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Damn the mosquitoes! Full speed ahead!

Dr Pedro Alonso
Director of the Global Malaria Programme

15 March 2016

Dr Pedro Alonso, Director of the Global Malaria Programme

At WHO, we are hoping that advances in technology will help stop the explosive spread of Zika virus throughout the Americas and, potentially, much of the rest of the world.

Zika, of course, is the virus spread by the Aedes aegypti mosquito, and is suspected of having caused thousands of children in northeastern Brazil to be born with abnormally small heads. Many of these children are likely to require specialized care throughout their lives.

Concern over that suspected link led WHO last month to declare an international health emergency. And on 14 and 15 March, we are holding an extraordinary meeting of our Vector Control Advisory Group. This group of external experts from the top echelons of academia and research will review the latest data, some of it unpublished, and provide recommendations to WHO on at least 3 experimental technologies:

  • A new tool led by Monash University (Australia) is infecting mosquitoes with Wolbachia bacteria, which reduce their ability to transmit dengue viruses to humans. Since the mosquito that carries dengue also carries Zika virus, it may protect against Zika, too.
  • A British biotech company is modifying the genes of male mosquitoes so that their offspring cannot reach adulthood.
  • Another approach involves releasing sterilized male mosquitoes so that fertilized eggs do not hatch into larvae and, as a result, cannot reach the biting, adult stage. The technology is called sterile insect technique.

If the advisory group concludes that any of the technologies appears promising and ready for use, WHO will recommend to fast-track their deployment.

But we are not waiting for a magic bullet, since we already know what works -- vector control. In lay terms: killing mosquitoes or eliminating their habitats. At present, we have no alternative: there is no vaccine; no treatment; and we do not even have good diagnostic tests.

WHO has established a vector-control group composed of experts from across the Organization to monitor and support the work of member countries in 6 regions, all of which are home to the Aedes aegypti mosquito. Luckily, we know how to get the job done. This week, WHO will release emergency guidelines for vector control and entomological surveillance – including monitoring of insecticide resistance. They include a list of measures we can -- and must -- take now:

Manage the environment

That means getting rid of any standing pools of water that can serve as a habitat for the Aedes aegypti mosquito.

Use larvicides

WHO recommends 12 compounds or formulations to control mosquito larvae, including pyriproxyfen, an insect-growth regulator that mimics a natural hormone found only in invertebrates. Some reports have speculated that pyriproxyfen could itself be causing the neonatal malformations. But a WHO review of toxicological studies has found nothing to support that suspicion. Biological control methods include use of small ornamental fish that have an appetite for larvae.

Kill the adults

You have seen the videos – people wearing backpacks connected to hoses that are belching out clouds of insecticide. They have been used against yellow fever, dengue, chikungunya and, now, Zika. A word of caution, though – this method needs to be applied strictly at dawn or dusk and not during the day.

Apply insect repellent, especially if you are pregnant

You will be protecting yourself and those around you by breaking the transmission cycle.

These are time-tested, low-tech and readily available tools. And they work. During much of the last century, we used each of them as part of large-scale, vector-control programmes to keep mosquito populations at bay throughout Latin America.

An extreme example and a singular achievement in the world of public health was the elimination of the Anopheles gambiae mosquito from tens of thousands of square kilometres of Brazil in the 1930s and early 1940s. The mosquito, which had been accidentally introduced from sub-Saharan Africa and was spreading malaria, was eliminated through a programme that relied overwhelmingly on larval control. More than 60 years later, it has not reappeared.

Alas, the resources for such efforts were slashed in the 1950s, when DDT proved highly effective at killing mosquitoes of all types – for long periods of time and at little cost. But indiscriminate use of the powerful insecticide caused widespread environmental damage and led to insecticide resistance. In 2001, at the Stockholm Convention on Persistent Organic Pollutants, more than 90 countries signed a treaty that put DDT on a restricted-use list and mosquito-control efforts languished.

The result: Aedes aegypti mosquitoes have spread across much of South and Central America, and beyond, causing major epidemics caused by the viruses it carries -- dengue, chikungunya, yellow fever and Zika.

Meanwhile, the field of vector control and medical entomology has shrunk massively. Even at WHO, the number of medical entomologists dropped from 38 in 1982 to 16 in 2015. Of them, only 7 who are based in WHO/HQ work full time on malaria and Neglected Tropical Diseases. The remaining 9, who are based in regional and country offices, are also responsible for other communicable diseases.

For WHO to spearhead the response against mosquito-borne diseases like Zika and malaria – which continues to claim the lives of more than 400,000 each year – this situation needs to be addressed urgently.

We are facing a challenge today that our forebears did not predict. Last year was the hottest on record on a worldwide basis. Has that affected the spread of mosquito-borne diseases? This is not yet clear, but it cannot have helped.

At WHO, we are hoping that our rapid action in the face of emerging health challenges such as Zika can serve as a template to protect against future threats that are sure to arise as the planet warms.

We have reason to hope that we will succeed, just as others are now. Cuba serves as a shining example of the power of community engagement when it comes to battling mosquitoes. By putting in place a programme to rid the island of the standing pools of water where the mosquito lay their eggs, Cuba has made dengue a rare disease on the island, with fewer than 700 cases in 2015.