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Zika virus

This is a transmission electron micrograph (TEM) of Zika virus, which is a member of the family Flaviviridae. Virus particles are 40 nm in diameter, with an outer envelope, and an inner dense core.
By CDC/ Cynthia Goldsmith, License: Public Domain


Definition of the Zika Virus

The Zika virus is a single-stranded DNA virus that belongs to the Flaviviridae family, the genus Flavivirus, which also includes dengue, yellow fever, West Nile, and Japanese encephalitis viruses. The viruses are considered arboviruses, which are transmitted by infected arthropod vectors (such as mosquitoes) when they feed on infected hosts; they carry the virus in their saliva and transmit it.

History of the Zika Virus

The story of Zika virus started in April 1947, when a group of scientists was observing the temperatures of rhesus monkeys as part of a study looking for a yellow fever virus in the Zika forest of Uganda near Lake Victoria. One of the rhesus monkeys (identified as Rhesus 766) developed mild pyrexia (39.7°C/103.4°F) and was kept under observation. A blood sample was taken from Rhesus 766 and injected intracerebrally into a group of mice. Signs of sickness began 10 days after injection, and the researchers isolated a filterable, transmissible agent from the brains of the mice.

In January 1948, researchers attempted to isolate the virus from Aedes africanus mosquitoes trapped in the Zika forest; they ground them up in a blood-saline solution and inoculated it subcutaneously into Rhesus 758, which developed a mild fever. The researchers isolated the same transmissible agent that had sickened Rhesus 766 and named it the Zika virus.

The first case identified a human infected with Zika virus was in Nigeria in 1954 during an epidemic of jaundice that was suspected to be yellow fever. A 10-year-old Nigerian girl developed fever and headache but no jaundice. Symptoms resolved spontaneously within 3 days. A sample of her blood was injected intracerebrally into a group of mice; they later developed signs of illness, and neutralization tests confirmed the presence of Zika virus.

From 1951 to 1981, human cases of Zika virus infection were reported across other African countries and parts of Southeast Asia.

From its discovery until 2007, only 14 people were diagnosed with Zika virus disease, in Africa and South Asia.

The first outbreak of Zika virus outside Africa and Asia was reported in 2007 in the Federated Republic of Micronesia, where 49 people were confirmed to have the disease.

In 2013, another outbreak occurred in French Polynesia, with 333 confirmed cases, and it was associated with an increased incidence of a serious neurological disease, Guillain-Barre Syndrome.

In 2015, a large outbreak of the virus was reported in Northeastern Brazil, and a few months later, clinicians started to notice an unusual increase in numbers of infants with microcephaly (small heads), which had been attributed to pregnant mothers infected with the Zika virus.

The virus started to spread from Brazil to much of South America, Central America, and the Caribbean.

Countries that have past or current evidence of Zika virus transmission

Image: Countries that have past or current evidence of the Zika virus transmission (as of January 2016). By CDC, License: Public Domain

Transmission of the Zika Virus

Vector

Zika virus is a mosquito-borne disease spread mainly by the Aedes genus (Aedes aegypti and Aedes albopictus). The virus has been spreading inside many African countries and outside to the Americas, mainly through infected mosquitoes biting healthy people. The mosquitoes are very active during the daytime but also bite people at night.

Zika virus causes only mild flu-like symptoms, which resolve spontaneously within days. Therefore, infected patients may not realize they have the Zika virus in their blood. Furthermore, an infected individual can be a source of infection to healthy mosquitoes.

Vertical Transmission

In 2015, more than 3,500 cases of microcephaly were reported in Brazil after the large outbreak of Zika virus there, and the virus was isolated from the amniotic fluid of at least 2 pregnant women, suggesting a strong relationship between Zika virus and the microcephalic infants. It has been suggested that mothers infected with Zika virus can transmit the virus to their infants through during pregnancy or during delivery.

Note: There are still not enough studies about the effects of Zika virus on pregnancy.

Sexual Transmission

Two cases of sexually transmitted Zika virus infections have been reported. It has been found that the virus lasts longer in semen than in blood.

Transmission via Blood Transfusion

Several people have been reported to be infected with Zika virus through blood transfusions in Brazil, but no such cases have been reported in the United States. The reports are being investigated.

Clinical Manifestations of the Zika Virus

Rash on an arm due to Zika virus

Image: Rash on an arm due to the Zika virus. By FRED, License: CC BY-SA 3.0

Zika virus can cause mild flu-like illness or even no symptoms. About 1 in 5 people infected with Zika virus will have non-specific symptoms, such as:

  • Mild pyrexia
  • Rash
  • Arthralgia (joint pain)
  • Myalgia (muscle pain)
  • Conjunctivitis (red eye)
  • Headache and malaise

No incubation period has been identified. The virus usually lasts 3 days to 1 week and subsides spontaneously without treatment. Infected patients usually acquire lifelong immunity against the virus.

No other complications have been reported with Zika virus disease, other than the large increase in cases of Guillain-Barré syndrome during the French Polynesia outbreak in 2013 and the large increase in microcephalic babies in pregnant mothers with a history of traveling to areas endemic with Zika virus infection. 

Diagnosis of Zika Fever

Clinicians should suspect Zika virus infection in any patient who presents with fever, rash, arthralgia, conjunctivitis, and headache, with a recent history of travel to an area endemic with Zika virus, especially pregnant mothers due to the risk of microcephaly.

The clinical manifestations of Zika virus disease are similar to dengue virus and Chikungunya disease, which also are transmitted by the same types of mosquitoes. Therefore, diagnosis can be confirmed only by a laboratory isolating Zika virus RNA from the blood or other body fluids, such as urine or saliva.

Prevention of Zika Fever

No vaccine has been established to protect against the Zika virus disease. The only effective preventive method is avoiding getting bitten by infected mosquitoes.

The Centers for Disease Control and Prevention (CDC) recommend the following to all individuals traveling to areas where Zika virus is endemic:

  • Wear clothes with long sleeves, pants, socks, and hats to cover the entire body.
  • Avoid sleeping outdoors; sleep in rooms with air conditioning and windows and doors that have physical barriers (screens) to prevent entry by mosquitoes.
  • If you have to sleep outdoors, use a mosquito net to avoid getting bitten by infected mosquitoes.
  • Use insect repellent.
  • Covering clothes with Permethrin, an insecticide, but avoid applying Permethrin on the skin.

The CDC recommends that pregnant women postpone travel to areas endemic with the Zika virus due to the risk of delivering babies with microcephaly. Pregnant women who must travel to those areas should strictly apply the previously mentioned preventive methods and tell their health care providers about the travel.

Because sexual transmission of Zika virus disease has been reported, individuals traveling to endemic areas should abstain from sex or use condoms.

Individuals who already have Zika virus should avoid getting bitten by mosquitoes in the first week, as the virus exists in their blood and can be transmitted to other people.

During outbreaks of the Zika virus, authorities should spray insecticides to kill mosquitoes.

Treatment of Zika Fever

Zika virus causes a mild illness that resolves spontaneously within 3–7 days. There is no specific treatment for Zika virus infection, but symptoms can be treated as follows:

  • Bed rest
  • Analgesics and antipyretics, such as acetaminophen to relieve fever and associated joint and muscle pain; not nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Fluid supplementation to avoid dehydration, especially in tropical areas
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