: Comprehensive information on Zika virus, from prevention, travel information, and mosquito control through symptoms, testing, and treatment
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