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
© Verywell, 2018 

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.

Overall, the risk of microcephaly in affected pregnancies is between one percent and 13 percent. 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 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.

On August 26, 2016, the U.S. Food and Drug Administration issued new guidelines on the screening of blood donations in the U.S. Today, any donation that tests positive for the Zika virus will be removed from the blood supply.

Regional Risk

According to a report from the World Health Organization (WHO), a total of 61 countries had experienced a Zika outbreak between 2007 and 2016. This includes three areas in the U.S. affected by the 2016 outbreak: Brownsville, Texas, Miami-Dade County in Florida, and Palm Beach County in Florida.

The WHO further reported Zika-associated disease complications in following areas:

  • Zika-associated microcephaly was reported in 12 countries or territories: Brazil, Cabo Verde, Colombia, El Salvador, French Polynesia, the Marshall Islands, Martinique, Panama, Puerto Rico, Slovenia, Spain, Venezuela, and the United States. Brazil represented the vast majority of these cases. By contrast, the United States reported three.
  • Zika-associated Guillan-Barré syndrome was constrained to 13 countries: Brazil, Colombia, the Dominican Republic, El Salvador, French Guiana, French Polynesia, Haiti, Honduras, Martinique, Panama, Puerto Rico, Suriname, and Venezuela.

Meanwhile, non-mosquito-borne infections (presumably sexually transmitted) were reported in 10 countries: Argentina, Canada, Chile, France, Germany, Italy, New Zealand, Peru, Portugal, and the United States.

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