January 28, 2016

expert reaction to WHO estimate of 3-4 million cases of Zika in the Americas over the next 12 months

The World Health Organization has predicted that there may be up to four million cases of Zika infection in the Americas this year. An outbreak of Zika virus is ongoing in Central and South America, with a possible link to microcephaly in babies.

Dr Anthony Wilson, Head of the Integrative Entomology group within the Vector-borne Viral Diseases Programme, The Pirbright Institute, said:

“The WHO’s comments in response to questions about the vector responsible for the current Zika outbreak in Brazil, that we know Aedes aegypti is implicated and that there’s no reason to suspect Culex mosquitoes of being involved, is perfectly reasonable – other Aedes-borne arboviruses spread rapidly in the area and there is no reason to expect Culex to be involved based on the current pattern of spread. Other Aedes mosquitoes could plausibly be involved in the Brazilian outbreak, such as Aedes albopictus (the Asian tiger mosquito), which does also occur in parts of southern Europe (although not in the UK), although there is no evidence for that yet and there are no outbreaks of Zika in southern Europe.

“Aedes aegypti, the yellow fever mosquito, does not occur in the UK. It’s too cold for Aedes aegypti to establish in the UK, although in ideal summer conditions introduced individual mosquitoes might be able to survive for a few days; there was a small outbreak of yellow fever in Wales (Swansea) in 1861 which is believed to have been spread via mosquitoes that were inadvertently introduced on a ship returning from Cuba. It is plausible that a related species, Aedes albopictus (the Asian tiger mosquito) could be playing a role in the transmission of Zika virus in the Brazilian outbreak; it has been implicated during other Zika outbreaks. Aedes albopictus is more cold-tolerant and individuals introduced into Belgium and Netherlands have been detected in the past, although they have failed to establish in those locations. Aedes albopictus is found in parts of southern Europe, but Zika has not been seen there.”

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.”

http://terrance.who.int/mediacentre/presser/WHO-RUSH_Zika_virus_presser_28JAN2016.mp3

Declared interests

Dr Anthony Wilson: “I currently lead the Integrative Entomology group within the Vector-borne Viral Diseases Programme at Pirbright, which studies insects and their ability to transmit high-impact viruses. I acted an EFSA scientific expert during the Schmallenberg outbreak, another recent outbreak of a relatively obscure (in that case, previously unknown) insect-borne virus, and have been involved in policy advice for bluetongue outbreaks as well. My current funded grants include one for which I spend a proportion of my time working with scientists at Oxitec on developing computer simulations to model strategies for releasing GM insects to control natural populations. I am a Fellow of the Royal Entomological Society.”

Prof. Jimmy Whitworth: Nothing to declare.

Dr Nathalie MacDermott: “I am undertaking a PhD in Ebola virus disease at Imperial College London funded by the Wellcome Trust.  I was employed by Samaritan’s Purse International Relief, working in Liberia as part of their Ebola response team, in July 2014 and from October 2014 to March 2015.”

Prof. Jonathan Ball: “No conflicts.”

Prof. Paul Reiter: “I have absolutely no competing interests that any third party could object to. I worked on this subject for my entire career, first at CDC, then the Pasteur Institute. I am now retired.”

Prof. Michael Bonsall: “I was the Specialist Adviser to the inquiry on Genetically Modified Insects by the House of Lords Science and Technology Committee, and am a member of ACRE (Defra’s committee on releases into the environment).  The Advisory Committee on Releases to the Environment is independent scientific committee that through EU derivatives and national UK legislation is mandated to provide the UK government/ministers with advice on the environmental risks (to wider receiving environments/human health) of non-native and GM releases. I have been a member of this committee since December 2007.  I have carried out occasional consultancy work for WHO, NIH, and EFSA, including work on GM-based control methods for mosquitoes.  I have received research funding from BBSRC (including working with Oxitec on GM insects), Oxford Martin School, NERC, the Royal Statistical Society, and the Royal Entomological Society.  For full details off all my interests, please see: http://www.parliament.uk/documents/lords-committees/science-technology/GMInsects/michael-bonsall-specialist-adviser-interests.pdf.”

None others received.