Zika virus is a mosquito-borne illness that is spread by the Aedes species of mosquito, the mosquito also responsible for the transmission of dengue and chikungunya viruses.1-3
Unlike malaria-carrying mosquitos, this species is mostly active during the day and so barrier methods such as mosquito nets are ineffective. These mosquitos can survive in both indoor and outdoor environments.1
The two known species responsible for Zika transmission are the Aedes albopictus, known as the Asian Tiger mosquito, and the Aedes aegypti species.2
The Zika virus was first identified in monkeys in Uganda in 1947. The first human cases were detected in Uganda and the United Republic of Tanzania in 1952, and following that there have been further outbreaks in Africa, South East Asia, and the Pacific Islands.
While the symptoms of Zika typically pass within the space of a week, there have been recent concerns about the virus are due to a potential link between Zika and birth defects such as microcephaly.
In light of a strongly suspected causal relationship, the World Health Organization (WHO) declared that the Zika virus outbreak constituted a Public Health Emergency of International Concern on 1 February 2016.
28 October 2016 update: The Centers for Disease Control and Prevention (CDC) has issued travel warnings for people traveling to areas and certain countries where Zika virus transmission is ongoing. These include:
- Central and South America: Argentina, Belize, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, El Salvador, French Guiana, Guatemala, Guyana, Honduras, Nicaragua, Panama, Paraguay, Peru, Suriname, Venezuela
- Caribbean: Anguilla, Antigua and Barbuda, Aruba, the Bahamas, Barbados, Bonaire, British Virgin Islands, Cayman Islands, Cuba, Curaçao, Dominica, Dominican Republic, Grenada, Guadeloupe, Haiti, Jamaica, Martinique, Puerto Rico, Saba, Saint Barthélemy, Saint Kitts and Nevis, Saint Lucia, Saint Martin, Saint Vincent and the Grenadines, Sint Eustatius, Sint Maarten, Trinidad and Tobago, Turks and Caicos Islands, US Virgin Islands
- Oceania: American Samoa, Fiji, Marshall Islands, Micronesia, New Caledonia, Papua New Guinea, Samoa, Tonga
- North America: Mexico
- Africa: Cape Verde
- Asia: Singapore
Fast facts on Zika
Here are some key points about Zika. More detail and supporting information is in the main article.
- Zika virus cases typically occur in tropical climates
- Infection in the US is linked to travel to and from tropical regions
- Symptoms of Zika virus infection can last for up to a week
- The majority of people infected with Zika virus do not display any symptoms
- Cases of Zika virus infection that result in hospitalization are uncommon
- A link between maternal Zika infection and infant microcephaly is currently being investigated
- As yet, there is not enough evidence to fully characterize the link between the two conditions
- Zika infection can spread from a mother to a fetus during pregnancy
- At present, there is no treatment for the virus
- Avoiding mosquito bites is a key aspect of Zika virus prevention
Where does Zika virus occur?
Zika virus is transmitted by the Aedes species of mosquito. This species is also responsible for the transmission of dengue.
While the majority of Zika virus cases occur in tropical regions such as Brazil, Colombia, Paraguay, Suriname, Venezuela and French Guiana, the possibility exists of mosquito infection in tropical-like climates in some cities, such as Houston and New Orleans in the US.1,2,4
Areas within the US which are of concern for potential Zika-infected mosquitos are those with wet lowlands, warmer temperatures and higher levels of poverty.2
Other countries with past or recent Zika virus infection include parts of Africa, Asia, the Americas, Oceania and the Pacific Islands.4 Infection in the US is currently linked to exposure of travelers who return from other countries.1,2
Due to the fact that the species of mosquito that transmits Zika virus can be found throughout the world, the Centers for Disease Control and Prevention (CDC) believe it is likely that outbreaks of the disease will spread to new countries.1
WHO expects the virus to rapidly spread through the whole of the Americas and some virologists and epidemiologists also believe that Asia will be at risk.
'Explosion' of Dengue fever reported in Brazil
A massive rise in cases of dengue has also been reported in Brazil. This is particularly relevant because dengue is carried by the same Aedes mosquito that carries Zika.
There has been a 50% increase in dengue cases in the last 3 weeks compared with the same period in 2015, with 74,000 cases having been described between 3rd and 23rd of January.
It seems likely that the escalation in dengue mirrors that for Zika and is evidence that attempts to control the mosquito are not working. Aedes aegypti lays its eggs in standing water, and the Brazilian military has been drafted in to try and help exterminate the mosquito in these areas.
The mosquito also transmits chikungunya which, like dengue, causes marked fever and joint pains, and if the incidence of this also rises, it gives further evidence that controlling and eradication techniques for the mosquito are failing.
Could Zika be transmitted by sexual intercourse?
Dr. Helen Webberley explained to Medical News Today about another way that the virus could potentially be transmitted:
What is also of great concern is that the virus has been identified in patients who have not visited these areas and, along with the fact that Zika has been also identified in saliva, urine and semen (where it can exist for up to 2 weeks), raises the distinct possibility that Zika can be transmitted by sexual intercourse.
Although this does seem to be a rare event, it does raise the possibility that Zika can spread in any country, not just those that have the Aedes mosquito.
Some countries are now recommending that men should use condoms for up to 28 days after visiting an "at risk" area if their partners are pregnant or likely to become pregnant, if they do not develop symptoms or for 6 months if they do.
The CDC state that men with a pregnant partner who have traveled to an "at risk" area should either "abstain from sexual activity (vaginal, anal, or oral) or consistently and correctly use condoms for the duration of the pregnancy."10
Evidence now shows that Zika infection can pass from a mother to a fetus during pregnancy.11
There is also a potential risk that the Zika virus could spread through infected blood. As a result, the US Food and Drug Administration (FDA) have advised people who have traveled to a region with active Zika virus transmission to defer from donating blood.12
Recent developments on Zika transmission from MNT news
Scientists have made headway in understanding how pathogens, such as the Zika virus, cross from the mother to the unborn child and cause birth defects, according to research published in Science Advances.
The Centers for Disease Control and Prevention are investigating 14 more cases of sexual transmission of Zika virus in the US. All the potential cases involve men infecting women - including some pregnant women.
Symptoms of Zika virus
Signs and symptoms of Zika virus are vague and can last for up to a week. Diagnosis of the virus is typically confirmed with a blood test.1
Symptoms of Zika virus include:1,2
According to the Pan American Health Organization (PAHO), only 1 in 4 people infected with Zika virus develop symptoms.5 In contrast, the CDC state the figure is 1 in 5.
In the past, there have also been reports of patients developing Guillain-Barré syndrome following a Zika virus infection. Guillain-Barré syndrome is a rare but serious autoimmune disorder that affects the central nervous system.6
Infection with the Zika virus is rarely severe enough to warrant hospitalization, and it is rarer still for an individual to die as a result.6
Current research on Zika virus
A growing concern that is currently under investigation is a possible link between maternal Zika virus infection and infant microcephaly.1-5,7 Brazil in particular has seen a surge in infants born with microcephaly since October 2015, at rates that have been reported to be 10 times higher than those in previous years.7
Many people are concerned that there may be a link between maternal Zika virus infection and infant microcephaly.
These infants have been tested for Zika virus with mixed results - some positive and some negative for the virus.7 Zika virus has been confirmed to be present in two amniotic fluid samples of microcephalic babies.2,3
To date, there have been no known transmissions of the virus from mother to infant during breastfeeding.1
Other regions such as French Polynesia have seen an increase in fetal and newborn brain and spine defects over the past year.3 According to PAHO, "the French Polynesia health authorities hypothesize that Zika virus infection may be associated with these abnormalities if mothers are infected with the virus during the first or second trimester of pregnancy."3
What is microcephaly?
Microcephaly is a neurological condition where an infant's head circumference is significantly smaller than the average size for infants of the same age. Microcephaly can lead to developmental delays in movement and speech among other complications.8
Complications of microcephaly include:
- Dwarfism or short stature
- Facial distortion
- Mental retardation
Microcephaly is believed to be caused by both environmental and genetic factors. While there is currently no direct treatment for microcephaly, supportive therapy can assist infant development.
Recent developments on Zika and microcephaly from MNT news
The Zika virus appears to infect a type of neural stem cell that is involved in the development of the brain's cerebral cortex, says research published in Cell Stem Cell.
A new study describes how amniotic fluid retrieved in Brazil from two pregnant women carrying fetuses diagnosed with microcephaly showed evidence of Zika virus.
Health officials in the US have concluded that the Zika virus causes microcephaly and other severe brain defects in babies.
On the next page, we look at treatment options for Zika virus and measures that can be taken to prevent transmission of the condition.