An epidemic of Zika fever, caused by Zika virus, began in Brazil and affected other countries in the Americas from April 2015 to November 2016. The World Health Organization (WHO) declared the end of the epidemic in November 2016, but noted that the virus still represents "a highly significant and long term problem". It is estimated that 1.5 million people were infected by Zika virus in Brazil, with over 3,500 cases of infant microcephaly reported between October 2015 and January 2016. The epidemic also affected other parts of South and North America, as well as several islands in the Pacific.
Zika virus spread to Brazil from Oceania in 2013 or 2014. Brazil notified the WHO of an illness characterized by skin rash in March 2015, and Zika was identified as the cause in May 2015. In February 2016, the 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 a pregnant woman to her fetus, and can cause microcephaly and other severe brain anomalies in the infant. Zika infections in adults can result in Guillain–Barré syndrome. 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. Prior to the outbreak, Zika was considered a mild infection, as most infections are asymptomatic, making it difficult to determine precise estimates of the number of cases.
The virus is spread mainly by the Aedes aegypti mosquito, which is commonly found throughout the tropical and subtropical Americas. It can also be spread by the Aedes albopictus ("Asian tiger") mosquito, which is distributed as far north as the Great Lakes region in North America. People infected with Zika can transmit the virus to their sexual partners.
A number of countries were issued travel warnings, and the outbreak was expected to reduce tourism significantly. Several countries took the unusual step of advising their citizens to delay pregnancy until more was known about the virus and its impact on fetal development. Furthermore, the outbreak raised concerns regarding the safety of athletes and spectators at the 2016 Summer Olympics and Paralympics in Rio de Janeiro.
Epidemiology
Number of Zika cases, 2015–16 Country Autochthonous cases Imported cases Total cases Deaths Confirmed Suspected Africa 2 2 2 7,457 7,459 10 10 1 1 2 2 1 1 Americas Confirmed Suspected Imported Total Deaths 7 44 1 52 14 393 2 409 26 1,821 27 1,874 28 614 7 649 21 3 24 37 653 690 49 537 586 5 5 129 718 4 851 60 60 109,596 200,465 310,061 6 374 374 29 201 10 240 29 29 8,826 96,546 105,372 1,453 2,618 32 4,103 3 30 33 820 820 79 1,150 1,229 327 4,893 5,220 812 2,722 15 3,549 51 11,353 11,404 483 9,700 10 10,193 108 314 422 379 30,845 31,224 466 2,785 3,251 6 6 5 2,955 2,960 298 31,863 32,161 186 7,052 7,238 12 36,680 36,692 6,642 15 6,657 2,044 3 2,047 517 2,089 42 2,648 12 583 595 145 18 163 34,070 121 34,191 5 9 9 61 900 961 26 532 558 50 822 872 200 2,825 3,025 38 156 194 16 16 62 168 230 723 2,755 3,478 4 643 1 644 12 115 3 130 236 5,590 5,826 1 1 2,380 59,147 61,527 38 51 89 603 803 2 1,408 Asia Confirmed Suspected Imported Total Deaths 23 23 1 1 9 9 7 7 8 2 10 1 1 1 1 39 6 45 435 1 436 14 14 13 13 680 680 3 141 141 Europe Confirmed Suspected Imported Total Deaths 42 42 121 121 3 3 13 13 8 8 7 7 1,141 1,141 56 56 2 2 2 2 16 16 101 101 2 2 2 2 2 2 109 109 30 30 2 2 18 18 3 3 9 9 3 3 7 7 311 311 35 35 28 28 244 244 Oceania Confirmed Suspected Imported Total Deaths 54 706 7 767 76 76 17 17 (FSM) 12 82 94 2 34 36 1 1 73 73 97 97 6 6 10 10 302 302 36 2,010 2,046 1 1 Total Confirmed Suspected Imported Total Deaths Worldwide 174,667 528,157 8,557 711,381 18
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 has 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. The 2015 -16 El Nino increased ocean and ground surface temperatures to above average . January 2016 brought about nine consecutive months with temperatures 1.04 °C above the global average.Global Climate Report - January 2016. Retrieved March 16, 2018, from https://www.ncdc.noaa.gov/sotc/global/201601 While South America had areas experiencing 2.0 °C above average temperatures (for 1981–2010), areas including Argentina, Southern Brazil and Uruguay experienced temperatures 0.5 °C below average.
Precipitation is another crucial factor to consider as Eastern Brazil and other areas in southern South America experienced high amounts of precipitation in early 2016. The environmental conditions of increased rainfall and higher average temperatures in the South American region, led to both a longer mosquito season and a higher mosquito density which created an environment in which the Zika carrying Aedes aegypti and Aedes albopictus mosquitoes can thrive. Looking forward, climate models suggest that regions favorable to the Aedes mosquitoes will grow, widening the range of Zika and other mosquito-carried diseases. The potential for epidemics will spread inland and into other regions of the world, not just in tropical environments.
Confirmed cases have been reported in 40 countries or territories in South America, North America, and the Caribbean, as well as 16 in the western Pacific and one in Africa since the beginning of 2015 (see table).
Many countries with no cases of mosquito transmission have reported travel-related Zika cases: people who moved or came home from a Zika-affected region before they showed symptoms (see table).
Transmission
Zika is a mosquito-borne disease. The resurgence of Aedes aegyptis worldwide distribution over the past 2–3 decades makes it one of the most widely distributed mosquito species. In 2015, Aedes albopictus was present in tropical, subtropical, and temperate regions of the Americas, reaching as far north as the Great Lakes of North America and, internationally, living alongside Aedes aegypti in some tropical and subtropical regions.
The Aedes aegypti mosquito usually bites in the morning and afternoon hours, and can be identified by the white stripes on its legs. The mosquito species (Aedes aegypti, mainly, and Aedes albopictus) that can spread Zika virus can also spread dengue, chikungunya, and yellow fever.
Zika can also be sexually transmitted between partners of both genders. Sexual transmission of Zika has been documented in nine countries—Argentina, Canada, Chile, France, Italy, New Zealand, Peru, Portugal, and the United States—during this outbreak.
Zika is transmitted from pregnant women to the fetus ("vertical transmission"), and causes microcephaly and other severe brain anomalies in infants born to women infected with the virus.
Zika infections in adults can cause Guillain–Barré syndrome.
Diagnosis
Symptoms are similar to other flaviviruses such as dengue fever or the alphavirus that causes chikungunya, but are milder in form and usually last two to seven days. It is estimated that 80% of cases are asymptomatic. The main clinical symptoms in symptomatic patients are low-grade fever, conjunctivitis, transient joint pain (mainly in the smaller joints of the hands and feet) and maculopapular rash that often starts on the face and then spreads throughout the body.
It is difficult to diagnose Zika virus infection based on clinical signs and symptoms alone due to overlaps with other arboviruses that are endemic to similar areas. The methods currently available to test for Zika antibodies cross-react with dengue antibodies. An IgM-positive result in a dengue or Zika ELISA test can only be considered indicative of a recent flavivirus infection. Plaque-reduction neutralization tests (PRINT) can be performed and may be specific. The Zika virus can be identified by RT-PCR in acutely ill patients.