Causes and Risk Factors of the Zika Virus

The Zika virus is transmitted by mosquito bites. It can also be passed from a pregnant mother to an unborn baby, transmitted from person to person through unprotected sex, and infected blood may pose a small risk as well.

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

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. The virus can quickly move from the skin into the bloodstream and spread throughout the body.

Pregnancy Risk

While a Zika infection is usually mild and uneventful, it can have serious consequences if it is passed to a developing fetus during the early stages of pregnancy.

The virus increases the risk of miscarriage and stillbirth, and it increases the risk of severe malformations in the newborn baby. 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.

While scientists do not yet fully understand how the infection causes these effects, it appears that the virus is able to breach the placenta during the early part of the first trimester of pregnancy when fetal stem cells are just starting to develop into the brain, heart, and other vital organs.

Overall, the risk of microcephaly in affected pregnancies is between 1% and 13%.

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 males to females.

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 cases in Brazil linked to platelet transfusions (typically used to treat people who have hemophilia or who are 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 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). The 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, presumed to be 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, fetal death or stillbirth, and congenital Zika syndrome (CZS), which can include microcephaly, abnormal brain development, limb contractures, eye abnormalities, brain calcifications, and other neurologic manifestations.