Zika virus was first discovered in the Zika forest of Uganda in 1947 in rhesus monkeys, but was not identified in humans until 1952 in Tanzania.[18][19] Since then, outbreaks have occurred sporadically in Africa, the Americas, Asia, and the Pacific. Until 2007, only 14 cases had been documented in humans worldwide.[20]

The first large outbreak was reported on the island of Yap (Federated States of Micronesia) in 2007.[21][22] The most likely source of this outbreak was introduction of the virus by travel or trade involving an infected person or an infected mosquito.[1] Another large outbreak was seen in the Pacific Islands (French Polynesia, Easter Island, the Cook Islands, New Caledonia) in 2013 to 2014. This was the first outbreak where congenital malformations (e.g., microcephaly) and neurologic complications, including Guillain-Barre syndrome (GBS), were linked to the infection, although this association was made retrospectively.[21][23][24]

In the current outbreak, the first reports of locally transmitted infection came from Brazil in May 2015, although there are data to suggest that the virus originated in the Americas in Brazil between October 2012 and May 2013.[25] Eighty-six countries, territories, and subnational areas have reported evidence of mosquito-borne Zika virus transmission.[26] Transmission is ongoing in the Americas, the Western Pacific region, the Southeast Asia region, and Africa.

As of August 1, 2018, 5716 cases have been reported in US states (5430 cases in returning travelers, 231 cases acquired through presumed local mosquito-borne transmission in Florida and Texas, and 55 cases acquired through other routes), and 37,262 cases have been reported in US territories (most via local mosquito-borne transmission).[27] As of July 17, 2018, the US Zika Pregnancy Registry and the Zika Active Pregnancy Surveillance System in Puerto Rico have reported 2474 pregnant women with any laboratory evidence of possible Zika virus infection in US states and the District of Columbia, and 4900 pregnant women in US territories.[28] Among 478 confirmed cases in Miami-Dade County, Florida, 6.9% of cases occurred in children ages 1 to 17 years.[29]

In the US, local transmission has previously been reported in Florida and Texas. Pregnant women and people living in or traveling to either of these areas should consult current Centers for Disease Control and Prevention (CDC) guidance. The CDC classifies geographic areas in the US and Hawaii as either red (active transmission areas) or yellow (cautionary areas), and this classification affects local guidance for pregnant women.[30]

CDC: advice for people living in or traveling to South Florida

CDC: advice for people living in or traveling to Brownsville, Texas

CDC: guidance for areas with local Zika virus transmission in the continental United States and Hawaii

Cases in returning travelers have been reported in, but not limited to, locations including the US, UK, Europe, Australia, New Zealand, Israel, Japan, and China.[27][31][32][33][34][35][36][37][38] As of November 2017, 305 cases were reported in the UK, all of them associated with travel.[39] Between June 2015 and January 2017, 21 countries in the European Union reported 2133 confirmed cases of infection (106 cases in pregnant women).[40]

An association between Zika virus infection and fetal microcephaly, as well as other birth defects, was first reported in the current outbreak in October 2015.[41]The prevalence of birth defects potentially related to Zika virus infection was reported to be 3 per 1000 live births in a birth cohort of nearly 1 million births in 2016.[42]Data from the US Zika Pregnancy Registry found that 10% of pregnancies with laboratory-confirmed infection resulted in fetuses/infants with birth defects. This figure increases to 15% when restricting the analysis to the first trimester.[43] This report covered cases reported in US states only. A more robust study of completed pregnancies in women with laboratory evidence of Zika virus infection in US territories found approximately 1 in 20 (5%) fetuses or infants had a possible Zika-associated birth defect. When the analysis was restricted to confirmed infection in the first trimester, the rate increased to 1 in 12 (8%).[44]

An association between Zika virus infection and GBS was first reported in the current outbreak in July 2015. Current evidence estimates the incidence of GBS to be 24 cases per 100,000 persons infected with Zika.[45] GBS has not been reported in children.[46]

CDC: Zika virus case counts in the US

WHO: Zika virus classification tables

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