The deadly bite of mosquitoes | Science| In-depth reporting on science and technology | DW | 29.06.2017
  1. Inhalt
  2. Navigation
  3. Weitere Inhalte
  4. Metanavigation
  5. Suche
  6. Choose from 30 Languages

Science

The deadly bite of mosquitoes

In Africa, every two minutes a child dies - of malaria. Mosquitoes and humans are engaged in a war. To help humans win, scientists are working on new strategies. Especially in Ifakara, the "Mosquito City" in Tansania.

Cooking, eating, fetching water, washing, chatting - in Ifakara, the huts are too small for so much action. People in this city in rural Tanzania are poor, most homes are only big enough to hold a bed or two. Except for sleeping, residents go about their lives outdoors - where the mosquitoes are, a deadly danger. One meaning of the word Ifakara is "the place where I die."

In Africa, a child dies of malaria every two minutes. Fever attacks, impaired consciousness and organ failure are typical symptoms. The parasites can multiply in about 30 different Anopheles mosquitoes - one bite, and they can enter a person's bloodstream.

The Kilombero Valley surrounding Ifakara is one of the African regions worst-hit by malaria.

"When I started working here, we couldn't count the mosquitoes we caught in light traps for our samples, we could only weigh them," says Gerry Killeen, an Irish researcher who has worked at the Ifakara Health Institute (IHI), a renowned center of malaria research, for 15 years. "Sometimes, the collection bags would be brimming with mosquitoes after a single night," he remembers.

Anopheles mosquito

More than 700,000 people die every year from mosquito bites because the insects transmit deadly diseases

Invincible anopheles mosquitoes

That has changed. Almost everyone in Ifakara sleeps under a mosquito net these days. Anopheles mosquitoes prefer to bite at night, and the nets rob them of their livelihood. Mosquito nets treated with insecticides are the main contributor to the fact that the death rate has dropped by 60 percent since 2000, and infections have decreased by a third. In the 1980s, a person living in Ifakara would suffer an average of 2,000 infectious mosquito bites per year. That figure has dwindled to 18.

Global figures, too, give reason for hope, even if a first glance paints a different picture: according to the WHO, 3.2 billion people live in areas at risk, and 212 million people fell ill with malaria last year - 18 percent fewer cases since 2000.

It seemed as if mankind might defeat malaria.

But the mosquitoes won't play along. Despite all efforts, they continue to fly, bite and kill. Malaria cases stagnate at the level once reached. It is like running into a wall, as if the opponent, believed defeated, is recovering. Researchers in Ifakara are intent on tracking down a mystery: malaria parasites can still be detected in the blood of more than ten percent of the population in the Kilombero valley although they sleep under nets. "If you take into account that Anopheles mosquitoes only bite at night, it is surprising that we still have so many cases of malaria here," says IHI Director of Science Fredros Okumu.

Human bait

Okumu is working to unravel the contradiction with the help of the residents of the Kilombero valley. For a small reward, they hunt for places where large numbers of mosquitoes gather, or act as human bait, sitting for hours beneath a mosquito net connected to a trap. They collect data, keep accounts of what exactly their family does between six in the evening and six in the morning.

IHI scientists in the lab check how many mosquitoes carry the parasite, whether the females have already bitten, and what species the insects belong to.

Helping science: Hamis Chande baits mosquitoes

Helping science: Hamis Chande baits mosquitoes

Bruse Willis among mosquitoes

The pieces of the puzzle are slowly coming together to form a picture that explains the remaining cases of malaria. Just a few years ago, nine out of ten mosquitoes were of the Anopheles gambiae sensu strictu species, "efficient transmitters of malaria that only bite at night and only indoors ," according to Okumu. "This species has almost vanished because the mosquito nets prevent the insects from reaching people."

The IHI scientists found that other Anopheles sub-species took their place, mainly Arabiensis and Funestus. They call them the "Bruce Willis among Anopheles mosquitoes" for their flexibility and versatility - and most of all for the fact they are the devil to kill.

Anopheles arabiensis also bites cows and chickens if humans are not available. Anopheles funestus is less common, but the mosquitoes more often carry malaria parasites. Both are active during dusk, and remain so until the sun rises.

The data shows how skillfully the insects have adapted to humans. In Ifakara, they are most plentiful between eight and nine p.m. - surveys show that most people there go to bed under their protective mosquito nets between nine and ten. The mosquitoes show up again at around five a.m., when many people rise.

Occurance of malaria in Africa (light colors mean more cases, from 0 to more than 685 taken ill per 1,000 people per year)

Occurance of malaria in Africa (light colors mean more cases, from 0 to more than 685 taken ill per 1,000 people per year)

Mosquitoes - at home on every continent except the Antarctic for 100 million years - are huge survival artists. Presumably, mosquitoes were around to bite dinosaurs, too. Their swift succession of generations lasting just two to three weeks and the large number of offspring - a female lays up to 300 eggs per deposition - make them extremely adaptable.

In order to completely push back malaria, a UN goal by the year 2030, humans must find a way to be safe from mosquito bites outside of their homes and beds,too. But how can that be achieved in Tanzania, where the average monthly income amounts to $75 (70 euros)? Many Tanzanians cannot even afford to buy candles to light their huts, much less an insect repellent. Ifakara farmers also cannot afford larger, mosquito-safe houses.

The time pressure is growing

IHI researchers are thus experimenting with various methods. They set up empty clay vases where mosquitoes like to hide during the day. The pots are treated with pyriproxifen, and the insects inadvertently carry the pesticide back to where they lay their eggs.

A hut in Ifakara

A hut in "Mosquito City," where scientists can study mosquito behavior under realistic conditions

The active agent prevents mosquito larvae from developing into adult mosquitoes. On the go, people can carry sisal mats saturated with an insecticide and set them up to keep the insects away, which more or less works up to five meters. Producing such mats costs about $2 apiece. Sandals treated accordingly also offer some protection.

It is a race against time, however - and against the mosquitoes' reaction. "Increasingly, they are becoming resistant against insecticides," says Nancy Matowo, a researcher who also works at the IHI. The insects have become resistant to at least one class of chemical agent in 60 out of 96 countries, the WHO warns. Ethiopia, Sudan and Afghanistan even report of mosquitoes that are resistant to all four available types of insecticides.

Welcome to "Mosquito City"

"For that reason, we wanted to develop a method that kills the mosquitoes immediately, without using an insecticide," Matowo says.

They came up with the Mosquito Landing Box, a black wooden box with adjustable louvers that hide netting linked to electricity, made partially from cheap electric bug swatters. A fan disperses an attractant that smells of human sweat, for instance used nylon socks. "When the mosquitoes fly into the box, they hit the electric grid, and die," Matowo says. Solar panels power the attractant dispensing unit during the day, at night the box uses a battery.

The scientists Nancy Matowo collect and examine mosquitoes in the Kilombero valley

Nancy Matowo demonstrates the Mosquito Landing Box (MLB.) The odor created by the attractant dispensing unit inside the box is distributed by a fan. The silver plate on the lid reflects the odor.

The boxes were first tried out on the premises of "Mosquito City," the IHI's special biospheres to study mosquitoes. The huge greenhouses hold ponds, banana trees and huts with sleeping mats - a replica of the mosquitoes' hunting grounds.

The results are promising. A landing box reduces the number of mosquitoes out to bite humans to a fraction. Matowo's colleague Arnold Mmbando is already working on taking the box a step further: an off-putting odor could deter mosquitoes from alighting on humans, with the Landing Box waiting to bait and kill them when they flee.

Mmbando's boss Okumu is convinced that it takes such methods to completely eradicate malaria. The next phase of the battle probably is no longer about large scale intervention by the state, he says. Instead, every region needs its own, tailored strategy to ensure that in the fight against mosquitoes, it's not just mankind's opponent who adapts.

DW recommends

Advertisement