Zika virus and complications: Questions and answers

Zika virus and complications: Questions and answers

Online Q&A
Updated 6 September 2016

Summary of the situation

The rise in the spread of Zika virus in Brazil has been accompanied by an unprecedented rise in the number of children being born with unusually small heads—identified as microcephaly. In addition, several countries, including Brazil, reported a steep increase in Guillain-Barré syndrome—a neurological disorder that could lead to paralysis and death. Based on research, there is scientific consensus that Zika virus is a cause of microcephaly and Guillain-Barré syndrome.

Zika virus

Updated! How do people catch Zika virus?

Zika virus is primarily transmitted to people through the bite of an infected Aedes mosquito, which also transmits chikungunya, dengue and yellow fever.

A study, conducted by Fiocruz Pernambuco, detected the presence of Zika virus in Culex quinquefasciatus mosquitoes. These samples were collected in Recife, Brazil, in houses where people had Zika. Recent laboratory studies have shown that Culex species were experimentally unable to transmit the Zika virus, and it is unlikely that they play a role in the current outbreak of Zika.

Zika virus can also be transmitted through sex and has been detected in semen, blood, urine, amniotic fluids, saliva as well as body fluids found in the brain and spinal cord.

Where does Zika virus occur?

Local transmission of Zika virus by Aedes mosquitoes has been reported on the continents of Africa, the Americas, Asia and the Pacific.

There are 2 types of Aedes mosquito known to be capable of transmitting Zika virus. In most cases, Zika is spread through the Aedes aegypti mosquito in tropical and subtropical regions. The Aedes albopictus mosquito also transmits the virus and can hibernate to survive regions with cooler temperatures.

Can El Niňo have an effect on Zika transmission?

The Aedes aegypti mosquito breeds in still water. Severe drought, flooding, heavy rains and temperature rises are all known effects of El Niño—which is the result of a warming of the central to eastern tropical Pacific Ocean. An increase in mosquitoes can be expected due to expanding and favourable breeding sites. Steps can be taken to prevent and reduce the health effects of El Niño.

More on El Niño and Zika

Can the Aedes mosquito travel from country to country and region to region?

The Aedes mosquito is a weak flyer; it cannot fly more than 400 meters. However it may be possible for the mosquito to be transported from one place to another accidentally and introduce Zika virus to new areas.

What are the symptoms of Zika virus disease?

Zika virus usually causes mild illness. Symptoms most commonly include a slight fever or rash, appearing a few days after a person is bitten by an infected mosquito. Although many will not develop any symptoms at all, others may also suffer from conjunctivitis, muscle and joint pain, and feel tired. The symptoms usually last from 2 to 7 days.

There is no known difference in the symptoms of infected pregnant and non-pregnant women.

How is Zika virus disease diagnosed?

Diagnosis is based on symptoms and the person’s recent history (e.g. mosquito bites, or travel to an area where Zika virus occurs). Laboratory testing can confirm the presence of Zika virus in the blood. However, this diagnosis may not be reliable as the virus could cross-react with other viruses such as dengue, West Nile and yellow fever. A reliable, rapid point-of-care diagnostic test is a research and development priority.

How is Zika virus disease treated?

The symptoms of Zika virus disease can be treated with common pain and fever medicines, rest and plenty of water. If symptoms worsen, people should seek medical advice.

Olympic and Paralympic Games Rio, 2016

What are the risks facing athletes and visitors attending the Olympic and Paralympic Games in Rio, Brazil in 2016 with regard to the current Zika outbreak?

Athletes and visitors will face risks similar to residents of Brazil. They are at risk of being infected with Zika if bitten by an infected mosquito or through sexual transmission of the virus.

Zika virus usually causes mild symptoms1 and most people may not develop any symptoms at all.

However, there is scientific consensus that Zika virus is a cause of microcephaly (children being born with unusually small heads) and other brain malformations and disorders in babies born to women who were infected with Zika virus during pregnancy. Thus WHO advises pregnant women not to travel to areas where Zika is circulating.

Zika is also a cause of Guillain-Barré syndrome, a rare but serious neurological disorder that could lead to paralysis and death.

The Games will take place during Brazil’s wintertime, when there are fewer active mosquitoes and the risk of being bitten is lower.

1 Symptoms include fever, skin rashes, conjunctivitis, muscle and joint pain, malaise, and headache.

What can athletes and visitors do to protect themselves?

While mosquitoes are the primary vectors, a person infected with Zika virus can also transmit the virus to another person through unprotected sex. Athletes and visitors to Rio de Janeiro, and other areas where Zika virus is circulating, are being encouraged to:

– Follow the travel advice provided by WHO and their countries’ health authorities, and consult a health worker before travelling;

– Whenever possible, during the day, protect themselves from mosquito bites by using insect repellents and by wearing clothing – preferably light-coloured – that covers as much of the body as possible;

– Use physical barriers such as regular or mesh screens or insecticide treated netting materials on doors and windows, or close doors and windows; and

– Avoid visiting over-crowded areas in cities and towns with no piped water and poor sanitation (ideal breeding grounds of mosquitoes) where the risk of being bitten is higher.

Should pregnant women attend the Rio 2016 Games? And what about their partners?

Pregnant women should not travel to areas with ongoing Zika virus transmission. This includes Rio de Janeiro. Pregnant women’s sex partners returning from areas with circulating virus should practice safer sex or abstain from sex throughout the pregnancy.

Mosquito protection

Updated! What can people do to protect themselves from mosquito bites?

The best protection from Zika virus is preventing mosquito bites. Women who are pregnant or planning to become pregnant and their sexual partners should take extra care to protect themselves from the bite of mosquitoes that transmit Zika. This can be done by:

Wearing clothes (preferably light-coloured) that cover as much of the body as possible.

Using insect repellent: repellents may be applied to exposed skin or to clothing, and should contain DEET (diethyltoluamide) or IR 3535 or Icaridin which are the most common biologically active ingredients in insect repellents. Repellents must be used in strict accordance with the label instructions. They are safe for use by pregnant women.

Using physical barriers such mesh screens or treated netting materials on doors and windows.

Sleeping under mosquito nets day and night.

Identifying and eliminating potential mosquito breeding sites, by emptying, cleaning or covering containers that can hold even small amounts of water, such as buckets, flower pots and tyres.

National programmes can target polluted water bodies and sewage wastes (septic tank outlets need to be covered) with water and sanitation interventions.

How can pregnant women protect themselves from mosquito bites?

Pregnant women living in areas of Zika virus transmission should follow the same prevention guidelines as the general population.

Pregnant women living in areas with ongoing Zika virus transmission should attend their regular antenatal care visits in accordance with national standards and comply with the recommendations of their health-care providers. They should also start antenatal care visits early for diagnosis and appropriate care and follow-up if they develop any of the Zika symptoms or signs.

Mosquito surveillance (monitoring)

What role does mosquito surveillance play in addressing Zika virus?

Monitoring the numbers and location (surveillance) of mosquitos is used for operational and research purposes. It helps to determine changes in geographical distribution of mosquitos, for monitoring and evaluating control programmes, for measuring mosquito populations over time, and for facilitating appropriate and timely decisions regarding interventions.

Surveillance can serve to identify areas where a high-density infestation of mosquitos has occurred or periods when mosquito populations increase. In areas where the mosquitos are no longer present, mosquito surveillance is critical in order to detect new introductions rapidly before they become widespread and difficult to eliminate. Monitoring of the mosquito population’s susceptibility to insecticide should also be an integral part of any programme that uses insecticides. Surveillance is a critical component of prevention and control programme as it provides the information necessary for risk assessment, epidemic response and programme evaluation.

Surveillance programs in use for Aedes aegypti mosquitos do not involve collecting thousands of mosquitoes to test them all for the Zika virus. This would be extremely difficult to do considering the huge number of mosquitoes in the environment and the actual very low rate of Zika infection, even in an epidemic. For example, published reports suggest that less than 1 mosquito is infected with Zika for every 1000 mosquitoes.

New! What are the differences between the Aedes and the Culex mosquitoes?

The Aedes mosquito transmits Zika, and other diseases it carries, more effectively than the Culex since it bites during the day and is more adapted to living in and around human settlements. Culex breeds in polluted waters, laying their eggs in the same breeding place and prefers to feed on one person. Whereas the Aedes lay their eggs in several places and feed off of many people – spreading the diseases it carries more widely in the locality.

Updated! Why is WHO focusing on the Aedes mosquito as the primary vector for Zika? Is enough known to warrant the current design of the vector control program?

All studies conducted to-date in Africa, Asia, the Pacific, and the Americas support the conclusion that Aedes aegypti is the main vector and a mosquito in the same family, Aedes albopictus, has the potential for transmission of Zika virus. Both species breed and live near or inside human dwellings, preferring to bite humans over other animal hosts, and extensive documentation has shown their competency in transmitting the Zika virus. However, recent research in Brazil shows that the Culex mosquito may also transmit Zika.

WHO’s vector control recommendations targeting the Aedes species are also very efficient against other mosquito vectors, including the Culex. The range of methods for reducing mosquitos include spraying the inside walls of houses, indoor space spraying, larval control, and eliminating breeding sites. Mosquito control is recommended alongside personal protection steps such as the use of insect repellents, bed nets during the day and at night, window and door netting, and wire mesh screens.

Is there national or international research underway into the Zika virus to look at all the possible causes?

There is a severe lack of international research and a shortage of funding to address the questions that have arisen during this outbreak. Available evidence indicates that Aedes mosquitoes are the main vectors in urban and semi urban areas, and comprehensive efforts are underway to control it. WHO is working closely with partners to better understand any additional factors that contribute to Zika virus transmission and complications.

Where can I find more information about mosquito surveillance?

More information on mosquito surveillance can be found in the publication below:

Entomological surveillance for Aedes spp. in the context of Zika virus: Interim guidance for entomologists
A review of Zika vectors – Aedes

Sexual transmission

What can people do to be protected from sexual transmission of Zika virus?

In regions with active Zika virus transmission, health programmes should ensure that:

All people (male and female) with Zika virus infection and their sexual partners (particularly pregnant women) receive information about the risks of sexual transmission of Zika virus.

Men and women get counselling on safer sexual practices (including correct and consistent use of male or female condoms, non-penetrative sex, reducing the number of sexual partners, and postponing sexual debut) and be offered condoms.

Sexually active men and women should be correctly counselled and offered a full range of contraceptive methods to be able to make an informed choice about whether and when to become pregnant, in order to prevent possible adverse pregnancy and fetal outcomes.

Pregnant women practice safer sex or abstain from sexual activity for at least the whole duration of the pregnancy.

Pregnant women should be advised not to travel to areas of ongoing Zika virus outbreaks.

In regions with NO active Zika virus transmission health programmes should ensure that:

Men and women returning from areas where transmission of Zika virus is known to occur should adopt safer sex practices or consider abstinence for at least 6 monthsc upon return to prevent Zika virus infection through sexual transmission.

Couples or women planning a pregnancy, who are returning from areas where transmission of Zika virus is known to occur, are advised to wait at least 6 months before trying to conceive to ensure that possible Zika virus infection has cleared.

Sexual partners of pregnant women, returning from areas where transmission of Zika virus is known to occur, should be advised to practice safer sex or abstain from sexual activity for at least the whole duration of the pregnancy.

Information for the general public:

Fact sheet: Family planning/Contraception

Information for policy-makers:

UNFPA, WHO and UNAIDS Statement on condoms & prevention of HIV, other STIs and unintended pregnancy

Information for health-care providers:

Prevention of potential sexual transmission of Zika virus, interim guidance

What should women do if they have been exposed to unprotected sex but do not wish to become pregnant because of possible infection with Zika?

All women and girls should have ready access to emergency contraception, including accurate information and counselling as well as affordable methods.

Advice for general public:

Fact sheet: Emergency contraception


What should people travelling to Zika-affected areas do?

Travellers should stay informed about Zika virus and other mosquito-borne diseases, such as chikungunya, dengue and yellow fever, and consult their local health or travel authorities if they are concerned.
Pregnant women should be advised not to travel to areas of ongoing Zika virus transmission; pregnant women whose sexual partners live in or travel to areas with Zika virus transmission should ensure safer sexual practices or abstain from sex for the duration of their pregnancy.

More on travelling in the context of Zika

Guillain-Barré syndrome, microcephaly and other neurological disorders

Is Zika virus a cause of microcephaly and Guillain-Barré syndrome?

Based on a growing body of preliminary research, there is scientific consensus that Zika virus is a cause of microcephaly and Guillain-Barré syndrome.

While intense efforts continue to reinforce and refine the link between Zika virus and a range of neurological disorders within a rigorous research framework, a stream of recent case report studies as well as a small number of case control and cohort studies, support the conclusion that there exists an association between Zika and microcephaly and Guillain-Barré syndrome.i

What events led WHO to investigate a causal link?

A Zika outbreak in Brazil, identified in early 2015, was followed by an unusual increase in microcephaly among newborns, as well as an increased number of cases of Guillain-Barré syndrome. In the light of these events in Brazil, it was determined that a previous outbreak of Zika virus in French Polynesia in 2013-14 was also associated with an increased number of cases of Guillain-Barré syndrome, microcephaly, and other neuro disorders on the islands. The scientific community responded with urgency to the rapidly evolving situation and began building a knowledge base about the virus and its implications extremely quickly.

Are there other explanations for microcephaly and Guillain-Barré syndrome?

Guillain-Barré syndrome and microcephaly are conditions with a number of underlying causes, triggers and neurological effects. Microcephaly can result from infections in the womb, exposure to toxic chemicals and genetic abnormalities, while Guillain-Barré syndrome is an autoimmune condition that can be triggered by specific infections.

Scientists do not exclude the possibility that other factors may combine with Zika virus infection to cause neurological disorders but more research is needed before any conclusions in this area can be reached.

Is there a link between Zika and other neurological disorders?

Recent case reports suggest there may be a link between Zika and other neurological abnormalities such as: myelitis (inflammation of the spinal cord) or brain abnormalities on scan in the absence of microcephaly. WHO and partners have acted on the emerging reports about the other neurological manifestations and assessment of the evidence for these conditions is on-going, as for microcephaly and Guillain-Barré syndrome.

What is Guillain-Barré syndrome?

Guillain-Barré syndrome is a rare condition in which a person’s immune system attacks his or her nerves. People of all ages can be affected, but it is more common in adult men. Most people recover fully from even the most severe cases of Guillain-Barré syndrome. In 20%-25% of people with the condition, the chest muscles are affected, making it hard to breathe. Severe cases of Guillain-Barré syndrome are rare, but can result in paralysis.

Fact sheet on Guillain–Barré syndrome

What is microcephaly?

Microcephaly is a condition where a baby’s head is smaller than those of other babies of the same age and sex. Microcephaly happens when there is either a problem in utero, causing the baby’s brain to stop growing properly, or after birth when the head stops growing properly. Children born with microcephaly often have developmental challenges as they grow older. In some cases, children with microcephaly develop entirely normally. Microcephaly can be caused by a variety of environmental and genetic factors such as Down syndrome; exposure to drugs, alcohol or other toxins in the womb; and rubella infection during pregnancy.

Fact sheet on microcephaly


Can women transmit Zika virus to their fetuses during pregnancy or childbirth?

Transmission of Zika virus from pregnant women to their fetuses has been documented.

Zika virus infection close to term could potentially be transmitted during both pregnancy and at childbirth, although this has not been scientifically proven to date.

Pregnant women in general, including those who develop symptoms of Zika virus infection, should see their health-care provider for close monitoring of their pregnancy.

Can mothers with Zika infection breastfeed their baby?

Zika virus has been detected in breast milk but there is currently no evidence that the virus is transmitted to babies through breastfeeding.

WHO recommends exclusive breastfeeding for the first 6 months of life.

Fact sheet: Breastfeeding

What should women do if they wish to postpone pregnancy because they are worried about microcephaly?

Whether and when to become pregnant should be a personal decision, on the basis of full information and access to affordable, quality health services.

Women wanting to postpone pregnancy should have access to a comprehensive range of reversible, long- or short-acting contraceptive options. They should also be counselled on the dual protection against sexually transmitted infections provided by condoms.

There are no known safety concerns regarding the use of any hormonal or barrier contraceptive methods for women or adolescent girls at risk of Zika virus, women diagnosed with Zika virus infection, or women and adolescents being treated for Zika virus infection.

Information for policy makers:

Ensuring human rights in the provision of contraceptive information and services – Guidance and recommendations

Advice for health-care providers on medical eligibility for contraception

How can women manage their pregnancy in the context of Zika virus and complications?

Most women in Zika-affected areas will give birth to normal infants.

Early ultrasound does not reliably predict fetal malformations. WHO recommends a repeat ultrasound of the fetus in the late second or early third trimester (preferably between 28 and 30 weeks) to identify fetal microcephaly and/or other brain abnormalities when they are easier to detect.

Where feasible, screening of amniotic fluid for abnormalities and congenital infections, including Zika virus, is recommended, especially in cases where women test negative for Zika, but their ultrasounds indicate fetal brain abnormalities.

Based on the prognosis of associated fetal brain abnormalities, the woman—and her partner if she wishes—should be offered non-directive counselling so that she, in consultation with her health-care provider, can make a fully informed choice about the next steps in the management of her pregnancy.

Women carrying their pregnancy to term should receive appropriate care and support to manage anxiety, stress and the birth environment. Plans for care and management of the baby soon after birth should be discussed with the parents in consultation with a paediatrician or paediatric neurologist where available.

Women who wish to discontinue the pregnancy should receive accurate information about their options to the full extent of the law, including harm reduction where the care desired is not readily available.

Women, whatever their individual choices with respect to their pregnancies, must be treated with respect and dignity.

Safe abortion: Technical & policy guidance for health systems. Legal and policy considerations – Key messages.

Zika virus response

What is the WHO response to Zika?

The Organization, working together with partners, has now developed a framework setting out the key questions that need to be answered to strengthen the body of evidence on a causal relationship between Zika and neurological complications. In addition, WHO convened several meetings among global experts on Zika and related topics during March 2016 to discuss evidence, answer pressing scientific questions and provide practical guidance to support countries responding to the outbreak and to cases of neurological disorders.

WHO will continue to lead the harmonisation, collection, review and analysis of data that will seek to answer these questions. As scientists unravel how Zika is implicated in conditions such as microcephaly and Guillain-Barré syndrome, we will gain a better understanding of the scope of the problem as well as greater clarity on which populations are likely to be affected and to what extent.

More on WHO’s Strategic Response Framework

What are WHO recommendations to countries?

Given the severity of the risk to public health, WHO has advised countries in areas where Zika virus is circulating to take decisive and immediate action to protect their populations. Reducing the risk of people being bitten by Zika-infected mosquitoes is the most effective way to prevent people from getting the virus. Affected countries are urged to scale up vector control efforts and ensure that individuals and communities are aware of how to protect themselves from bites and how to eliminate mosquito breeding sites. There is currently no vaccine for Zika.

Social support and care before, during and after pregnancy need to be provided as well as clinical care for individuals who develop Guillain-Barré syndrome.

All of WHO’s guidance on Zika virus

How can the Zika response keep up with all the new research being published?

New studies on Zika and its complications are being published daily and the pace of research will continue to increase. WHO and partners will systematically evaluate new studies to track any changes in the direction of the evidence base and to identify knowledge or research gaps.

To allow publication of the latest evidence at regular intervals, a living systematic review is being developed. This will be an online summary of health research, which will be updated as new research becomes available. This system can be adapted for future outbreaks of new and emerging communicable diseases to generate real time evidence-based information.


References: i
Besnard M, Lastere S, Teissier A, Cao-Lormeau V, Musso D. Evidence of perinatal transmission of Zika virus, French Polynesia, December 2013 and February 2014. Eurosurveillance 2014 Apr 3;19(13):20751. Available from: https://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20751

Dick GWA. Zika virus. II. Pathogenicity and physical properties. Trans R Soc Trop Med Hyg 1952 Sep;46(5):521–34. Available from: https://ovidsp.ovid.com/ovidweb.cgi?T=JS&PAGE=reference&D=emed1aa&NEWS=N&AN=12995441

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