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Zika Virus Infection Explained—Commentary

02/02/16 Matthew E. Levison, MD, Drexel University College of Medicine|Drexel University;

Recent cable news and newspaper headlines include “WHO sounds strong alarm on Zika virus,” “…linked to severe birth defects in thousands of babies in Brazil,” which is "spreading explosively," “could infect as many as 4 million people in the Americas,” and WHO declares "Global Health Emergency." What is this all about?

What is Zika Virus?

Zika virus is similar to the viruses that cause dengue, yellow fever, West Nile fever, and chikungunya disease. Zika virus is transmitted by mosquitoes. The mosquito that transmits most cases of Zika virus, the yellow fever mosquito, is restricted, in the United States, to the deep South, but another type, the Asian tiger mosquito, has adapted to colder climates and is present across a large portion of the southeastern United States (see map). It is not clear whether this mosquito will be able to transmit Zika virus infection in the more temperate climate of the United States.

Aedes mosquito distribution in US

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Where is the Zika Virus Found and How is it Transmitted?

Zika virus was first isolated from monkeys in the Zika Forest in Uganda in 1947, but it was not considered an important cause of disease in humans until the first large-scale outbreaks appeared in the South Pacific islands in 2007. By March 2014, tourists were believed to have carried Zika virus to Easter Island, about 2,180 miles west of continental Chile. In May 2015, local transmission was first reported in South America, then in Central America, and then in the Caribbean, finally reaching Mexico by late November 2015.

Currently, ongoing local transmission of Zika virus has been reported in: 1 North American country (Mexico); 9 South American countries (Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Suriname, and Venezuela); 6 Central American countries (Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama); and 9 Caribbean islands (Barbados, Curacao, Dominican Republic, Guadeloupe, Haiti, Martinique, the Commonwealth of Puerto Rico, Saint Martin, and the U.S. Virgin Islands), for which the US Centers for Disease Control and Prevention (CDC) has issued travel alerts. Also, the CDC has issued a travel alert for Cape Verde, a nation of islands off the northwest coast of Africa, and one for Samoa and American Samoa.

Areas of Zika virus transmission_tn

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Cases of Zika virus infection have appeared in travelers returning home after visits to places where local transmission is occurring. There have been over 30 documented "travel-associated cases" of Zika virus infection in 11 US states.

During the first week of infection, Zika virus can be found in blood and passed from an infected person to mosquitoes. If travelers from regions where Zika virus is common have Zika virus in their blood on arrival in their home country and mosquitoes that can transmit Zika virus are present, local transmission of Zika virus is possible. But, because contact between mosquitoes and people is infrequent in most of the continental United States and Hawaii (due to mosquito control and people living and working in air-conditioned environments), local transmission of Zika virus is expected to be rare and limited.

Although Zika virus infection is transmitted primarily by mosquitoes, other modes of transmission are possible. There has been one report of possible spread through blood transfusion and one report of possible spread through sexual contact. Transmission of Zika virus can also occur from mother to child during pregnancy. Breast milk transmission has been reported for related viruses, so it is possible Zika virus may also be transmitted in breast milk.

What are the Manifestations of Zika Virus Infection?

Only one in five people who become infected with Zika virus will feel sick. Symptoms of Zika virus infection include fever, rash, pink eye, pains in joints, pain behind the eyes, headache, and muscle pain. Symptoms last 4 to 7 days. Most Zika virus infections are mild, but very uncommonly Guillain-Barre syndrome (muscle weakness that affects the legs and then progresses up to the arms) may follow Zika virus infection.

The manifestation responsible for most of the alarming headlines is microcephaly (a birth defect associated with incomplete brain development), which has been linked with Zika virus infection that occurs during pregnancy. The time frame and geographic location of reports of infants with microcephaly coincides with the outbreak of Zika virus infections in Brazil. Only a few cases of microcephaly and congenital Zika virus infection have been confirmed by laboratory tests. The first case of microcephaly linked to Zika virus within the United States was reported recently in Hawaii. The child’s mother was probably infected when she lived in Brazil in May 2015, early in her pregnancy.

Diagnosis of Zika Virus Infection

Many tropical diseases cause symptoms that are similar to those of Zika virus infection, so laboratory testing is needed when Zika virus infection is suspected because of a person’s symptoms and travel history. However, there are no commercially available diagnostic tests for Zika virus infection. In the United States, testing must be done at the CDC. 

The CDC has issued specific guidelines to doctors about when people, especially pregnant women, should be tested for Zika virus infection.

What’s the Bottom Line?

How likely is there actually an increased incidence of microcephaly? Undoubtedly there is a true increase given the numerous reports of Brazilian pediatricians who are witnessing a dramatic increase in the number of cases of microcephaly.

How likely do we think the cause of this increase in microcephaly is Zika virus infection?  The sharp rise in the incidence of microcephaly in Brazil over a short period of time suggests a common cause. The coincidence of the rise in microcephaly with the Zika virus outbreak in time and place suggests that Zika virus infection may be the cause. Much more work has to be done urgently to firm up this association. Pending definitive proof, it is reasonable to take precautions.

How likely is the Zika virus epidemic to continue to grow? Zika virus infection is similar to other infections transmitted by the same mosquitoes, so Zika virus will likely spread in the same way, unless its transmission is interrupted by an effective Zika virus vaccine or control of the mosquito population.

Prevention: Until more is known, the CDC. has recommended that pregnant women consider postponing travel to places with active Zika virus transmission. (

Canadian Blood Services is asking all potential donors who have traveled anywhere other than Canada, the United States or Europe to delay donating blood until one month after their return. The American Red Cross has not yet taken a similar precaution

There is currently no vaccine to prevent Zika virus infection. Control of this infection depends on control of  the mosquitoes that transmit the virus and prevention of mosquito bites when traveling to countries where Zika virus is endemic. The following precautions should be taken:

  • Wearing long-sleeved shirts and long pants
  • Staying in places with air conditioning or that use window and door screens to keep mosquitoes out
  • Sleeping under a mosquito bed net in non-air conditioned environments
  • Using DEET insect repellent on exposed skin surfaces
  • Treating clothing and gear with permethrin insecticide

Dr. Levison is The Manuals' Editorial Board reviewer for Infections.