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. After spreading rapidly through the Americas starting in 2015, mosquitoes carrying the virus had reached 84 countries and territories around the world by March 2017. It has caused a jump in the number of birth defects and is strongly suspected in an increase in cases of a rare neurological disorder.
The Zika 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, a rare condition in which an infant’s head is abnormally small. Subsequently, the condition, as well as birth defects including brain irregularities, vision problems and hearing loss, were connected to Zika infections in pregnant women. Of the 250 pregnant women in the U.S. with a confirmed Zika infection in 2016, 24 — or about 1 in 10 — had a fetus or baby with Zika-related defects, according to the U.S. Centers for Disease Control and Prevention. Both figures are rising. The World Health Organization reports that 31 countries or territories have registered microcephaly or related cases that may be linked to Zika. Twenty-three have reported either an increased incidence of Guillain-Barre syndrome, a sometimes deadly nerve disorder that can cause paralysis, or Zika infection among Guillain-Barre cases. A 2013 Zika outbreak in French Polynesia coincided with a spike in the syndrome, and researchers strongly suspect a link. 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.
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 spread so rapidly in the Americas is that people there were encountering it for the first time and had 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 U.S. Food and Drug Administration approved the first such experiment in the U.S. for Key Haven, Florida, but residents rejected the trial in a 2016 referendum, sending British biotech company Oxitec searching for an alternative location. 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, explores the implications of the Zika outbreak in the New England Journal of Medicine.
- The World Health Organization’s website offers information on Zika as does that of the Pan-American Health Organization.
- A Businessweek article lays out the arguments for and against deploying genetically modified mosquitoes to fight Zika.
First published Feb. 4, 2016
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