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Our pandemic plight: Combating the spread of neglected tropical diseases

March 25, 2019

Mosquitoes are the world’s most deadly animal, rapidly spreading lethal diseases around the world. What can we do to stop them?

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At some point 4,000 to 6,000 years ago in western Africa, the Aedes aegypti mosquito effectively became domesticated. It evolved to live in human habitats, breed in the water humans stored, and feed off its reluctant human hosts.

As it spread around tropical and sub-tropical parts of the world, Aedes aegypti adapted perfectly to live in urban environments, spending its 1-2 week lifespan buzzing in and around the houses near where it emerged as an adult.

These insects are experts at survival. The conditions they need to reproduce are exceedingly modest – Aedes aegypti have created breeding grounds in discarded soda cans, plant pots, even used tires. Indeed, Aedes aegypti larvae can complete their development in just a bottle cap filled with rainwater.

Unlike most mosquitoes that draw sufficient blood in a single bite, the female Aedes aegypti is a ‘sip feeder’ taking multiple little sips during multiple bites – and it has a particular taste for human blood. And following each blood feed, Aedes aegypti will lay batches of around 100 eggs. 

This all makes it an especially effective conduit for transmitting diseases. It is no surprise then that Aedes aegypti is more widely known as the Yellow Fever mosquito. But it could equally be called the dengue fever mosquito, the chikungunya mosquito or the Zika mosquito. It is a tiny insect with a devastating impact on human life.

“These hordes of blood-sucking insects are more dangerous than any enemy army.”

– Dr Kurt Vandock, Senior Scientist with Environmental Science at Bayer

Femmes fatales

Every year more than 500,000 people die as a result of mosquito bites. It is by far the world’s most deadly animal. In comparison, crocodile attacks kill around 1,000 people a year.

The vast majority of these deaths derive from the tropical diseases passed on by just two types of mosquito – Aedes aegypti and Anopheles. The latter is a genus of mosquito that is the only carrier of one of the deadliest diseases in human history – malaria.

While the diseases carried by the Aedes aegypti are typically not as fatal as malaria, in poorer areas that lack access to proper medical care, epidemics can be devastating.

The initial symptoms of dengue, chikungunya and yellow fever are all similar to a severe flu – fever, acute joint pain, vomiting and fatigue. But for those who are at higher risk – infants, young children and the elderly – more severe symptoms can quickly develop and can ultimately prove fatal. An outbreak of Yellow Fever in Angola in 2016 infected more than 3,625 people, killing 357.

Based on WHO estimation, 2018.

Dengue is the fastest growing mosquito-borne disease in the world. The WHO estimates that there are close to 390 million cases of dengue every year. And it is spreading. Before 1970 only nine countries had experienced severe dengue epidemics, now the disease impacts more than 100 countries with over 3.9 billion people at risk globally. 

A combination of a rapidly growing global human population, urbanization, increased tourism and climate change is making it easier for mosquitoes to colonize new regions, multiply rapidly and spread disease. The Zika pandemic is a prime example of this.

The fightback

“These hordes of blood-sucking insects are more dangerous than any enemy army,” says Dr Kurt Vandock, Senior Scientist with Environmental Science at Bayer. “Over the past decade, the population and geographical range of Aedes aegypti have exploded globally. If you want to stem dengue, chikungunya or Zika, you have to target mosquitoes.”

The public health strategy of combating most tropical diseases involves a mix of medicinal and vector control efforts. While some infections can be controlled or even eliminated through approaches such as mass drug administration (MDA) of safe and effective medicines, this is not always possible. For most arboviruses (viruses transmitted by insects) the most effective tactic is prevention ­– reducing the transmission by targeting the carrier.

For mosquitoes, there are different forms vector control – for example long-lasting insecticide treated bed-nets (LLINs), indoor residual spraying (IRS) and larvicides that target the breeding sites of mosquitoes. When properly used, LLINs reduce the risk of malaria infection by up to 50%. The WHO estimates that 54% of the population in countries at risk of malaria in sub-Saharan Africa slept under an LLIN in 2016, compared to 5% in 2005. IRS programs, meanwhile, can protect whole communities if at least 80% of the houses in a targeted area.

However, an emerging challenge is that mosquitoes are becoming resistant to existing insecticides. Since 2010, 60 of the 73 malaria endemic countries that monitor insecticide resistance have reported mosquito resistance to at least one insecticide class used on nets and indoor spraying.

In response, scientists continue to develop new insecticides and better, longer lasting nets. But it is a constant battle. Therefore, global health and development organizations have partnered with the world's 13 leading pharmaceutical companies and committed to control, eliminate or eradicate 10 neglected tropical diseases by 2020. In doing so they hope to improve the lives of over a billion people and perhaps finally beat one our oldest foes.

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