Zika declared public health emergency

08 February 2016

Interview with

Peter Horby, Oxford University

The World Health Organisation has declared a public health emergency aroundAedes aegypti in Dar es Salaam, Tanzania the outbreak of Zika virus in Brazil where as many as a million people could have been infected with this agent since last year. The virus has been linked with a birth defect known as microcephaly, which means an undersized brain in children born to women who pick up the infection when they're pregnant. Zika virus was first described in the 1940s in one part of Africa and it's transmitted by mosquitoes but scientist don't know what's causing the sudden flurry of cases in Brazil, or the true scale of the disease.  Next week the UK Science & Technology Committee have announced that they're convening a special hearing on Zika to examine the threat. Peter Horby specialises in emerging infectious diseases at Oxford University and he'll be giving evidence to that panel, as he explained to Chris Smith...

Peter - The mosquitoes don't fly very far at all; probably only a few hundred metres, up to 500 metres, maybe, so it's actually people that spread it.  So someone will get infected and they will travel while their still infectious, and then they'll be bitten by a mosquito at the other end, and then they will then infect the mosquito populations and humans at the other end, so it's people that are moving the virus around the world.

Chris - And when a person succumbs to Zika virus, what happens to them?  How does it make people ill?

Peter - Well it's generally a mild illness actually.  Probably 60-80% of infections are, actually, asymptomatic so people don't really get sick and those that do get sick, they'll get fever, muscle aches, joint pain and a rash.  It wouldn't really be a concern if it hadn't been for the abnormalities in the babies born towards the end of this year.

Chris - How long does a person incubate the infection before they become symptomatic?

Peter - Not really known but it's probably three days, possibly up to two weeks; we need to really look into that.

Chris - You say that this is a mosquito spread infection, therefore the infections likely to go where the mosquitoes are.  So where around the world are those mosquitoes and, in other other words, which territories are at most risk of ending up with this as an endemic infection?

Peter - Unfortunately, the mosquitoes are all over the place.  The Aedes mosquitoes really live in the tropical band but that  tropical band is all the way around the world.  So it's across Central Africa, South America, most of Asia; so huge populations are at risk of being bitten by these mosquitoes.  There are two types that are likely to be transmitting this virus; there's the Aedes aegypti, which is the principal vector of Dengue, so we're likely to Zika virus in the same distribution we see Dengue virus.  But there's another mosquito called Aedes albopictus which is a bit more tolerant of cold, so we actually have that mosquito in parts of southern Europe and it's also in reasonably large parts of the southern parts of the United States.

Chris - One of the most concerning aspects of this outbreak has been that people have reported an increase in microcephaly; these are babies being born with brains that are undersized.  What's the evidence that  Zika virus is to blame for that?

Peter - The reports of microcephaly, which many of us will have seen on the TV and are quite distressing, is the reason that the WHO have declared this public health emergency.  What we've seen is, in Brazil, the virus really took off in February last year and then, almost exactly nine months later, there was an increase in the number of babies born with microcephaly and then there was biological evidence as well.  So some women who had abnormalities on ultrasound, they took a sample of the amniotic fluid that surround the baby in the womb, and found the virus there, and then in a handful of babies that were born with microcephaly and then died very quickly, and in a couple of babies there were spontaneous abortion, they also identified the virus in those children.

Chris - Do we know how it damages the developing brain?

Peter - No we don't.  We know that babies can be infected in the womb by several different organisms and, traditionally, that does interfere with the development of the child.   One of the worries is that the microcephaly we're seeing is just one presentation of the problem; there may be more subtle changes in developmental or neurological development of children that are harder to detect that might be due to the Zika virus.

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