By Siouxsie Wiles 04/02/2016

Earlier this week the WHO declared the Zika virus outbreak in the Americas a Public Health Emergency of International Concern and yesterday the CDC reported that someone in Dallas had become infected with the virus by having sex with someone infected overseas. Meanwhile, the internet is awash with claims that genetically-modified mosquitoes are to blame for the outbreak in Brazil. 

In case you need reminding, Zika is a virus spread by mosquitoes which is currently suspected of being responsible for a cluster of microcephaly cases – a condition in which babies are born with a small head and brain. I’ve previously posted an FAQ about Zika but here is another based on these latest developments.

What is a Public Health Emergency of International Concern?

By calling the outbreak a Public Health Emergency of International Concern, the WHO can call on a binding international agreement to fast track research and aid to tackle the infection. This is only the fourth time the WHO have made such a declaration; the first was for swine flu in 2009, the second for polio in 2014 and the last was for the Ebola outbreak in West Africa.

There are so many unanswered questions regarding Zika so this declaration will hopefully speed up the search for answers. Questions like: which species of Aedes mosquitoes can carry the virus? Is the link with microcephaly real? If so, when are pregnant women at risk and how many infected women will pass the virus to their developing baby? Do the women need to be symptomatic? Is the link with Guillain-Barré Syndrome real? How far will the virus spread? Will it be possible to develop a vaccine?

While the research gets underway, the first thing has to be to provide access to contraception and to tackle the mosquitoes. More on tackling the mosquitoes later, but there are interesting times ahead for the many women at risk of being infected with Zika in countries with almost no access to contraception and abortion.

Will the fact that Zika can be sexually transmitted mean the virus could establish itself in countries without the right mosquitoes?

I think this is the question most on people’s minds with the announcement that the CDC has confirmed that someone in Dallas became infected with Zika by having sex with someone who had caught Zika while overseas. I think its safe to say that this mode of transmission is going to play a minor role in spreading Zika. The major route of transmission is from mosquitoes feeding on infected people and then spreading it to other people when they feed again. When infected people travel to countries without the right mosquitoes, cases usually end there. Yes, the sexual transmission route could mean infected people cause small clusters of cases in countries without the mosquitoes, like this case in Dallas. The number of cases will be related to how long the virus stays in a man’s semen and how many sexual partners he has. From all indications so far, infection with the Zika virus is for a relatively short period of time so its likely that men are only infectious for a few weeks. Contrast this with HIV which causes a chronic infection where people are infectious for years without treatment. Once there are better ways to detect the virus in those who have been infected, scientists will be better able to answer questions about how many men end up with the virus in their semen and how long it stays there.

Why has the outbreak exploded in the Americas?

It’s likely this outbreak is the culmination of a perfect storm of factors. Mosquito behaviour will be part of it – with the mosquito species most involved in Brazil, the female takes a blood meal before laying a batch of eggs and can produce about five batches in her life time, spreading the virus between people with each bite. Contrast this with other species of mosquitoes that only have one big blood meal, then lay one batch of eggs. Another important factor is the number of mosquitoes around. The strong El Niño in the Pacific has caused flooding in Brazil, Paraguay and other countries. This abundance of clean water has caused a mosquito population boom. The more mosquitoes there are, the more virus will be transmitted, meaning more infected people for mosquitoes to feed on. The virus has spread to other countries in the region because the disease doesn’t really incapacitate those infected. If they travel to somewhere with the right mosquitoes and still have virus in their blood when they arrive, then the mosquitoes will pick it up and start transmitting it to other people when the mosquitoes feed.

Whats all this about genetically modified mosquitoes in Brazil?

One of the best defenses against Zika is going to be control of the mosquito population. Over the last few years Brazil has been trialing the use of genetically-modified mosquitoes to control cases of Dengue fever, another viral infection spread by the same mosquito. The GM mosquitoes are a high tech version of a well established technique called the sterile insect technique. The usual way to do this is to irradiate a population of male mosquitoes in the lab so they become sterile and then release them into the environment in huge numbers to outcompete the fertile males for breeding females. This way the mosquito population crashes as less offspring are born. The problem with the irradiation technique is that it can affect more than just a male mosquito’s fertility. It can also affect their overall ‘fitness’ – their ability to survive in the environment and compete with normal males.

A company called Oxitec have used genetic engineering as a smarter way to make male Aedes mosquitoes for the sterile insect technique and they’ve been trialing them in Brazil over the last few years. Cue maps showing the sites where the mosquitoes are being trialed overlapping with the epicentre of the outbreak (they don’t) and cries that the GM males being released are spreading the virus (they can’t – male mosquitoes don’t bite!). Pure anti-GM propaganda. There is a great take down of all the claims here.

I spoke about all of these things with Kathryn Ryan on Nine to Noon yesterday.