Epidemiology

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 neurological 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.

Cases in returning travellers have been reported in, but not limited to, locations including the UK, Europe, US, Australia, New Zealand, Israel, Japan, and China. [27] [28] [29] [30] [31] [32] [33] [34] [35] As of November 2017, 305 cases were reported in the UK, all of them associated with travel. [36] Between June 2015 and January 2017, 21 countries in the European Union reported 2133 confirmed cases of infection (106 cases in pregnant women). [37] WHO has warned that the risk of Zika virus transmission in Europe is low to moderate, although 3 areas are at high risk: the island of Madeira in Portugal, Georgia, and the southern part of the Russian Federation. [38]

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. [39] 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. [40] 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. [41] 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%). [42]

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. [43] GBS has not been reported in children. [44]

CDC: Zika virus case counts in the US

WHO: Zika virus classification tables

European Centre for Disease Prevention and Control: threats and outbreaks of Zika virus disease

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