For decades after its discovery, the Zika virus was considered no great threat to anyone. It had caused only occasional documented cases of disease apart from several outbreaks in remote Pacific islands, and even so was thought to produce only mild flu-like symptoms. Zika’s days of obscurity are over. The mosquito-borne virus has spread rapidly through the Americas, and poses a growing threat in the Asia-Pacific region. It has caused a jump in the number of babies born with an alarming birth defect and is strongly suspected in an increase in cases of a rare neurological disorder. Health officials in several countries have taken the unusual step of suggesting women put off plans for pregnancy.
The World Health Organization reported in October that outbreaks of Zika virus had been reported in 56 countries and territories from 2015 onwards. The disease has spread to countries making up most of the Americas, with the first cases of mosquito-borne transmission within the continental U.S. confirmed in August. More than a dozen countries in Asia reported locally acquired cases in 2016. There are no specific drugs to treat Zika and no vaccine to prevent it, though drugmakers and the U.S. government are working on both. The virus, which is carried by the Aedes mosquito, produces symptoms such as rash, fever, joint pain and pinkeye in about 1 in 5 people infected. As it spread, Brazil recorded a 20-fold increase in the incidence of microcephaly. Infants with the condition, which has been connected to Zika infections in pregnant women, have abnormally small heads and, usually, brain defects. According to the WHO, 22 countries have registered cases that may be linked to Zika. Almost as many have reported a rise in cases of Guillain-Barre syndrome, a nerve disorder that can cause temporary paralysis. A 2013 Zika outbreak in French Polynesia coincided with such a spike, and researchers strongly suspect a link.
Zika, which is related to other mosquito-borne viruses, including dengue and chikungunya, was first identified in rhesus monkeys in the Zika forest in Uganda in 1947 and then in humans in 1952 in Uganda and Tanzania. One theory for why Zika has spread so rapidly in the Americas is that people there have encountered it for the first time and have no natural immunity. Zika is transmitted by mosquitoes that have bitten a human infected by the virus. It can move to new areas when Zika-carrying mosquitoes hitch a ride on travelers or cargo, or when someone who is infected travels to new territory and is bitten by an Aedes mosquito there. During the latest epidemic, scientists confirmed that Zika also can be transmitted through sexual contact. The rise in global travel has increased the threat from mosquito-borne disease.
In the absence of a vaccine, which could take years to develop, the best way to control Zika is to control mosquitoes. The city of Piracicaba, near Sao Paulo, has expanded a pilot program to release male Aedes mosquitoes that have been genetically modified so that their offspring die early. The first such experiment in the U.S., in Key Haven, Florida, was approved by the Food and Drug Administration, but local officials say they will respect the outcome of a pair of referendums on the test scheduled in November. Advocates for such programs argue that they have the potential to be much more effective than traditional measures: fumigation, reducing sources of standing water where bugs lay eggs, and advising residents to use repellents, cover up and stay in air-conditioned or screened rooms or sleep under bed nets. According to studies published in 2012 and 2015, the release of modified mosquitoes achieved an 80 percent reduction in Aedes mosquitoes populations in locations in the Cayman Islands and Brazil. Opponents fear unknown consequences of putting modified mosquitoes, which are food for other animals, into the wild.
The Reference Shelf
- Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, and his senior director, David Morens, explore the implications of the Zika outbreak in the New England Journal of Medicine.
- The Pan-American Health Organization website offers information about Zika.
- A Businessweek article explores the arguments for and against deploying genetically modified mosquitoes to fight Zika.
First published Feb. 4, 2016
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