Zika Virus

Updated: Aug 01, 2017
  • Author: Bhagyashri D Navalkele, MD, MBBS; Chief Editor: Michael Stuart Bronze, MD  more...
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Overview

Background

Zika virus (ZIKV) belongs to the Flavivirus genus; like other flaviviruses, Zika virus is an icosahedral, enveloped, single-stranded RNA virus. [1] The lipid envelope is covered with dense projections that consist of a membrane and envelope glycoproteins. [1]

In most cases, Zika virus infection causes a mild, self-limited illness. The incubation period is likely 3-12 days. [2] Owing to the mild nature of the disease, more than 80% of Zika virus infection cases likely go unnoticed. [2] The spectrum of Zika virus disease overlaps with other that of arboviral infections, but rash (maculopapular and likely immune-mediated) typically predominates. [2]

In April 2016, a deputy director at the Centers for Disease Control and Prevention (CDC) warned that the risk of Zika virus infection in the United States may have been previously underestimated, citing the increased range of the mosquito vectors (now in 30 US states, up from 12 as previously thought) and the travel risks associated with the 2016 Olympics in Brazil. [3]

Zika virus was first described in a febrile rhesus monkey in the Zika forest of Entebbe, Uganda, and was reported in a human field worker shortly thereafter. [1] Currently, Zika virus is known to be widely distributed outside of Africa. Outbreaks have been described previously in Micronesia and French Polynesia. [4, 5]

The Centers for Disease Control and Prevention (CDC) currently lists the following countries as areas of active virus transmission: Aruba, Barbados, Bolivia, Bonaire, Brazil, Colombia, Commonwealth of Puerto Rico (US territory), Costa Rica, Cuba, Curacao, Dominica, Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica, Martinique, Mexico, Nicaragua, Panama, Paraguay, Saint Martin, Saint Vincent and the Grenadines, Sint Maarten, Suriname, Trinidad and Tobago, US Virgin Islands, Venezuela, American Samoa, Fiji, Kosrae (Federated States of Micronesia), Marshall Islands, New Caledonia, Samoa, Tonga, and Cape Verde.

Zika virus infection is among the nationally notifiable diseases in the United States. State and local health departments should be informed by healthcare professionals of suspected cases of Zika virus infection to facilitate diagnosis and to reduce the risk of local transmission.

The CDC has released a map of potential Zika virus spread in the United States based on the estimated range of Aedes aegypti and Aedes albopictus mosquitoes. The maps can be found at http://www.cdc.gov/zika/pdfs/zika-mosquito-maps.pdf.

For the latest information concerning Zika virus, see also Medscape’s Zika Virus Resource Center.

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Pathophysiology

Like many other flaviviruses, Zika virus is transmitted by an arthropod: the Aedes mosquito, including Aedes aegypti,Aedes africanus, Aedes luteocephalus, Aedes albopictus, Aedes vittatus, Aedes furcifer, Aedes hensilli, and Aedesapicoargenteus. [1, 5, 6, 2] Sexual transmission among humans has also been described. [1, 7]

Zika virus is well-adapted to grow in various hosts, ranging from arthropods to vertebrates. Viral attachment to unidentified cellular receptors is mediated by the E (envelope) glycoprotein. This is followed by endocytic uptake and then uncoating of the nucleocapsid and release of viral RNA into the cytoplasm. A viral polyprotein is produced and modified by the endoplasmic reticulum. Immature virions collect both in the endoplasmic reticulum and in secretory vesicles before being released. [1]

Sirohi et al described the structure of mature Zika virus based on cryoelectron microscopy. The virus resembles other known flavivirus structures with the exception of approximately 10 amino acids surrounding the Asn154 glycosylation site in each of the 180 envelope glycoproteins comprising the icosahedral shell, the carbohydrate moiety of which may be the attachment site of the virus to host cells. [8]

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Epidemiology

The global prevalence of Zika virus infection has not been widely reported owing to asymptomatic clinical course, clinical resemblance to other infection with other flaviviruses (dengue, chikungunya), and difficulty in confirming diagnosis.

Based on sporadic case reports, entomological surveys, and seroprevalence surveys, Zika virus infection had been reported in various hosts, including humans, primates, and mosquitoes, in 14 countries across Africa, Asia, and Oceania, as of 2014. [9] The prevalence of Zika virus infection in Uganda was 6.1% in 1952 among a population of 99 residents. [10] The prevalence of Zika virus infection was 7.1% in Java, Indonesia, from 1977-1978 among patients who were hospitalized for fever. [11] Since Zika virus was first isolated in 1947, the disease has spread outside of Africa, mainly into Southeast Asia. Until 2007, sporadic cases of Zika virus illness in humans were reported. In 2007, Yap Island in Micronesia reported an outbreak of Zika virus infection transmitted via Aedes hensilli. [5] Subsequently, in 2013 and 2014, epidemics of Zika virus infection occurred in French Polynesia, New Caledonia, Cook Islands, and Easter Islands. [12]

In May 2015, Brazil reported the first outbreak of Zika virus infection in the Americas. The Brazil Ministry of Health estimated around 440,000-1,300,000 suspected cases of Zika virus infection in December 2015. [13] Aedes aegypti and Aedes albopictus were recognized as vectors for transmission of Zika virus. Since then, the infection has spread rapidly to several other countries, becoming a pandemic. The association of Zika virus infection with Guillain-Barré syndrome (GBS) and congenital birth defects (particularly microcephaly) amid the ongoing outbreak of Zika virus infection in Brazil is still under investigation. [13, 14]

In March 2016, the WHO reported that Zika virus was actively circulating in 38 countries and territories, 12 of which have reported an increase in GBS cases or laboratory evidence of Zika virus among patients with GBS. [15]

As of June 2016, a total of 591 laboratory-confirmed travel-associated Zika virus infections have been reported in the United States, with none acquired via local vector-borne transmission. Eleven cases have been transmitted sexually, and one case of associated Guillain-Barré syndrome has been reported. [16] US territories such as Puerto Rico and the US Virgin Islands have 935 laboratory-confirmed local cases of Zika virus infection, and 4 cases have been attributed to travel. [16] Five cases of associated Guillain-Barré syndrome have been reported. See the images below.

Laboratory-confirmed Zika virus disease cases repo Laboratory-confirmed Zika virus disease cases reported to ArboNET by state or territory — United States, 2015–2016 (as of May 25, 2016). Courtesy of the CDC.
All countries and territories with active Zika vir All countries and territories with active Zika virus transmission. Courtesy of the CDC.
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Prognosis

Most cases of Zika virus infection are mild and self-limited. Owing to the mild nature of the disease, more than 80% of Zika virus infection cases likely go unnoticed. [2] However, serious complications have been reported in rare cases, including Guillain-Barré syndrome. [2, 9] In addition, great concern is emerging over congenital malformations due to transplacental transmission of Zika virus, including microcephaly and various ophthalmologic abnormalities. [11, 12]

For more details, see History.

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Patient Education

Certain patients should be educated concerning travel risks associated with Zika virus and prevention of mosquito bites and mosquito-control measures.

The World Health Organization (WHO) and CDC recommend that mothers with Zika virus infection still breastfeed their infants, including those born with microcephaly. Zika virus transmission via breast milk has not been documented, although more research is needed for confirmation. [17]

For more details, see Prevention.

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