Zika virus (ZIKV) is a member of the Flaviviridae virus family and the Flavivirus genus. In humans, it 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 Central America, the Caribbean, and South America, where the Zika outbreak has reached pandemic levels. Zika virus is related to dengue, yellow fever, Japanese encephalitis, and West Nile viruses, all of which are arthropod-borne flaviviruses.
The illness it causes is similar to a mild form of dengue fever, is treated by rest, and cannot be prevented by drugs or vaccines. There is a possible link between Zika fever and microcephaly in newborn babies of infected mothers. 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 considering postponing travel, and guidelines for pregnant women. Other governments or health agencies soon issued similar travel warnings, while Colombia, 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 with 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 1947 from a rhesus macaque in the Zika Forest of Uganda and in 1968 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.
The pathogenesis of the virus is hypothesized to start with an infection of dendritic cells near the site of inoculation, followed by a spread to lymph nodes and the bloodstream. Flaviviruses generally replicate in the cytoplasm, but Zika virus antigens have been found in infected cell nuclei. 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 published. 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.
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. 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". Global Aedes aegypti predicted distribution. The map depicts the probability of occurrence (blue=none, red=highest occurrence).
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.
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, Guatemala, Guadeloupe, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint Martin, Suriname, and Venezuela.
In 2009 Brian Foy, a biologist from the Arthropod-borne and Infectious Diseases Laboratory at 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 fetal infection. On 20 January 2016, 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.
Common symptoms of infection with the virus include mild headaches, maculopapular rash, fever, malaise, pink eye, 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 preventive drug is available. Symptoms can be treated with 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. 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. 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. In December 2015 it was suspected that a transplacental infection of the foetus may lead to microcephaly and brain damage. The Brazilian Ministry of Health has since confirmed the relation between the Zika virus and microcephaly.
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, 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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