Zika virus (ZIKV) is a member of the Flaviviridae virus family and the Flavivirus genus, transmitted by daytime-active Aedes mosquitoes, such as A. aegypti.
In humans, the virus causes a mild illness known as Zika fever, Zika, or Zika disease, which since the 1950s 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.
General
Zika virus is related to dengue, yellow fever, Japanese encephalitis, and West Nile viruses.The illness it causes is similar to a mild form of dengue fever,is treated by rest, and cannot yet be prevented by drugs or vaccines There is a possible link between Zika fever and microcephaly in newborn babies by mother-to-child transmission, as well as a stronger one with neurologic conditions in infected adults, including cases of the Guillain–Barré syndrome.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.Other governments or health agencies soon issued similar travel warnings,while Colombia, the Dominican Republic, Ecuador, El Salvador, and Jamaica advised women to postpone getting pregnant until more is known about the risks.Along with other viruses in this family, 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. 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.When the monkey developed a fever, researchers isolated from its serum a transmissible agent that was first described as Zika virus in 1952. In 1968, it was isolated for the first time from humans in Nigeria. From 1951 through 1981, evidence of human infection 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.There are two lineages of Zika virus, the African lineage and the Asian lineage.Phylogenetic studies indicate that the virus spreading in the Americas is most closely related to French Polynesian strains Complete genome sequences of Zika viruses have been publishedRecent 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.
Transmission
Zika virus is transmitted by daytime-active mosquitoes and has been isolated from a number of species in the genus Aedes, such as A. aegypti, and arboreal mosquitoes such as A. africanus, A. apicoargenteus, A. furcifer, A. hensilli, A. luteocephalus, and A. vitattus. Studies show that the extrinsic incubation period in mosquitoes is about 10 days. Zika virus can migrate between humans through sexual contact and it can also cross the placenta, affecting an unborn fetus. A mother already infected with Zika virus near the time of delivery can pass on the virus to her newborn around the time of birth, but this is rare.
The vertebrate hosts of the virus are primarily monkeys and humans. Before the current pandemic, which began in 2007, Zika virus "rarely caused recognized 'spillover' infections in humans, even in highly enzootic areas".
The potential societal risk of Zika virus can be delimited by the distribution of the mosquito species that transmit it (its vectors). The global distribution of the most cited carrier of Zika virus, A. aegypti, is expanding due to global trade and travel.A. aegypti distribution is now the most extensive ever recorded – across all continents including North America and even the European periphery.[23] 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.
Recent news reports have drawn attention to the spread of Zika in Latin America and the Caribbean.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.
In 2009, Brian Foy, a biologist from the Colorado State University, sexually transmitted Zika virus to his wife. He visited Senegal to study mosquitoes and was bitten on a number of occasions. A few days after returning to the United States, he fell ill with Zika, but not before having had unprotected intercourse with his wife. She subsequently showed symptoms of Zika infection with extreme sensitivity to light. Foy is the first person known to have passed on an insect-borne virus to another human by sexual contact.
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.
Clinical
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. The first well-documented case of Zika virus was described in 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.
As of 2016, no vaccine or preventative drug is available. Symptoms can be treated with rest, fluids, and paracetamol, while aspirin and other nonsteroidal anti-inflammatory drugs should be used only when dengue has been ruled out to reduce the risk of bleeding.
In December 2015, it was suspected that a transplacental infection of the fetus may lead to microcephaly and brain damage. In December 2015, the European Centre for Disease Prevention and Control issued a comprehensive update on the possible association of Zika virus with congenital microcephaly and this syndrome.Data suggests that newborn babies of mothers who had a Zika virus infection during the first trimester of pregnancy are at an increased risk of microcephaly.The Brazilian Ministry of Health confirmed in January 2016 the relation of the Zika virus and microcephaly; scientists from the state of Paraná, Brazil, detected genetic material of Zika virus in the placenta of a woman who had undergone an abortion due to the fetus's microcephaly, which confirmed that the virus is able to pass the placenta.
Other neurological complications
In a French Polynesian epidemic, 73 cases of Guillain–Barré syndrome and other neurologic conditions occurred in a population of 270,000, which may be complications of Zika virus.
Vaccine development
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 has just recently become available for use.Work has begun towards developing a vaccine for Zika virus, according to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.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.Nikos Vasilakis of the Center for Biodefense and Emerging Infectious Diseases predicted that 10 to 12 years may be needed before an effective Zika virus vaccine is available for public use.
History
In 1947, scientists researching yellow fever placed a rhesus macaque in a cage in the Zika Forest (zika meaning "overgrown" in the Luganda language), near the East African Virus Research Institute in Entebbe, Uganda. The monkey developed a fever, and researchers isolated from its serum a transmissible agent that was first described as Zika virus in 1952. It was subsequently isolated from a human in Nigeria in 1954. From its discovery until 2007, confirmed cases of Zika virus infection from Africa and Southeast Asia were rare.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.However, 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.More recently, epidemics have occurred in Polynesia, Easter Island, the Cook Islands, and New Caledonia. 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 CDC issued a level 2 travel alert for people traveling to regions and certain countries where Zika virus transmission is ongoing.The agency also suggested that women thinking about becoming pregnant should consult with their physicians before traveling.Governments or health agencies of the United Kingdom,[10 Ireland, New Zealand, Canada, and the European Union 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. 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.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.In the worst affected region of Brazil, approximately 1 percent of newborns are suspected of microcephaly.
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