Causes and Risk Factors of the Zika Virus

Most people understand that the Zika virus is transmitted by mosquito bites, and that, once bitten, the virus can be passed to an unborn baby. But that's not the only way that infection can be spread. Evidence now shows that it is possible to transmit the virus from person to person through unprotected sex and that even infected blood may pose a potential, albeit small, risk.

By understanding how the Zika virus is passed, you will be better able to protect yourself and others from harm.

Zika virus risk factors
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Mosquito Transmission Risk

The Zika virus is a member of the virus family Flaviviridae and is closely related to other mosquito-borne viruses such as those that cause dengue fever, yellow fever, and Japanese encephalitis.

The primary carrier of the virus, the Aedes aegypti mosquito, is unusual in that it is most active during the daytime hours. It thrives in subtropical and tropical climates and can be found in much of South America, Central America, Central and East Africa, India, Southeast Asia, and northern Australia. In the United States, the mosquito is found mostly along the Gulf Coast running from Florida to Texas.

Mosquito bites occur most commonly during the spring and summer months when the insects are actively breeding. It only takes one bite for infection to occur. Once the surrounding skin cells are inoculated, the virus can quickly move into the bloodstream and spread throughout the body.

While most cases of Zika are mild or asymptomatic (without symptoms), the virus can, on rare occasion, lead to a serious complication known as Guillain-Barré syndrome in which the body attacks its own nerve cells. The disorder is believed to be caused when a Zika infection lasts for more than a week and is accompanied by persistent fever.

Pregnancy Risk

While a Zika infection is usually mild and uneventful, it can turn serious if passed to a developing fetus during the early stages of pregnancy. While scientists do not yet fully understand the pathway of the disease, it appears that the virus is able to breach the placenta during the early part of the first trimester when fetal stem cells are just starting to specialize into the brain, heart, and other vital organs.

The virus' impact on these cells can be devastating, causing serious malformations and increasing the risk of miscarriage and stillbirth. The most serious concern is microcephaly, a rare and irreversible birth defect in which a baby is born with an abnormally small head and brain.

The risk of microcephaly appears to be limited to the first trimester. By the second and third trimesters, the risk will have decreased to near-negligible levels, according to research from the Center for Disease Control and Prevention. Still, babies born to mothers with Zika can have serious neurological issues regardless of the trimester she was infected.

Overall, the risk of microcephaly in affected pregnancies is between 1% and 13%. No other contributing risk factors have yet been identified.

Sexual Transmission Risk

While the Zika virus is considered a mosquito-borne illness, early surveillance of the disease revealed that some infections had occurred in climates where mosquito infestations were unlikely. Further investigation revealed that many of these infections were being passed between sexual partners, and mostly from men to women.

According to evidence published in the New England Journal of Medicine, the Zika virus is able to persist in semen even longer than it can in mosquitoes, increasing the potential for male-to-female transmission. By contrast, the virus cannot thrive in either saliva or vaginal secretions, making it less likely to transmit the infection from women to men.

Based on the current evidence, the Zika virus can be passed from a recently infected partner through oral, vaginal, or anal sex whether there are symptoms or not. The sharing of sex toys may also pose a risk.

Blood Transfusion Risk

The risk that the Zika virus poses to the blood supply is unclear. While there have been several credible cases in Brazil linked to platelet transfusions (typically used to treat hemophiliacs or persons undergoing cancer chemotherapy), there have been no similar occurrences elsewhere.

In July 2018, the U.S. Food and Drug Administration announced revised guidelines on the testing of donated blood and blood components for Zika virus. Given the significant decrease in cases of Zika virus infection in the U.S. and its territories, instead of testing individual donations, the FDA recommends testing pooled donations.

Regional Risk

As of July 2019, the World Health Organization (WHO) reported that 87 countries and territories have had evidence of autochthonous mosquito-borne transmission of Zika virus (ZIKV) distributed across four of the six WHO Regions (African Region, Region of the Americas, South-East Asia Region, and Western Pacific Region). Incidence of ZIKV infection in the Americas peaked in 2016 and declined substantially throughout 2017 and 2018. Zika virus transmission has been found in all countries in the Region of the Americas except mainland Chile, Uruguay, and Canada.

Meanwhile, as of July 2017, non-mosquito-borne infections (presumably sexually transmitted) were reported in 13 countries: Argentina, Canada, Chile, France, Germany, Italy, Netherlands, New Zealand, Northern Ireland, Peru, Portugal, Spain, and the United States.

The WHO further reported that infection with ZIKV continues to carry the risk of Guillain-Barré Syndrome and adverse pregnancy outcomes including increased risk of preterm birth, foetal death and stillbirth, and congenital malformations collectively characterized as congenital Zika syndrome (CZS), including microcephaly, abnormal brain development, limb contractures, eye abnormalities, brain calcifications, and other neurologic manifestations.

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Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Zika Transmission. Updated July 24, 2019.

  2. Centers for Disease Control and Prevention. Congenital Zika Syndrome & Other Birth Defects. Updated May 8, 2019.

  3. McCarthy M. Microcephaly risk with Zika infection is 1-13% in first trimester, study shows. BMJ. 2016;353:i3048. doi:10.1136/bmj.i3048

  4. US Food and Drug Administration. FDA announces revised guidance on the testing of donated blood and blood components for Zika virus. 2018.

  5. World Health Organization. Zika Epidemiology Update. Updated July 2019.

  6. World Health Organization. Situation Report: Zika Virus, Microcephaly, Guillan-Barré Syndrome. 2016.

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