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Zika and Other Emerging Viruses Transmitted by Aedes Mosquitos

The chikungunya, dengue, and Zika viruses are transmitted by Aedes mosquitoes. All 3 viruses have overlapping clinical symptoms and may be found in the same geographic locations.

Outbreaks have been reported throughout the world.1 Since May 2015, when Zika virus was first reported in Brazil, it has been detected in most regions of South America, the Caribbean, and Central America; parts of the South Pacific, Southeast Asia, India, and Pakistan; as well as many African countries. The most recent information on global Zika virus risk can be found at

Local transmission of the Zika virus has occurred in the United States. Centers for Disease Control and Prevention (CDC) reports provisional Zika virus disease case counts in the United States and its territories on the first Thursday of each month. As of May 2, 2018, there have been a total of 5,700 reported Zika virus cases in the 50 US states and the District of Columbia:

  • Presumed locally acquired mosquito-borne cases: 231
  • Travel-associated cases:  5,414
  • Sexually transmitted cases: 55
  • Laboratory-acquired cases: 2
  • Person-to-person transmission through unknown route:1

In addition, there have been 37,229 reported cases in US territories.

Anyone living in or traveling to an endemic area is at risk if bitten by an Aedes mosquito. Persons in areas where Aedes mosquitoes are found may also be at risk as the virus expands its geographic range.

Additional modes of transmission may also put people at risk (see Questions 2 and 5).

Most (80%) people infected with Zika virus are asymptomatic.1 When symptoms do occur, they are usually mild and may include fever, maculopapular rash, arthralgia, or nonpurulent conjunctivitis. Symptoms typically last from several days to a week. Severe disease requiring hospitalization is uncommon, and fatalities are rare.

The virus may be associated with Guillain-Barré syndrome, a rare paralytic condition that sometimes occurs after certain infections. Microcephaly and other congenital anomalies have been associated with Zika virus infection in pregnant women (see below). The association with microcephaly has not been seen in other viral illnesses transmitted by the Aedes mosquitoes.

In addition to mosquito-to-human transmission, Zika virus infection can be transmitted from mother to child during pregnancy, resulting in a congenital infection (see Question 6). The virus can also be spread via sexual activity, blood transfusion, and laboratory exposure. The CDC recommends that pregnant women abstain from sex, or use condoms, for all sexual activity if their sexual partner has traveled to, or lives in, an area with active Zika virus transmission. This recommendation should be followed for the duration of the pregnancy.3

There is a theoretical concern that transmission could occur through organ or tissue transplantation. Although Zika virus RNA has been detected in human breast milk, there have been no reported health problems due to Zika infection acquired through breastfeeding. Because the known benefits of breastfeeding are thought to outweigh the risks of Zika infection acquired through breastfeeding, the CDC recommends breastfeeding even in areas with risk of Zika virus acquisition.4

The Zika virus can be transmitted from a woman to her baby during pregnancy or around the time of birth, leading to microcephaly and other severe fetal brain defects. Congenital Zika virus syndrome is characterized by severe microcephaly, decreased brain tissue, damage to the back of the eye, joint problems such as club foot, and increased muscle rigidity. Not all babies born with congenital Zika virus infection have this typical presentation; some babies may experience slow head growth and develop postnatal microcephaly.

Approximately 1 in 10 pregnancies with laboratory-confirmed Zika virus infection results in a fetus or infant with Zika virus–associated defects.5 The proportion of fetuses and infants with Zika virus–associated defects has been reported to be highest (15%) among those with first-trimester Zika virus infection.5 The full clinical spectrum of congenital Zika virus infection is not yet known. Additional information is available at

It is estimated that Zika virus infection in a woman who is not pregnant would not pose a risk for birth defects in future pregnancies after the virus has cleared from the blood.5

Yes. The CDC has issued a travel alert (Level 2, Practice Enhanced Precautions) for people traveling to certain regions and countries where Zika virus transmission is ongoing. Please refer to the CDC’s travel notices web page for updates (

Women who are pregnant, or are planning to become pregnant, are advised to postpone travel to affected areas if possible. Some countries are recommending that women try to postpone pregnancy until the outbreak is contained. Pregnant women in Zika virus-affected areas should protect themselves from mosquito bites by using air conditioning and screens to keep mosquitoes outdoors, using bed nets if unable to keep mosquitoes outdoors, wearing long sleeves and pants, and using appropriate insect repellants, and using permethrin-treated clothing and gear (except in Puerto Rico, where permethrin should not be used). Pregnant and lactating women can use all US Environmental Protection Agency (EPA)-registered insect repellents according to the product label.

For additional information about pregnancy and the Zika virus, please refer to these Web pages:

Symptomatic persons: Zika virus testing is recommended only for those who live in or have traveled to an area with risk of Zika transmission or have had sex without a condom with a partner who lives in or has traveled to an area with risk of Zika infection.

Asymptomatic persons: Zika virus testing is recommended only for pregnant women who 1) have ongoing exposure to Zika virus (eg, those who live in or frequently travel to an area with risk of Zika); have physician-identified Zika-associated abnormalities on an ultrasound; or give birth to a baby with birth defects that may be related to Zika infection.6Therefore, all pregnant women in US states, the District of Columbia, and US territories should be asked about possible Zika virus exposure and symptoms before and during the current pregnancy, including the periconceptional period (the 6 weeks before last menstrual period or 8 weeks before conception). They should also be asked whether they have had sex (vaginal, anal, or oral) or have shared a sex toy without a condom with anyone who has traveled to or lives in an area with risk of Zika transmission.

It is now advised to ask patients about potential exposure to Zika virus before their current pregnancy; this will help guide decisions about testing and interpretation of test results.

For more information, visit:

For information on evaluating and testing infants, visit:

Paired serum and urine are the primary diagnostic specimens for Zika virus infection.7

Multiple assays and sample types are often needed to establish a definitive laboratory diagnosis of Zika virus infection. The recommended tests include Zika virus RNA and/or IgM antibody tests. It is further recommended that all persons suspected of having Zika infection should also be tested for chikungunya and dengue infections. The symptoms overlap, and the viruses are transmitted in the same regions by the same mosquito vector.

Zika virus RNA testing should be performed during the acute phase of infection, generally up to 2 weeks after onset of symptoms. However, viral RNA may be detectable in urine longer than 2 weeks after symptom onset. Data suggest that Zika virus RNA may be detectable longer in pregnant women; therefore, guidelines recommend RNA testing in symptomatic pregnant women through 12 weeks after symptom onset.7

Zika virus IgM antibodies rise shortly after symptom onset and persist for up to 12 weeks or longer for months after infection, making it difficult to use Zika virus IgM tests to determine if women might have been infected before or after they became pregnant.7 The Zika virus IgM antibody test can exhibit cross-reactivity with antibodies from other flavivirus infections, such as dengue, West Nile, and yellow fever. Thus, confirmation of a Zika virus IgM non-negative test result should be conducted according to CDC algorithms. As the prevalence of Zika virus disease declines, the lower prevalence increases the probability that positive test results are false-positives.7

Refer to for the CDC’s guidance for laboratory testing.

Additional information concerning the Zika virus can be found at For the latest information from the CDC, see “CDC Guidelines for Health Care Providers” at

Additional information about the chikungunya virus and related available tests can be found at:

For more information about testing for these 3 mosquito-borne viruses, refer to the CDC memorandum “Updated Diagnostic Testing for Zika, Chikungunya, and Dengue Viruses in the US Public Health Laboratories” (January 13, 2016) at


  1. Hennessey M, Fischer M, Staples JE.Zika virus spreads to new areas—region of the Americas, May 2015-January 2016. MMWR Morb Mortal Wkly Rep. 2016;65:55–58.
  2. World map of areas with risk of Zika. Centers for Disease Control and Prevention website. Updated March 9, 2018. Accessed July 23, 2018.
  3. Centers for Disease Control and Prevention.Men and Zika. Updated September 7, 2017. Accessed July 23, 2018.
  4. What we know about Zika: after birth. Centers for Disease Control and Prevention website. Updated March 14, 2018. Accessed July 23, 2018.
  5. Reynolds, MR, Jones MA, Petersen EE. Vital Signs: Update on Zika virus-associated birth defects and evaluation of all US infants with congenital Zika virus exposure—US Zika pregnancy registry, 2016. MMWR Morb Mortal Wkly Rep. 2017;66:366-373.
  6. Microcephaly & other birth defects. Centers for Disease Control and Prevention website. Updated March 1, 2018. Accessed July 23, 2018.
  7. Updated guidance for US laboratories testing for Zika virus Infection. Centers for Disease Control and Prevention website. Updated July 24, 2017. Accessed July 23, 2018.


This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.


Document FAQS.178 Version: 3
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