Facts You Should Know About Zika Virus
Picture of an infant with microcephaly
- Zika virus is a virus that can be transmitted to humans by mosquitoes or via sexual contact.
- It is transmitted by the Aedes mosquito, which also transmits dengue fever, Chikungunya, and yellow fever. With climate warming, the habitat for this mosquito is expanding.
- Zika virus causes Zika fever, which is very similar to dengue but is usually milder. Zika fever symptoms and signs include
- Zika virus typically resolves on its own.
- Zika virus has rapidly spread from Africa and Southeast Asia into the Americas.
- Although Zika fever is mild, it can cause severe birth defects in unborn children. Severe birth defects are likely to be a risk from Zika virus infection throughout the duration of pregnancy.
- Microcephaly is one of these severe birth defects, in which the brain is underdeveloped, causing the head to be abnormally small. This defect cannot be outgrown and is associated with delayed development, neurological abnormalities, and intellectual disability.
- Other severe birth defects are also associated with Zika virus.
- Guillain-Barré syndrome and other neurologic problems have also been linked to Zika virus infections. Guillain-Barré syndrome is a paralytic condition that is triggered in certain individuals after various types of infections. Recovery may take months, and there may only be partial improvement.
- In January 2016, the U.S. Centers for Disease Control and Prevention (CDC) recommended pregnant women postpone travel to areas where Zika virus is reported, and if they do travel, to strictly avoid being bitten by mosquitoes in those areas. Zika virus has been detected in semen, and sexual transmission to a non-traveling partner has been documented from men to women, as well as men to men. Men who have lived or traveled in active Zika transmission areas are advised to avoid sex or use condoms if their female partner may be pregnant; they should do this during the entire pregnancy because it is not known how long the virus remains in semen.
- Guidelines for health care providers caring for pregnant women exposed to Zika virus have been issued.
- Zika virus is unlikely to infect a person more than once.
- The situation is evolving. Researchers and public-health authorities are investigating Zika virus closely. Vaccines and tests are being developed.
What Is the Zika Virus?
Zika virus is a Flavivirus, or a virus belonging to the Flaviviridae family, which includes many viruses, including dengue virus, West Nile virus, and yellow fever virus. The Aedes aegypti mosquito is the main insect "vector" (transmitter) of Zika virus, but Aedes albopictus has been documented, as well. These mosquitoes also transmit viruses such as dengue, Chikungunya, yellow fever, viral encephalitis, and some blood parasites.
Where Have Zika Virus Infection Outbreaks Occurred?
Zika virus was isolated from rhesus monkeys in Africa in the 1940s. Human infection was identified in the early 1970s. Zika virus epidemics have occurred in the Pacific, Africa, and Asia, however, it was first transmitted locally in the Western hemisphere in early 2015 in Brazil. Local transmission by mosquitoes has now been documented in South and Central America, the Caribbean, the U.S. territory of Puerto Rico, and Florida. Travelers with Zika virus have been diagnosed in several U.S. states. Locally acquired Zika virus was first documented on the U.S. mainland in Florida. Updated information about Zika virus and areas of active virus transmission can be found at the CDC (http://www.cdc.gov/zika/index.html).
How Is the Zika Virus Transmitted? Is a Zika Virus Infection Contagious?
Zika virus is mainly transmitted by the bite of an infected Aedes mosquito. It is not spread by food, water, or air.
Documented cases of transmission by sexual contact before, during, and after symptoms of Zika fever have shown Zika virus to be a sexually transmitted disease (STD), as well as mosquito-borne. Sexual contact includes vaginal, anal, and oral sex and potentially sharing sex toys. Sexual transmission to a non-traveling partner has been documented between men and women and between men. Transmission has been documented by both vaginal and anal sex. While it has not been documented due to sexual contact between women, it seems likely. Saliva does not appear to shed the virus.
The duration of infectious Zika virus in semen is an important topic of study. Zika virus has been documented to be present in semen after symptomatic illness for at least six months. In European case reports in August 2016, scientists found evidence of Zika virus genes for up to 181 and 188 days; no actual virus grew in cultures of these specimens, and follow-up tests are ongoing. It is not known if the virus is actually transmissible for that long. In addition, sexual transmission of Zika virus has been documented from man to woman, both of whom never had Zika symptoms (they were tested as part of fertility screening). Zika genes were detected in semen over a month after vacationing in Martinique. Studies are ongoing to identify the full duration of Zika virus shedding in semen and how long it may be communicable to a sexual partner, but public-health experts recommend that men who have had Zika virus avoid sex or use strict barrier precautions (condoms) for six months. These developments may also affect the handling of donated sperm and eggs for human fertility treatments.
Studies are ongoing to determine how long Zika may be present in vaginal fluids. Menstrual blood may be a risk factor.
Zika virus has been found in the blood of infected people, and infection has been documented via blood transfusion. This suggests infection could be transmitted via blood-to-blood exposure (such as sharing needles for drug use). The virus is believed to be found in blood for less than one week in most cases. However, a case of prolonged Zika virus detection in blood was documented in a woman after she developed Zika virus infection at around the third week of pregnancy. Zika virus was detected in her blood five and 10 weeks later. The fetus was found to have severe abnormalities other than microcephaly, and high levels of Zika virus were detected in the brain. It is thought the Zika virus in her blood might have come from the placenta or the fetus. It is, however, possible that a woman infected during pregnancy may transmit Zika via blood exposure long after her own symptoms resolve.
A mother infected in pregnancy may transmit Zika virus infection to the developing fetus or newborn. Zika virus has been found in amniotic fluid and fetal brain tissue. Breastfeeding has not been reported to transmit Zika virus to babies, and the benefits of breastfeeding outweigh known risks at this time.
What Are Zika Virus Infection Risk Factors?
The main risk factor for acquiring Zika virus is exposure to Aedes mosquitoes in an area where local transmission has been recorded or suspected. These mosquitoes are found on all continents (except for Antarctica), wherever favorable conditions of warm climate, rain, floods, and standing water may occur.
The second risk factor for acquiring Zika virus is any sexual contact with a person who is or has been traveling in an area of local transmission of Zika virus within the last three months (possibly longer).
An infected mother is a risk factor for transmission of Zika virus to an unborn child ("vertical transmission"). Getting blood transfusions or sharing needles for intravenous drug use in an area of local Zika virus transmission, or perhaps with a woman carrying an infected fetus, are further potential risk factors.
Puerto Rico is the first U.S. territory to experience local Zika virus transmission. The U.S. Food and Drug Administration (FDA) recommended that U.S. territories experiencing local transmission stop local blood collection until all blood donations can be screened for Zika virus or until FDA-approved pathogen reduction technology is in place; donated blood must be imported from transmission-free areas. Local collections resumed in Puerto Rico in April 2016 with blood screened for Zika virus using FDA-approved testing. Florida is the first state to document local transmission via mosquitoes in Miami.
What Is the Contagious Period for Zika Virus?
Most people get Zika virus from mosquito bites. Zika virus is only contagious between people via sexual contact. The CDC suggests women assume they are contagious for up to eight weeks, and men for up to six months, if they have had Zika virus symptoms. The contagious period via sex is still being investigated, however. (See How Is the Zika Virus Transmitted? Is a Zika Virus Infection Contagious?)
What Are Zika Virus Infection Symptoms and Signs?
Most people do not know they are infected with Zika virus. Only 20% of people who are infected develop any symptoms or signs. Zika fever begins with mild headache, followed by a maculopapular rash (pink spots and bumps) starting on the head and upper body and spreading to the palms of the hands and soles of the feet. Very few infections produce a rash on the palms and soles, so this may help identify the infection. Weakness and muscle and joint aches (myalgia and arthralgia) occur, as well as a short period of low-grade fever. Conjunctivitis (red eyes) may occur, as well as stomach pain and diarrhea. These symptoms are usually mild and resolve within a week without specific treatment. Most cases do not require hospitalization. Several of the symptoms of Zika infection are similar to dengue and Chikungunya, which can occur at the same time and may be transmitted by the same mosquitoes in the same areas. In some cases, more serious illness may occur, such as meningoencephalitis. The symptoms of meningoencephalitis are headache, eye sensitivity to light (photophobia), confusion, and inability to bend the neck forward (cannot touch the chin to the chest; stiff neck).
Infants infected in the womb of mothers who were infected with Zika virus at any point in pregnancy may have severe birth defects such as microcephaly (abnormally small head and abnormal brain development), vision and hearing problems, intracranial calcifications, and other physical deformities.
What Types of Specialists Treat Zika Virus Infections?
People with Zika fever symptoms can be evaluated and managed by any primary-care provider, including internists, family practice doctors, pediatricians, or nurse practitioners. If severe symptoms or signs of complications occur, the person may go to an emergency room or urgent-care facility, where evaluation may be performed by an emergency medicine specialist. Admission to a hospital for IV fluids or tests may be necessary. Depending on the situation, an infectious-disease doctor may be consulted either in the hospital or for a clinic visit.
Women who are pregnant and have been exposed to Zika virus, or who have had symptomatic Zika virus while pregnant, are usually evaluated and managed by an obstetrician, sometimes together with a primary-care provider. If laboratory testing confirms Zika virus infection in a pregnant woman, she should be referred to a maternal-fetal medicine specialist.
Babies born with defects due to Zika virus may require treatment by a pediatrician, pediatric neurologist (nervous system specialist), and physical therapist for complications of microcephaly alone. Other specialists may be needed depending on other defects.
How Do Health Care Professionals Diagnose a Zika Virus Infection?
Initial diagnosis of Zika virus is made by a doctor based on symptoms and signs and history of possible exposure to Zika virus. The diagnosis is confirmed by blood, urine, or saliva tests. The specimens are tested for the presence of genetic material of Zika virus with a reverse transcriptase polymerase chain reaction (RT-PCR) test. Because other diseases may also be present in Zika virus areas, tests are also done for other conditions, such as dengue virus and Chikungunya.
The tests for Zika virus are available to any doctor via certain commercial laboratories. Testing is also done by local health-department laboratories and the CDC.
Can People Who Have Returned from a Country with a Zika Outbreak Be Tested for the Infection?
Zika virus testing is not recommended on a returning traveler who has no symptoms unless
- she is found to be pregnant (because her pregnancy needs special monitoring) or
- he/she has symptoms of Zika fever (because he/she may be infectious to mosquitoes and sexual partners).
Zika virus levels may come and go in someone without symptoms, so no one can say the person is not infected and is safe to have sex without precautions. It is simply best to take precautions to avoid infecting a pregnant woman or becoming pregnant while a partner may be infectious.
What Is the Incubation Period for the Zika Virus?
The incubation period is the time between infection and the start of symptoms and signs. For Zika virus, the incubation period is about one to two weeks. For the fetus of a woman infected with Zika virus, the incubation period is not known and may be quite long.
How Is a Zika Virus Infection Treated?
There is no specific medicine that cures Zika virus. The virus must run its course. Treatment includes supportive care for fever, body aches, and stomach symptoms. Rest and oral rehydration therapy (drinking plenty of fluids) is important. Acetaminophen (Tylenol) is the preferred pain reliever (analgesic) for fever and aches. Aspirin, ibuprofen (Advil), naproxen (Aleve), and other nonsteroidal anti-inflammatory drugs (NSAIDs) are avoided, because they may increase the risk of bleeding if dengue virus is also present. A doctor must closely monitor fetal development in women who have been infected during the pregnancy. (See Zika Virus Infections During Pregnancy.)
Are There Zika Virus Infection Home Remedies?
There are no home remedies for the Zika virus other than general supportive treatments as described above. If infected, it is very important to stay indoors for one week, to keep from spreading the virus to mosquitoes and starting a local epidemic. If you are pregnant and believe Zika virus has infected you during pregnancy, see your health care provider right away. (See Zika Virus Infections During Pregnancy.)
Zika Virus Infections During Pregnancy
Zika virus in areas of local transmission highlighted the risk of severe birth defects in unborn children, and the World Health Organization (WHO) declared a public-health emergency in 2015. The CDC issues guidance for U.S. travelers and health care providers of pregnant women exposed to Zika virus. Pregnant women themselves do not seem to have more severe disease or be more susceptible to infection.
Women who may become or are trying to become pregnant should especially postpone travel to areas where Zika transmission is occurring until after pregnancy, or until the area has been declared Zika-free by public-health authorities. If travel cannot be avoided to these areas, both women and men should
- take strict mosquito-avoidance precautions, and
- use strict barrier precautions (male or female condom, dental dam) during sex until several weeks after returning to a Zika-free area.
Women may wish to discuss using a second form of effective birth control with their doctor before traveling to be extra safe. (See Can Zika Virus Infections Be Prevented?)
Women who may have been infected during pregnancy should have the pregnancy closely monitored by a doctor for the possibility of birth defects or other problems. It is not possible to predict if Zika virus will or will not affect an exposed woman or her unborn child at this time. The CDC recommends the following:
- Pregnant women should consult with their health care providers about travel during the pregnancy. Health care providers should also ask about travel during pregnancy or family planning.
- Male partners of women who are pregnant or may become pregnant should avoid sex or use condoms for the duration of pregnancy.
- Zika virus testing is currently available through the CDC and state health departments in the U.S. Health care providers should consult their local health department about testing for Zika virus in the following situations:
- A pregnant woman has traveled to an area with Zika virus transmission and has had two or more symptoms of Zika fever (fever, maculopapular rash, joint pains, or conjunctivitis) within two weeks of travel.
- A woman has the above history, and uterine ultrasound of the fetus shows microcephaly or intracranial calcifications.
- A blood test for Zika virus PCR may be used to test the mother or the newborn.
- It is not known how reliable a Zika virus PCR is when it is performed on amniotic fluid, but a positive result may be useful information, so the risks and benefits of this testing (amniocentesis) may also be considered.
- Testing of the placenta, umbilical cord, and umbilical cord blood should be performed at birth or if a miscarriage occurs.
- If laboratory testing confirms Zika virus infection in a pregnant woman, periodic ultrasound exams should be considered to monitor the fetus. She should also be referred to a maternal-fetal medicine specialist.
- Women who have been exposed to Zika virus have probably been exposed to dengue and Chikungunya, which have similar symptoms, so testing for other infections may also be considered.
- Once the Zika infection has resolved, it does not affect future pregnancies.
- More information and details about Zika virus and pregnancy may be found at the CDC (http://www.cdc.gov/zika/pregnancy/question-answers.html).
What Are Zika Virus Infection Complications?
Most Zika virus infections do not cause lasting effects or complications. However, Zika virus infection can be severe in some cases. Zika virus infection may cause meningoencephalitis.
Zika seems to be a "neurotropic" virus, meaning it targets nervous system tissue. Guillain-Barré, a serious neurologic syndrome, has been reported with Zika virus outbreaks. Most devastating, Zika virus infection during pregnancy has been associated with severe birth defects, including microcephaly and physical deformities. Microcephaly causes an abnormally small head due to stunted brain development.
The eyes and ears are part of the central nervous system, and there have been reports of hearing problems in adults with Zika virus, as well as in infants with microcephaly. In December 2015 in Brazil, a study of 29 babies with microcephaly found eye abnormalities and vision problems in about one-third of them. Other deformities have been documented, such as deformed joints, and it appears that these are also associated with nerve damage during development. Infection at any point throughout pregnancy can be associated with severe defects found at birth or later. The earlier in pregnancy infection occurs, the smaller the head and more severe the defects. Very early infection has been associated with miscarriage.
In Brazil, late-onset microcephaly has been described in up to one in five babies of mothers with Zika virus. The head appears normal at birth but then fails to grow properly. By six months, the brain has stopped growing and severe problems become apparent.
Babies with complications of Zika virus during pregnancy can have many medical needs and may need nursing support at home.
What Is the Prognosis for Zika Virus Infections?
The prognosis, or predicted course of infection and outcome, is generally good for the person infected. Most people fully recover from Zika virus infection after a few days of symptoms and have no further problems.
The prognosis for the fetus or newborn of a woman infected with Zika in pregnancy is not predictable and is actively being investigated. (See Zika Virus Infections During Pregnancy.) Pregnant women themselves do not seem to have more severe disease or be more susceptible to infection. Once the infection has resolved, it does not affect future pregnancies.
Is It Possible to Prevent a Zika Virus Infection?
There is no vaccine against Zika virus, and there is no medication known to prevent it. Avoidance of infected mosquitoes and bites in areas where Zika is active is the primary way to prevent Zika virus. Avoiding sexual transmission, especially to women who are or may become pregnant, is also critical. There is no safe period of time during pregnancy where Zika virus does not cause severe birth defects.
The CDC issues travel advisories that recommend avoidance or enhanced precautions for territories where Zika virus has been reported. This information is updated as new information is received and can be found at the CDC (http://www.cdc.gov/zika/index.html).
Until further studies can be done, women who are pregnant should postpone travel until after delivery. (See Zika Virus Infections During Pregnancy.)
The most reliable prevention requires avoidance of areas where Zika virus has been reported. If traveling or staying in these areas cannot be avoided, mosquito-avoidance precautions should be taken.
Mosquito avoidance precautions include the following:
- Avoid mosquito bites throughout the day and night. Aedes mosquitoes are most active during the day, and they bite indoors as well as outdoors.
- Use long-lasting EPA-registered insect repellents that contain DEET, picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol. Follow the label directions carefully, and apply the repellent on top of any sunscreen or other product you may be using.
- Wear loose long-sleeve shirts and pants when possible. Treat clothing with permethrin or buy permethrin-treated clothing because mosquitoes can bite through clothing.
- Sleep in lodgings with screened windows and doors, or air conditioning, or use mosquito netting around the bed at night. If this is not possible, sleep in a screened and zipped-up tent.
- If you are staying a long time or are at home, mosquito control is important in your environment. Even a bottle cap full of water left outside for a week is a perfect place for mosquitoes to breed. Aedes mosquitoes are adapted to humans and prefer to lay eggs in containers. The eggs can also survive drying out for several months, only to hatch with the next rain, so any collection of water will breed mosquitoes.
- Get rid of any possible breeding areas outside. Remove standing water from around the home. Dispose of tires that may be sitting outdoors. They are excellent breeding grounds because they are hard to empty. Think of any objects that may collect water outside and remove them or turn them over. Make sure all drains are free of clogs, including downspouts.
- People who have Zika fever should stay indoors and avoid mosquito bites for three weeks. Infected people become a source of Zika virus when mosquitoes bite them and then go on to bite another person. The disease then continues to spread. These precautions also help to prevent infection with dengue, Chikungunya, and viral encephalitis, which are found in the same areas as Zika virus and the Aedes mosquito. More detailed information on preventing mosquito bites can be found at the CDC, Zika Virus Prevention (http://www.cdc.gov/zika/prevention/index.html).
Blood-to-blood transmission may occur by transfusion, especially in areas of active transmission where blood donations are not screened for Zika virus. Blood transfusion may not be avoidable, so it is important to consider screening for Zika virus in such cases so appropriate precautions can be advised. Avoid blood-to-blood transmission by not sharing needles with someone who has traveled to such areas and is infected with Zika virus; this may include pregnant women who have been infected several weeks earlier.
Both men and women can be infected by a man or woman with active Zika virus infection. Transmission has been documented by anal and vaginal sex, but other types of sexual transmission, such as mouth to penis, have not been ruled out.
For people returning from an area where Zika virus is active, and who do not have symptoms, the CDC advises the following:
- Both men and women should avoid sex for at least eight weeks after return.
- If you have sex during this time, use barriers to protect against infection, every time during vaginal, anal, and oral sex.
- If the partner of a returning traveler, is pregnant, they should avoid sex or use barriers during sex until the end of pregnancy. (Couples planning pregnancy should discuss this with their health professional.)
- Do not share sex toys during this time.
For people returning from an area where Zika virus is active who develop Zika symptoms, the CDC advises the following:
- Men should avoid sex or use barriers for at least six months after symptoms end.
- Women should avoid sex or use barriers for at least eight weeks after symptoms end.
- Do not share sex toys during this time.
For people living in an area of active Zika virus, the CDC advises the following:
- Avoid sex or use barriers until the area is declared Zika-free.
- If planning pregnancy or if pregnancy occurs, see a health professional right away.
- If not taking precautions and symptoms of Zika fever occur,
- men should avoid sex or use barriers for at least six months after symptoms end and
- women should avoid sex or use barriers for at least eight weeks after symptoms end.
It is very important to use barriers properly and every single time to prevent sexual transmission. Barriers that protect against infection include male and female condoms and dental dams. Dental dams are latex or polyurethane sheets used between the mouth and vagina or anus during oral sex. If not used exactly as intended, they will fail. The CDC provides information about "Condom Effectiveness" in order to help the public protect themselves; this includes links to materials to illustrate correct use of different barriers. (This content is graphic and may not be appropriate for all viewers.)
Because there is continually new information arising about Zika virus and areas of active transmission, it is important to check the CDC Zika web site periodically for updates.
Reviewed on 3/17/2022
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