Causes and Risk Factors of the Zika Virus

Mosquitoes are not the only route of infection

man and woman lie in bed, the woman is asleep the man is awake and thinking.
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Zika is a mosquito-borne virus that has caused widespread infection in parts of Central and South America, spreading northward into the southern United States. In 2016, the rate of infection had become so alarming that the World Health Organization issued an international public health warning in an effort to stave further spread of the disease.

How Zika Is Transmitted

While the Zika virus is primarily transmitted by mosquitoes, most predominately the Aedes aegypti mosquito prevalent in most parts of the world, there are other ways you can get infected.

This includes passing the virus from mother to child during pregnancy. If this happens during early gestation, it can have a devastating impact on the developing fetus. In some cases, it can lead to miscarriage and, in others, a serious birth defect known as microcephaly in which the baby is born with an abnormally small head and brain.

Another, lesser known route of infection is sexual contact resulting from unprotected vaginal, oral, or anal sex. Current research suggests that the virus is more commonly passed from man to woman.

Donated blood may also pose a health risk, although it appears to be less effective in transmitting the virus. To date, there have been no cases of infection via blood transfusions in the U.S., while only a small handful have been linked to platelet transfusions in Brazil.

Transmission by Mosquitoes

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

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

The mosquito is most active in the spring and summer when it is breeding. Only the female bites, the blood of which she uses to mature her eggs. The mosquito itself can be recognized by white markings on its legs and a marking on its back in the shape of a lyre.

It only takes one bite to cause infection. Once the local skin cells are infected, the virus will moves swiftly into the bloodstream and spread throughout the body.

As many as 80 percent of people infected with the Zika virus will not have any symptoms. Those who do may experience mild, flu-like symptoms including:

  • Headache
  • Minor fever
  • Muscle aches
  • Joint pain
  • A widespread, bumpy rash

If traveling to an area with Zika is endemic, preventive measures should include protective clothing and the use of an insect repellent (ideally with a DEET concentration of greater than 10 percent). Mosquito bed netting should also be considered.

Pregnancy Risk

While a Zika infection is rarely serious in adults, the opposite may be true if the virus is passed from mother to child during the first trimester of pregnancy.

Oftentimes, the mother will not even know she been infected. In some cases, the infection may occur before the woman even realizes she's pregnant.

While scientists do not yet fully understand the pathway that the virus takes from mother to child, it appears that that virus able to breach the placenta in the early stages of pregnancy when fetal stem cells are just starting to specialize.

The virus' impact on these cells can be devastating, causing serious malformations and lifelong disabilities. The most common concern, microcephaly, is believed to affect anywhere from one percent and 13 percent of pregnancies where an Zika infection is involved.

The risk appears to be greatest during the early part of the first trimester. By the second and third trimesters, the risk will have decreased to near-negligible levels.

Symptoms of microcephaly can vary but may include:

  • Seizures
  • Developmental delays
  • Intellectual disability
  • Problems with movement and balance
  • Feeding problems (including difficulty swallowing)
  • Hearing loss
  • Vision problems

Symptoms can range from mild to severe and are typically related to the reduced size of the child's head. Breastfeeding does not appear to pose a risk. Because the infection is relatively short-lasting, having had a prior Zika infection poses no risk to future pregnancies.

In terms of disease avoidance, the same rules regarding mosquito prevention would apply to women who are pregnant or are trying to get pregnant. Special attention would also be needed to identify and understand the role of sex in transmitting the virus.

Sexual Transmission Risk

While the Zika virus is largely considered a mosquito-borne illness, the surveillance of the virus across four continents revealed that some infections were occurring in climates where the risk of infection was low.

Further investigation revealed that many of these infections were being passed between sexual partners and mostly from men to women. Before long, scientists were able to confirm that the virus was not only active in semen but able to persist even longer than it could in mosquitoes. (By contrast, the virus does not appear to thrive in either saliva or vaginal secretions.)

Based on the current evidence, transmission of the Zika virus can occur as a result of oral, vaginal, or anal sex whether the partner has symptoms or not. The sharing of sex toys is also considered a risk.

The Centers for Disease Control and Preventions currently advises couples to abstain from sex or to use condoms for at least six months if the male partner has been to a region where Zika is endemic. This is especially true if the female partner is pregnant or of child-bearing years.

The partner should also use mosquito repellent for at least three weeks after returning home. This can help prevent the further spread of infection by mosquitos.

Blood Transfusion Risk

The risk that the Zika virus poses to the blood supply is unclear. While there have been a few 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.

How to Reduce Your Risk

News stories about Zika can be scary, particularly if you're pregnant or planning to get pregnant. However, it is important to remember that the risk of the disease can be significantly reduced if you take the time to identify your personal risks.

If you or your partner are planning to travel abroad, check for travel advisories on the CDC website. In some cases, it may be wise to adjust or postpone your travels, particularly during the spring or summer months.

If you believe you have been exposed to the virus, a simple blood or urine test can be used to check for infection. On the off-chance the test is positive, you can take measures to prevent infecting others or make an informed choice as to whether you should delay pregnancy.

Whether at home or abroad, knowledge is your best defense against the Zika virus.


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Oster, A.; Brooks, J.; Stryker, J. et al. "Interim Guidelines for Prevention of Sexual Transmission of Zika Virus - United States, 2016." MMWR. 2016; 65(5):120-1. DOI: 10.15585/mmwr.mm6505e1.

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