Zika Virus - Mosquitoes

In Brief

  • Zika virus is an insect vector-borne disease that can be transmitted via exposure to infected blood, through sexual contact, or from mother to child during pregnancy.
  • Outbreaks have been linked to increased incidence of microcephaly and Guillain-Barré syndrome.
  • There is no vaccine for Zika, so responding to outbreaks involves stepped-up public-health efforts, including vector control of mosquitoes, work to expand public awareness, staff training, and development of treatment protocols.

In Focus

About the Zika Virus

Zika virus is an arbovirus, an insect vector-borne disease most commonly transmitted through Aedes (aegypti and albopictus) mosquitoes. Less commonly, Zika can be transmitted from mother to child during pregnancy. In some cases, Zika virus transmission also has been reported through exposure to infected blood or through sexual contact.

Zika is endemic to parts of Africa and Asia. Since early 2015, a large number of locally acquired cases (estimated to be as many as 1.5 million cases by the World Health Organization) have been discovered in Brazil, where the disease was previously unknown. The high number of cases and the novel locations of the outbreak made Zika a rapidly growing epidemic threat. Zika developed into a region-wide pandemic as human and mosquito travelers transmitted the disease to more than a dozen more countries throughout the Americas, leading the WHO to label it an international health emergency.

Zika Virus Symptoms

Symptoms of Zika virus in most people are similar to those of other arboviruses such as Chikungunya, including fever, headache, and fatigue. Zika, however, is distinctively marked by detection of the disease in amniotic fluid and increased cases of microcephaly (small head size) in newborns. The vast majority of cases of microcephaly in Brazil have been reported in the country’s northeastern region. Given the virus’ novel introduction to the Americas, some epidemiologists have expressed concern that resistance may be low, leading to heightened transmission rates and severity of symptoms.

In rare cases, Zika symptoms have been associated with Guillain-Barré syndrome, in which the body’s immune system attacks nerve cells in the peripheral nervous system. This can lead to weakness, numbness, tingling, and paralysis. A study published by The New England Journal of Medicine looked at the incidence of both Zika and Guillain-Barré across 7 countries: Brazil, Colombia, the Dominican Republic, Honduras, El Salvador, Suriname, and Venezuela. The study looked at the 1,474 cases of Guillain-Barré syndrome that were reported in those countries from April 2015 to March 2016. Comparing the timeline of reporting of Zika and Guillain-Barré during this period showed a close coincidence of the two diseases. For more information, please see “Zika Virus and the Guillain–Barré Syndrome—Case Series from Seven Countries.”

Environmental Factors in Zika Transmission

Zika virus was first identified in rhesus monkeys from Uganda’s Zika Forest in 1947 during the course of routine surveillance for yellow fever. Thereafter it was diagnosed in humans starting in 1952. During the 20th century, Zika became endemic in Africa and parts of Asia, only making the jump to the Americas beginning in 2015.

Significant circumstantial evidence links the spread of Aedes mosquitoes in the 2015–16 outbreak with a confluence of environmental factors in part attributable to climate change and the influence of the El Niño phenomenon. This confluence of environmental factors manifested in the form of increased heat and precipitation, particularly at lower elevations, which opened up new breeding areas for Aedes mosquitoes. With Zika virus now endemic to regions within the range of Aedes mosquitoes, additional cases are likely to emerge on a seasonal basis.

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