Zika virus

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This article is about the virus. For the disease, see Zika fever. For the 2015–16 outbreak, see Zika virus outbreak in the Americas (2015–present).
Zika virus
Zika EM CDC 280116.tiff
Electron micrograph of Zika virus. Virus particles are 40 nm in diameter, with an outer envelope and a dense inner core (source: CDC).
Virus classification
Group: Group IV ((+)ssRNA)
Family: Flaviviridae
Genus: Flavivirus
Species: Zika virus

Zika virus[pronunciation?] (ZIKV) is a member of the virus family Flaviviridae and the genus Flavivirus, transmitted by daytime-active Aedes mosquitoes, such as A. aegypti and A. albopictus. Its name comes from the Zika Forest of Uganda, where the virus was first isolated in 1947.[1]

The infections, known as Zika fever, often causes no or only mild symptoms. Since the 1950s it has been known to occur within a narrow equatorial belt from Africa to Asia. In 2014, the virus spread eastward across the Pacific Ocean to French Polynesia, then to Easter Island and in 2015 to Mexico, Central America, the Caribbean, and South America, where the Zika outbreak has reached pandemic levels.[2]

Zika virus is related to dengue, yellow fever, Japanese encephalitis, and West Nile viruses.[3] The illness it causes is similar to a mild form of dengue fever,[3] is treated by rest,[4] and cannot yet be prevented by drugs or vaccines.[4] There is a possible link between Zika fever and microcephaly in newborn babies by mother-to-child transmission,[5][6][7] as well as a stronger one with neurologic conditions in infected adults, including cases of Guillain–Barré syndrome.[8]

In January 2016, the U.S. Centers for Disease Control and Prevention (CDC) issued travel guidance on affected countries, including the use of enhanced precautions, and guidelines for pregnant women including considering postponing travel.[9][10] Other governments or health agencies soon issued similar travel warnings,[11][12][13] while Colombia, the Dominican Republic, Ecuador, El Salvador, and Jamaica advised women to postpone getting pregnant until more is known about the risks.[12][14] On February 2, 2016, Dallas County, Texas health officials confirmed the first case of transmission in the United States, which came from sexual contact.[15][16]

Virology[edit]

A video explanation of Zika virus and Zika fever

The Zika virus belongs to Flaviviridae and the genus Flavivirus, and is thus related to dengue, yellow fever, Japanese encephalitis, and West Nile viruses. As other Flaviviruses, Zika virus is enveloped and icosahedral and has a nonsegmented, single-stranded, positive-sense RNA genome. It is most closely related to the Spondweni virus and is one of the two viruses in the Spondweni virus clade.[17][18]

There are two lineages of the Zika virus: the African lineage, and the Asian lineage.[19] Phylogenetic studies indicate that the virus spreading in the Americas is most closely related to the Asian strain, which circulated in French Polynesia during the 2013 outbreak.[19][20] The complete genome sequence of the Zika virus has been published.[21] Recent preliminary findings from sequences in the public domain uncovered a possible change in nonstructural protein 1 codon usage that may increase the viral replication rate in humans.[22]

Transmission[edit]

Aedes aegypti predicted distribution
Global Aedes aegypti predicted distribution. The map depicts the probability of occurrence (blue=none, red=highest occurrence).

The vertebrate hosts of the virus were primarily monkeys in a so-called enzootic mosquito-monkey-mosquito cycle, with only occasional transmission to humans. Before the current pandemic began in 2007, Zika virus "rarely caused recognized 'spillover' infections in humans, even in highly enzootic areas". Infrequently, other arboviruses have become established as a human disease though, and spread in a mosquito–human–mosquito cycle, like the yellow fever virus and the dengue fever virus ( both flaviruses), and the chikungunya virus (a togavirus).[8]

Vector[edit]

The Zika virus is transmitted by daytime-active mosquitoes as its vector. It is primarily transmitted by Aedes aegypti, but has been isolated from a number of arboreal mosquito species in the Aedes genus, such as A. africanus, A. apicoargenteus, A. furcifer, A. hensilli, A. luteocephalus and A. vittatus with anextrinsic incubation period in mosquitoes of about 10 days.[23]

The true extent of the vectors is still unknown, as the Zika virus has been detected in many more species of Aedes, along with Anopheles coustani, Mansonia uniformis, and Culex perfuscus; though this alone does not incriminate them as a vector.[24]

Transmission by Aedes albopictus, the tiger mosquito, was reported from a 2007 urban outbreak in Gabon, where it had newly-invaded the country and become the primary vector for the concomitant Chikungunya and dengue virus outbreaks.[25] there is concern for autochthonous infections in urban areas of European countries infested by A. albopictus since the first two cases of laboratory confirmed Zika virus infections imported into Italy were reported from viremic travelers returning from French Polynesia.[26]

The potential societal risk of Zika virus can be delimited by the distribution of the mosquito species that transmit it . The global distribution of the most cited carrier of Zika virus, A. aegypti, is expanding due to global trade and travel.[27] A. aegypti distribution is now the most extensive ever recorded – across all continents including North America and even the European periphery.[28] A mosquito population capable of carrying the Zika virus has been found in a Capitol Hill neighborhood of Washington, D.C., and genetic evidence suggests they survived at least the last four winters in the region. The study authors conclude that mosquitos are adapting for persistence in a northern climate.[29]

Since 2015 news reports have drawn attention to the spread of Zika in Latin America and the Caribbean.[30] The countries and territories that have been identified by the Pan American Health Organisation (PAHO) as having experienced "local Zika virus transmission" are Barbados, Bolivia, Brazil, Colombia, the Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint Martin, Suriname, and Venezuela.[31][32]

Sexual[edit]

As of February 2016 five cases, reported only in symptomatic men, suggest Zika virus can be sexually transmitted. The first case was published by a biologist who after visiting Senegal in 2009 to study mosquitoes and was bitten, fell ill a few days after returning to the United States, but not before having had unprotected intercourse with his wife. She subsequently showed symptoms of Zika infection, and Zika antibodies confirmed the diagnosis in both the biologist's and his wife's blood.[33][34] The second case was reported in early February 2016 by the Dallas County Health and Human Services department after sexual contact with an ill person returning from a high risk country and is still under investigation.[35][36] The third case was the finding of Zika virus in semen and urine by RT-PCR of a Tahitian man two weeks and possibly up to 10 weeks after he fell ill with Zika, and had noticed blood in his semen. Zika virus grew from semen samples, but not blood or urine.[37]

Cases four and five are of Irish nationals who returned to Ireland from Dallas in February 2016.[38]

It is unknown whether women can transmit Zika virus to their sexual partners. As of February 2016, the CDC recommends that men living or having traveled to an area of active Zika virus transmission "should abstain from sexual activity or consistently and correctly use condoms during sex", while infected men and their non-pregnant sex partners should only "consider". The CDC did not specify how long to do so after the infection, as both incidence and duration of seminal shedding are unknown and that "testing of men for the purpose of assessing risk for sexual transmission is not recommended".[39]

During pregnancy[edit]

In 2015, Zika virus RNA was detected in the amniotic fluid of two fetuses, indicating that it had crossed the placenta and could cause a mother-to-child infection.[40] There is a possible link between Zika fever and abnormalities of brain (microcephaly) and eye in newborn babies by mother-to-child transmission.[5][6][7] On February 5, 2016 the CDC updated its health care provider guidelines for pregnant women in various scenarios of potential Zika virus exposure (travel, residence), newly recommending serologic testing to asymptomatic pregnant women with the start of prenatal care and follow-up testing mid-second trimester.[41]

Zika fever[edit]

Main article: Zika fever
Rash on an arm due to Zika virus

Common symptoms of infection with the virus include mild headaches, maculopapular rash, fever, malaise, conjunctivitis, and joint pains. Three well-documented cases of Zika virus were described in brief in 1954, whereas a detailed description was published 1964; it began with a mild headache, and progressed to a maculopapular rash, fever, and back pain. Within two days, the rash started fading, and within three days, the fever resolved and only the rash remained. Thus far, Zika fever has been a relatively mild disease of limited scope, with only one in five persons developing symptoms, with no fatalities, but its true potential as a viral agent of disease is unknown.[23]

As of 2016, no vaccine or preventative drug is available. Symptoms can be treated with rest, fluids, and paracetamol (acetaminophen), while aspirin and other nonsteroidal anti-inflammatory drugs should be used only when dengue has been ruled out to reduce the risk of bleeding.[42]

There is a link between Zika fever and neurologic conditions in infected adults, including cases of the Guillain–Barré syndrome.[8]

Vaccine development[edit]

Effective vaccines exist for several flaviviruses. Vaccines for yellow fever virus, Japanese encephalitis, and tick-borne encephalitis were introduced in the 1930s, while the vaccine for dengue fever only became available for use in the mid-2010s.[43][44][45]

Work has begun towards developing a vaccine for Zika virus, according to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.[46] The researchers at the Vaccine Research Center have extensive experience from working with vaccines for other viruses such as West Nile virus, chikungunya virus, and dengue fever.[46] Nikos Vasilakis of the Center for Biodefense and Emerging Infectious Diseases predicted that it may take two years to develop a vaccine, but 10 to 12 years may be needed before an effective Zika virus vaccine is approved by regulators for public use.[47]

History[edit]

Virus isolation in monkeys and mosquitoes, 1947[edit]

The virus was first isolated in April 1947 from a rhesus macaque monkey that had been placed in a cage in the Zika Forest of Uganda, near Lake Victoria, by the scientists of the Yellow Fever Research Institute. A second isolation from the mosquito A. africanus followed at the same site in January 1948.[48]

When the monkey developed a fever, researchers isolated from its serum a transmissible agent that was first described as Zika virus in 1952.[23]It is rumor that a tribe of small headed monkeys still occupy the Zika forest. The cesarean section rate of effected pregnant women is expected to lower, with the increased risk of shoulder dystocia's in full term pregnancies.

First evidence of human infection, 1952[edit]

Zika virus had been known to infect humans from the results of serological surveys in Uganda and Nigeria. A serosurvey of 84 people of all ages showed 50 had antibodies, all above 40 years of age were immune.[49]

It was not until 1954 that the successful isolation of Zika virus from a human was published. This came as part of a 1952 outbreak investigation of jaundice suspected to be yellow fever; It was found in the blood of a 10 year old Nigerian female with low grade fever, headache and evidence of malaria, but no jaundice, who recovered within 3 days. Blood was injected into the brain of laboratory mice, followed by up to 15 mice passages. The virus from mouse brains was then tested in neutralization tests using rhesusmonkey sera specifically immune to Zika virus. In contrast no virus was isolated from the blood of two infected adults with fever, jaundice, cough, diffuse joint pains in one and fever, headache, pain behind the eyes and in the joints. Infection was proven by a rise in Zika virus specific serum antibodies.[49] A 1952 research study conducted in India had shown a "significant number" of Indians tested for Zika had exhibited an immune response to the virus, suggesting it had long been widespread within human populations.[50]

Spread from equatorial Africa to Asia, 1951-1981[edit]

From 1951 through 1981, evidence of human infection with Zika virus was reported from other African countries, such as the Central African Republic, Egypt, Gabon, Sierra Leone, Tanzania, and Uganda, as well as in parts of Asia including India, Indonesia, Malaysia, the Philippines, Thailand, and Vietnam.[23] From its discovery until 2007, confirmed cases of Zika virus infection from Africa and Southeast Asia were rare.[51]

Micronesia, 2007[edit]

In April 2007, the first outbreak outside of Africa and Asia occurred on the island of Yap in the Federated States of Micronesia, characterized by rash, conjunctivitis, and arthralgia, which was initially thought to be dengue, chikungunya or Ross River disease.[52] Serum samples from patients in the acute phase of illness contained RNA of Zika virus. There were 49 confirmed cases, 59 unconfirmed cases, no hospitalizations, and no deaths.[53] More recently, epidemics have occurred in Polynesia, Easter Island, the Cook Islands and New Caledonia.[51]

Americas, 2015-present[edit]

Since April 2015, a large, ongoing outbreak of Zika virus that began in Brazil has spread to much of South and Central America, and the Caribbean. In January 2016, the U.S. CDC issued a level 2 travel alert for people traveling to regions and certain countries where Zika virus transmission is ongoing.[54] The agency also suggested that women thinking about becoming pregnant should consult with their physicians before traveling.[55] Governments or health agencies of the United Kingdom,[11] Ireland,[12] New Zealand,[56] Canada,[13] and the European Union[13] soon issued similar travel warnings. In Colombia, Minister of Health and Social Protection Alejandro Gaviria Uribe recommended to avoid pregnancy for eight months, while the countries of Ecuador, El Salvador, and Jamaica have issued similar warnings.[12][14]

Plans were announced by the authorities in Rio de Janeiro, Brazil, to try to prevent the spread of the Zika virus during the 2016 Summer Olympic Games in that city.[13]

According to the CDC, Brazilian health authorities reported more than 3,500 microcephaly cases between October 2015 and January 2016. Some of the affected infants have had a severe type of microcephaly and some have died. The full spectrum of outcomes that might be associated with infection during pregnancy and the factors that might increase risk to the fetus are not yet fully understood. More studies are planned to learn more about the risks of Zika virus infection during pregnancy.[57] In the worst affected region of Brazil, approximately 1 percent of newborns are suspected of being microcephalic.[58]

See also[edit]

References[edit]

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