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Zika is a mosquito-borne virus that is closely related to the dengue viruses and within the same taxonomic family as yellow fever virus. Although the majority of human Zika virus infections are asymptomatic or so mild that they may go unrecognized, ~20% of infections result in dengue-like illnesses with fever, myalgia, and rash. Long an obscure virus of little human consequence, Zika has since 2013 spread pandemically and directionally around the globe from Southeast Asia to the South Pacific Islands; in 2015–2016, it spread to South and Central America and the Caribbean (Figure 108-1), including the U.S. Commonwealth of Puerto Rico and the Territory of the U.S. Virgin Islands (USVI). From these epidemics, travel-related cases and exported cases were observed in 2016 in all 50 U.S. states and the District of Columbia as well as in many other nations. Many primary care and other providers thus face diagnosing Zika cases and counseling patients about both actual Zika risks and unfounded fears. During this recent pandemic spread, Zika has been temporally and geographically linked in some countries to fetal development of microcephaly and other birth defects. In adults, Zika infection has been linked to Guillain-Barré syndrome and possibly to apparently rare cases of acute meningoencephalitis and myelitis.
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HISTORY AND BACKGROUND
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Discovered incidentally in 1947 during mosquito and primate surveillance in Uganda, Zika long remained an obscure enzootic arbovirus (i.e., an arthropod-borne virus) that initially was confined to Africa’s equatorial belt and then, at some indeterminate time in the past, spread to equatorial Asia. Zika has long circulated enzootically between wild primates and arboreal mosquitoes. Neither teratogenic nor neurologic complications of Zika were described until 2013.
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Since 2013, however, Zika virus has spread around the globe, arriving back in the area of its presumed origin—West Africa—to cause an epidemic in Cabo Verde. The last 10,000-mile leg of that journey, from the islands of the South Pacific to West Africa, was made in about 2 years or less. It remains unclear how an obscure enzootic virus that long had been unable to adapt to human spread suddenly changed completely in its epidemiologic behavior and clinical expression to cause a deadly pandemic.
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With the exception of dengue, no other arthropod-borne virus has ever expanded so far geographically, and no arbovirus, including dengue, has spread so far so quickly. The reasons for the sudden and explosive emergence of Zika virus are not known, since the virus existed in Africa and parts of Asia/Southeast Asia for decades without causing recognized outbreaks. Likely co-factors in this explosive spread include human movement (air, boat, and land travel of millions of people), population susceptibility, ...