Updated: Zika virus outbreak – Expert reaction

Concern over the current outbreak of the mosquito-spread Zika virus in the Americas is growing.

Aedes_aegypti_CDC-GathanyThe virus is likely to spread to all countries in the Americas except for Canada and Chile, the World Health Organization said yesterday. The US Centres for Disease Control currently lists 22 countries with active transmission of the virus, including Samoa.

The virus, initially thought only to cause mild fever and rashes, has now been tentatively linked to increases in the incidence of microcephaly – incomplete brain development – in newborns.

The New Zealand Ministry of Health has updated its information and advice on the Zika virus on the Ministry website.

UPDATE: The WHO has announced it will convene an Emergency Committee under the International Health Regulations. The Committee will meet in Geneva on Monday, 1 February. See updated UK SMC comments on this development below.

The Science Media Centre contacted New Zealand experts for comment on the situation.

Dr José G B Derraik, Senior Research Fellow, Liggins Institute, University of Auckland, comments:

“There are a number of possible reasons for the major outbreak of Zika virus in Latin America. One important factor is that the populations in this region had never been exposed to Zika virus and therefore have no immunity to it, so that it was able to spread rather quickly. However, it is also possible for example, that the virus has mutated since its discovery in Uganda in 1947, becoming more infective.

“The causative association between Zika and microcephaly has not been conclusively proven. However, the virus has been detected in the amniotic fluid from pregnancies with microcephaly, and the latest evidence from Brazil indicates that the virus can indeed cross the placenta and infect the developing fetus. The reports from Brazil indicate that Zika virus infection in pregnant women can lead to microcephaly in the unborn child as well as other adverse pregnancy outcomes, such as miscarriage.

“There have been outbreaks of Zika virus in the South Pacific already, including New Caledonia, Cook Islands, and Samoa. Travellers infected with exotic viruses (including Zika) arrive in New Zealand regularly. However, the risk of an outbreak of Zika virus in New Zealand is low compared to many nations, as the main mosquito vector of the virus (Aedes aegypti) or other efficient vectors such as Aedes albopictus are not present in the country. However, we do not know whether the mosquitoes present in New Zealand can transmit Zika virus.”

Dr Siouxsie Wiles, Senior Lecturer, Faculty of Medical and Health Sciences, University of Auckland, put together a FAQ on Zika for Sciblogs.co.nz. She writes:

What are the symptoms of Zika infection?

“Until recently, the Zika virus was thought to give people a mild infection. About 2-7 days after a mosquito bite, 1 in 4 people develop symptoms which could include a mild fever, conjunctivitis, a headache, joint pain and a rash. But since October 2015, there have been over 3,500 babies born in Brazil with microcephaly – a quite uncommon complication of pregnancy in which babies brains don’t develop properly and they are born with a smaller head and brain.

“The WHO is also investigating an increase of Guillain-Barré Syndrome (GBS) cases in El Salvador. GBS is an autoimmune disease caused by the body’s immune system mistakenly attacking the peripheral nerve.

“There is currently no treatment or vaccine for Zika. The main way to avoid infection is to prevent being bitten by mosquitoes and to try to stop Aedes mosquitoes breeding. Women in outbreak countries are currently being advised to delay getting pregnant.

What’s the risk to NZ?

“According to the Ministry of Health website, we don’t have the right mosquitoes in New Zealand for the Zika virus to flourish here. The main risk to New Zealanders is travelling to countries with widespread Zika transmission. This list of countries seems to be growing by the day, but most relevant to New Zealanders is probably Samoa.

“The CDC is advising women who are pregnant, or trying to get pregnant, to postpone travelling to countries with Zika, and if this can’t be done, to take all precautions to avoid being bitten by mosquitoes. This means using mosquito nets, covering up arms and legs with long clothing, and using proper insect repellents that contain DEET or picaridin. Now is not the time to be relying on ‘chemical-free’/homeopathic repellents or nonsense vitamin B patches.”

Read the full FAQ on Sciblogs.co.nz 


Updated commentary from the UK SMC in response the WHO’s announcement of an Emergency Committee:

Prof James Whitworth, Professor of International Public Health, London School of Hygiene & Tropical Medicine, said:

“The WHO’s decision to set up an emergency team for Zika virus infection is most welcome. This outbreak in south and central America is unprecedented and has caught the world unprepared once again, with no vaccine, no drugs and limited anti-mosquito measures. On Monday the WHO may well declare a ‘public health emergency of international concern’ as this epidemic has the potential to spread even more widely in tropical and sub-tropical regions. This action by WHO will stimulate international interest, funding and research to help tackle this outbreak on the ground and in laboratories around the world.”

Dr Nathalie MacDermott, Clinical Research Fellow, Imperial College London, said:

“There are no known reports of imported invasive Aedes mosquitoes including Aegypti or Albopictus in the UK. Nor have these mosquitoes established themselves in the UK, in other words they have not taken up residence and are not breeding in the UK. The Albopictus has in recent years spread to Southern Europe (Italy, Southern France and Spain) but our climate in the UK is still not ideally suited to the Albopictus, although it is not impossible for it to establish itself in the UK. There are occasional sightings of imported Aedes Albopictus in many countries in Europe but they do not necessarily establish themselves in these countries. The climate in the UK is not suited to the Aedes Aegypti mosquito and based on our current climate, it would not establish itself here.

“Even though a mosquito vector may be present in a country it does not mean the Zika virus is present. The mosquito would need to become infected with the virus and this would only happen by taking a blood meal from someone infected with the virus, for instance an infected returning traveller.

“The recent reporting of mosquitos in the South East of the UK in the media likely refers to the Culex Modestus mosquito which was identified in nature reserves in Kent in 2010. There is currently no evidence the Culex mosquito can be infected with or transmit Zika. This is being investigated by Brazilian scientists on the basis that Zika has spread so rapidly and the Culex mosquito is more prevalent than the Aedes. The prevalence of the Aedes mosquitoes in South America and their propensity for biting however is more than adequate to result in an outbreak of this proportion. We should not speculate until the results of this study are available at the end of February.”

Prof Jonathan Ball, Professor of Molecular Virology, University of Nottingham, said:

“The numbers likely to be infected by Zika in the current Americas outbreak are immense, but not so surprising. The virus has been unleashed in an area where its insect vector is widespread and the human population has never been exposed in the past – they don’t have any immunity and so the mosquito can pass the virus from person to person unhindered.

“Eventually the outbreak is likely to burn itself out as people become exposed then immune. But it is unlikely to disappear completely. In future it will probably survive by causing sporadic outbreaks and by infecting people who haven’t been exposed to the virus, for example children. An infection of children wouldn’t be a bad thing as it would probably mean that they are immune to later infection, particularly when they are at a child-bearing age. So by a natural process we would hope that the really serious effects that Zika might be having on unborn children will dwindle in those areas where the virus does become endemic.

“But these are possible scenarios but until we know how the virus behaves, in this and in previous outbreaks, and where it is endemic, we won’t be able to make predictions with any degree of certainty.

“The viruses survive by infecting their human host and also the insect that spreads them. Maintaining an ability to infect both species puts a lot of genetic pressure on the virus to stay the same. So whilst not impossible the virus is unlikely to change its insect vector. And you have to ask, why would it anyway? There are enough Aedes mosquitos around to enable it to spread.”

Prof Paul Reiter, Consultant on mosquitoes and mosquito-borne diseases and Professor of Medical Entomology, Pasteur Institute, said:

“There are many uncertainties with WHO’s estimate of the number of Zika virus infections there may be in the Americas. An important one is the ratio of asymptomatic to symptomatic cases. This can be very high (many more asymptomatic than symptomatic cases), and may vary with the strain.

“Four million clinical cases may sound a lot but may well be an underestimate.  For comparison, in one dengue epidemic that we investigated in Guayaquil, Ecuador, we estimated 405,000 cases in about four months. That city had 2 million people so was smaller than many urban areas in the Americas and in Asia.

“Regarding Culex mosquitoes and Zika, I totally agree with the WHO – there is no evidence in the field or in the laboratory that Zika or closely related viruses (e.g. dengue, yellow fever) can replicate in any species other than the urban Aedes (Aedes aegypti and Aedes albopictus) that are implicated in all outbreaks. The hyper-abundance of Aeges aegypti (which we already know is implicated in this Zika outbreak) is more than enough to sustain the very high incidence of Zika virus that we are seeing.  So there’s nothing to suggest that any Culex species are involved.

“Aedes aegypti has never been established in Britain. It has no diapause or cold-hardiness and so is unable to survive our climate. Moreover, it is an urban species, closely associated with human habitation-that is why it is such a good vector in cities throughout the tropics-and is never found in the countryside.”

Prof Michael Bonsall, Professor of Mathematical Biology, University of Oxford, said:

“WHO have estimated there may be 3-4 million cases of Zika in the Americas over the next 12 months, but I think we would need to see the model details before being able to be clear on the predictions.  WHO’s estimate was based on previous dengue outbreaks – the epidemiology of Zika is different from dengue but because transmission is by the same Aedes vector the estimate is probably sensible.  Fast and effective vector control will be absolutely essential here. Aedes aegypti is day-flying and urban-dwelling so appropriate vector controls to prevent ‘explosive spread’ are paramount.

“Regarding questions from the media about whether Culex mosquitoes could be involved with this Zika outbreak, I agree with WHO that Aedes aegypti is quite enough to be responsible for the rate of infection we are seeing (Aedes aegypti is urban and likes humid places, hence why Recifie is at such high risk).

“Aedes aegypti is the mosquito implicated in the current epidemic in Brazil.  It is very unlikely that these mosquitoes could live in the UK because they are a tropical and subtropical beast.  There are around 30 other species of mosquitoes in the UK of which about 1/2 are in the Aedes genus (group) but are different sepcies.  Species of Culex mosquito can be easily mis-identified as Aedes.”


Earlier commentary collected by the UK SMC.

Prof Laura Rodrigues, Professor of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, said:

Where in the world do people contract Zika, and how?

Zika is a mosquito transmitted virus; it has been isolated in countries in Africa, South Asia, Southeast Asia, Micronesia, Polynesia and Latin America. There have been outbreaks in Micronesia and French Polynesia and Latin America. Transmission could happen anywhere there is dengue (as Zika is transmitted by the dengue vector, Aedes Egypti) if the virus is introduced, for  example by someone returning from a country where the virus is circulating.

What does Zika virus do to a healthy (and non-pregnant) adult and what would their symptoms be?

Zika is normally very mild; symptoms are similar to mild dengue: malaise, rash, low fever, itching and red eyes. Extremely rarely there maybe neurological complications (Guillain-Barré syndrome) but the concern is transmission to the fetus in pregnant women.

What are the potential complications if Zika is contracted during pregnancy, and how strong is the evidence for an association / causative relationship with microcephaly?

“The evidence for a link is relatively strong, and considered strong enough to warrant public health measures. Evidence includes temporal association between Zika outbreaks and microcephaly outbreaks both in Brazil and French Polynesia. The virus is neurotropic – it grows in the brain of the fetus and destroys brain structures so the developing brain is malformed – small – that’s the cause of the microcephaly.

Is Zika contagious between people?
“We know for sure that most transmission is by Mosquitos. There is no evidence of person to person transmission yet but the virus has been isolated in semen so this is a possibility.

Do we know why cases of Zika are on the rise in South America and the Caribbean?

“Once a virus with a competent vector (the mosquito) is introduced in a totally susceptible population it is to be expected that transmission there would be epidemic. There were earlier outbreaks in Micronesia and Polynesia. The mystery is why there were no outbreaks in the other countries were Zika was isolated. Potential factors enhancing transmission are the high population density (large cities) with many breeding sites for the mosquito – dengue is a domestic mosquito and is rarely found more than 100 metres from human habitations.

What does it mean for the UK that three returning travellers have tested positive for Zika – could they pass it onto others? Does the mosquito that transmits the virus live in the UK?

“Because Aedes does not live in the UK (it does not survive cold winters) there is almost no risk of transmission unless there are transmission mechanisms we do not know about yet. Infected cases coming to the UK are therefore not going to transmit infection. All the risk is for pregnant women travelling to areas in the world were there is Zika and being infected there.

“This is a very new situation. Until a few months ago we did not know that Zika could cause congenital infections and microcephaly. It caught us all by surprise. There was very little research as Zika was seen as a virus of no public health importance. There is no vaccine, we do not know if a case becomes immune, there is no good diagnostic test and no treatment. Much research needs to be done – will be done – with a major international effort we hope to have better knowledge and maybe a vaccine in a couple of years.”

Prof Andrew Easton, Professor of Virology, University of Warwick, said:

Zika virus was first detected in Uganda in 1947 in a monkey with the first human cases identified in Nigeria in the late 1960s. The virus was then found in humans in small sporadic irregular outbreaks in many parts of Africa, particularly West Africa and subsequently South East Asia. In the cases reported at that time the disease was seen in approximately 20% of infected individuals with the remainder showing no obvious disease. The disease was mild with fever, rash and headaches and some additional symptoms and that remains the most common type of disease.  The full range of symptoms is not always seen.

“The virus continued its spread reaching the islands of Micronesia and ultimately French Polynesia in recent years.

“The virus is transmitted by mosquitoes belonging to the Aedes species.  Aedes aegypti is a very common mosquitos of this family found in urban areas in tropical and sub-tropical regions and is likely to be the primary species responsible for transmitting Zika virus, and many other viruses.  Zika virus is transmitted when the infected mosquito bites a person to take a blood meal and if the person is already infected the mosquito may pick up the virus at this time and transfer it to the next person they bite.  The virus must be introduced into the bloodstream to be able to establish an infection and is not transmitted by other routes such as aerosols. There has been a report suggesting that Zika virus may very rarely be transmitted sexually but more work on this is required and even if possible it is very unlikely to be coming and is certainly not a major route of infection.

“The virus was introduced into South America in probably in late 2014.  This was most likely due to an infected person (who may have had no symptoms of disease) being bitten by a mosquito which then transferred the virus to other people.  Mosquitoes are a major concern in that part of the world as they transmit a number of other diseases and they are extremely numerous.

“In parts of the world where mosquitoes do not survive well or are relatively uncommon transmission of the virus by a returning infected traveller is unlikely to represent a problem.

“It has only been in the last few months that there have been suggestions of Zika virus infection leading to problems with newborns. In French Polynesia there was a significant increase in the incidence of microcephaly and, as we have seen recently in Brazil, this appears also to be the case there with up to 4000 cases in a very short period.  Some studies have detected Zika virus in the fluid that surrounds the embryo during pregnancy and this suggests that infection of foetus in the womb can lead to these problems, though this is still an area of active investigation.  These investigations will become more urgent in light of the huge increase in incidence of complications.

“The link between Zika virus infection of pregnant women and microcephaly has not yet been definitively proven but the increase in spread of the virus incidence alongside the increase in microcephaly is a concern. Until the situation is clear pregnant women should seek advice about travel plans to parts of the world where Zika virus is found.  There is no treatment or vaccine available for Zika virus and the only options available are to reduce the risk of acquiring the infection.  All travellers to these regions should take precautions to reduce the risk of bites from mosquitoes by using repellents and covering their skin as far as possible with e.g. long sleeves.”

Dr Alain Kohl, MRC Programme Leader, MRC Centre for Virus Research, University of Glasgow, said:

Where in the world do people contract Zika, and how?

“The virus is presents in parts of sub-saharan Africa but also SE Asia and more recently Pacific Ocean Islands. The virus has now spread to the Americas and local transmission has occurred in several countries.

Does the mosquito that carries Zika live in the UK?

Aedes aegypti and Aedes albopictus – which are likely to be involved in transmission in Brazil – are not present in the UK.

What does Zika virus do to a healthy (and non-pregnant) adult and what would their symptoms be?
“Most people are asymptotic or develop fever, rash etc.

What are the potential complications if Zika is contracted during pregnancy, and how strong is the evidence for an association / causative relationship with microcephaly?

“Data is increasing that there may a link between microcephaly and Zika but this isn’t certain yet. The virus has been found in foetal tissues. There will certainly be strong research efforts to investigate this possibility.

Is Zika contagious between people?

“There is no evidence of airborne transmission and the main route of infection is through mosquitoes. There has been a case of sexual transmission, and theoretically transmission by transplantation or transfusion cannot be ruled out.

Do we know why cases of Zika are on the rise in South America?

“There are several possibilities – firstly there is no pre-existing immunity in a very large population exposed to mosquitoes that can transmit the virus, or potentially changes in the virus. There may be other factors that we don’t know about.

Is there advice for what pregnant women should do if they are planning travel to affected areas?=

“Protecting themselves from mosquito bites is important and precautions should taken.”

Prof Trudie Lang, Director of the Global Health Network, University of Oxford, said:

Zika virus is spread by the same mosquito as Dengue virus, and so can occur in the same countries where aedes mosquitos occurs. It has been know about for many years and was first observed in the Zika forest in Uganda. Typically it causes a mild illness, often with a fever and a rash, and 3 out of 4 people might never have symptoms.

“However there has been a increase in the number of people in Brazil and the wider region who are infected with Zika virus. At the same time there has been a dramatic increase in babies born with microcephaly, which means smaller than normal heads. These seem likely to be related, but it has not been proven yet; hence the advice in the USA from the CDC to pregnant women to avoid these regions.

“The mosquito that carries Zika does not live in the UK.

“This virus is not like Ebola in that there is no evidence for human to human transmission; Zika is not contagious.

“However, this is an important emerging disease outbreak situation and we really must apply the lessons that we learnt from Ebola because Zika could be a major public health issue in these countries.  There are many unknowns and so research is urgently needed to understand what is happening and how to prevent further cases.

“There is no treatment and no vaccine and so this would need addressing through clinical trials as quickly as possible. The international research community are pulling together through the ISARIC network to support local health research groups to set up studies as quickly as possible by sharing research methods and protocols.

“During the Ebola outbreak the research response was too slow and lagged behind the immediate medical humanitarian action. Research simply must be embedded in the global response to emerging outbreaks such as this in order that the impact to public health is limited by understanding the disease and evaluating interventions such as prevention strategies, treatments and vaccines.”

Declared interests

Prof Laura Rodrigues: No conflicts to declare.

Dr Andrew Easton: No interests to declare.

Dr Alain Kohl: “I work on this virus and have MRC funding on the subject.

Prof Trudie Lang: “Trudie is Professor of Global Health Research at the University of Oxford, is a member of the ISARIC network and director of the Global Health Network.”