page_title: 2015–2016 Zika virus epidemic
section_title: Epidemiology
hierarchical_section_title: 2015–2016 Zika virus epidemic / Epidemiology
caption_attribution_description: English: Photograph taken during a visit to the Uganda Virus Research Institute and the adjacent Zika Forest on 2 March 2018.
context_page_description: In early 2015, a widespread epidemic of Zika fever, caused by the Zika virus in Brazil, spread to other parts of South and North America. It also affected several islands in the Pacific, and Southeast Asia. In January 2016, the World Health Organization said the virus was likely to spread throughout most of the Americas by the end of the year. In November 2016, WHO announced the end of the Zika epidemic.
  In February 2016, WHO declared the outbreak a Public Health Emergency of International Concern as evidence grew that Zika can cause birth defects as well as neurological problems. The virus can be transmitted from an infected pregnant woman to her fetus, then can cause microcephaly and other severe brain anomalies in the infant. Zika infections in adults can result in Guillain–Barré syndrome. Prior to this outbreak, Zika was considered a mild infection, as most Zika virus infections are asymptomatic, making it difficult to determine precise estimates of the number of cases. In approximately one in five cases, Zika virus infections result in Zika fever, a minor illness that causes symptoms such as fever and a rash.
context_section_description: As early as August 2014, physicians in Natal, in northeastern Brazil began to investigate an outbreak of illness characterized by a flat pinkish rash, bloodshot eyes, fever, joint pain and headaches. While the symptoms resembled dengue fever, testing ruled out this and several other potential causes. By March 2015, the illness had spread to Salvador, Bahia  and had appeared in three different states. Then, in May 2015, researchers from the Federal University of Bahia and the Evandro Chagas Institute determined, using the RT-PCR technique, that the illness was an outbreak of Zika virus. Although, the first confirmed Zika virus infection in Brazil were diagnosed in a returning traveller in March 2015.
  The Zika virus was first isolated in 1947, in a rhesus monkey in a forest near Entebbe, Uganda. Although serologic evidence indicated additional human exposure during subsequent decades in parts of Africa and Asia, before the 2007 Yap Islands Zika virus outbreak, only 14 cases of human Zika virus disease had been documented.
  Researchers generally believe the virus was brought to Brazil by an infected traveler who had been exposed to the virus in French Polynesia, who was then bitten by a mosquito that then infected others. Phylogenetic analysis of the first Brazilian infections have strongly indicated that the circulating virus is the Asian, rather than African, strain of the virus, and was genetically similar to the virus found in the outbreak in French Polynesia. It appears Zika's route – from Africa and Asia to Oceania and then the Americas – may mirror that of chikungunya and dengue, both of which are now endemic in a large portion of the Americas.
  The specific event that brought the virus to Brazil was uncertain until March 2016. Brazilian researchers had suggested that the Zika virus arrived during the 2014 FIFA World Cup tournament. French researchers speculated the virus arrived shortly afterwards, in August 2014, when canoeing teams from French Polynesia, New Caledonia, Easter Island, and the Cook Islands, which had been or were experiencing Zika outbreaks, attended the Va'a World Sprint Championships in Rio de Janeiro. However, the outbreak in French Polynesia is known to have peaked and declined precipitously by February 2014, lending doubt to the suggestion the virus arrived later that year in Brazil with spectators and competitors. In March 2016, a study published in Science, which developed a "molecular clock" based on the count of virus mutations in a relatively small sample, suggested Zika virus arrived in the Americas (most likely in Brazil) from French Polynesia between May and December 2013, well before the World Cup and Va'a Championships. In the Science article, Faria and colleagues managed to trace the origins of the virus strain that is circulating in Brazil and found that this strain has little genetic variability when compared to the strain of French Polynesia; after relating the number of travellers arriving in Brazil from French Polynesia with the cases reported and the events happening in that year, the team was able to deduce that the virus arrived in Brazil in 2013 during the Confederation Cup, when Tahiti's team played against other teams in a few Brazilian cities, which attracted many tourists from both places. Zika virus usually has very mild, or no symptoms, so it took almost a year for Brazil to confirm the first case of the disease. By then the outbreak was already widespread.
  Factors associated with the rapid spread of Zika virus in Brazil include the non-immune population, high population density, tropical climate and inadequate control of Aedes mosquitoes in the country. The Zika virus epidemic also revealed structural problems of the health system, in particular in public health services and basic sanitation in Brazil.
  The above average warm temperatures of 2015–2016 caused by a strong El Nino created an environment conducive to the spread of the Zika Virus in Brazil[2010]. The 2015 -16 El Nino increased ocean and ground surface temperatures to above average [2010]. January 2016 brought
caption: The Uganda Virus Research Institute conducts research near the Zika Forest from which the virus takes its name.
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