Mosquitoes against humans

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Mosquitoes against humans

The fight against diseases carried by mosquitoes and other small organisms really needs new insecticides. But it’s unlikely that they will be ready in time writes Julia Gross.


A mosquito trying to feed on a human. © Anthony James

A couple of years ago local authorities in Boa Vista, north Brazil, found someone with Dengue fever, a serious, painful viral disease from mosquito bites. It was DENV-4. There are four types of Dengue viruses but there had been no DENV-4 in Brazil for almost 30 years. If people get several Dengue infections with different virus types, this can be serious and kill. So health officials in Boa Vista immediately started intensive mosquito control. They looked for places where mosquitoes breed, used larvicides (chemicals to kill the larva) and ordered a lot of spraying of the insecticide Deltamethrin against adult mosquitoes. Brazil’s Dengue Control program (following WHO guidelines)said this was a good example of what to do in a case like this.

But they found nearly as many mosquito eggs after this as before. In Boa Vista, there were about as many Dengue cases as in the years before. And after three months, the mosquitoes were twice as resistant to Deltamethrin. After half a year, they were three times as resistant. Seven months afterwards, DENV-4 was found in nine of the 27 Brazilian states. Two years later, it was in 23 of the 27 states.


Testing insecticide resistance in the lab: you put the insecticide in with some mosquitoes, leave it and then look later to see how many of them are still alive. Ifakara Health Institute in Tanzania. © Eliza Powell

In the Boa Vista case, they followed the information about resistance. But the same is happening around the world. Mosquitoes can get resistance by gene mutations – and they have done this – they now have resistance to almost every poison humans have ever used against them. They have a short reproduction time of about two weeks; and females lay several hundred eggs; so they adapt quickly. In South America and Asia, most tiger mosquitoes (that can give Dengue, Zika, yellow fever and Chikungunya) are resistant to all classes of insecticides. WHO data shows that malaria mosquitoes are resistant to at least one class of insecticides in 60 of 96 countries. Ethiopia, Sudan and Afghanistan have mosquitoes which adapt to all four classes of insecticides. The WHO even thinks this might have caused more malaria in some countries recently (they thought transmission had gone down by 37 per cent between 2000 and 2015).


Cause: parasites infect and destroy red blood cells. Vector (insect that carries it from one person to another): Anopheles mosquitoes Symptoms: Fever, vomiting, headaches. Global population at risk: 3.2 billion Important facts: WHO information says there were 212 million new cases and 429,000 deaths in 2015, most (88 per cent) in Africa. The number of people with malaria fell by 37 per cent since 2000. Death from malaria in populations at risk fell by 60 per cent, because they are using insecticidal mosquito nets and more people get antimalarial drugs.


Cause: Dengue Virus (4 different types) Vector: Aedes aegypti, Aedes albopictus Symptoms: very bad headaches, pain in muscles and joints, fever and, sometimes, internal bleeding Global population at risk: 3.9 billion Important facts: a recent estimate said there are 390 million Dengue infections per year. If there is good medical care, less than 1 per cent of these die. A lot more people have got Dengue in recent decades and it is also spreading to new areas. There is no specific treatment for Dengue.


Cause: Zika virus Vector: Aedes aegypti mosquito. Sexual transmission from human to human is possible Symptoms: mild fever, skin rash, conjunctivitis, muscle and joint pain Global population at risk: The 2015/2016 outbreak so far has affected 65 countries in the Americas and Southeast Asia Important facts: Zika was first found in humans in 1952 in Uganda and Tanzania. Since then, there have been several isolated outbreaks. Since the 2015 outbreak in Brazil scientists have concluded that Zika virus infection in pregnancy can cause congenital brain abnormalities, and that Zika virus is a trigger of Guillain-Barré syndrome with mostly reversible paralysis.

Yellow fever

Cause: Yellow fever virus Vector: Aedes and Haemagogus mosquitoes Symptoms: fever, muscle pain with a lot of backache, in serious cases: jaundice, dark urine, vomiting, bleeding Global population at risk: There is yellow fever in 47 countries in Africa and Central-/South America. An estimate shows that in 2013 there were as many as 170 000 serious cases and 29 000–60 000 deaths. Important facts: Many people have light cases and do not have symptoms. But 50 per cent of people die with severe yellow fever. There is a very efficient vaccine against the disease which protects for life.


Cause: Chikungunya virus Vector: Aedes aegypti and Aedes albopictus Symptoms: fever, bad joint pain Global population at risk: There is chikungunya in more than 60 countries in Asia, Africa, Europe and the Americas. Important facts: There is no global reporting system for Chikungunya cases. In the Americas, there were about 730,000 cases in 2015. There have been some cases in Italy and France in recent years. There is no specific medication and no vaccine against Chikungunya. In some cases, joint pain can continue for several months or years.

The blood meal

The mosquitoes and other insects that suck blood take in the microorganisms that produce the disease when they take blood from one human. They then inject them into the next person when they suck blood.

Most of the money to fight against these diseases goes to looking for more vaccines and new medicines. But this is only part of the solution: the new vaccines against malaria and Dengue do not work as well as they expected. The malaria parasite Plasmodium is now resistant to at least one class of drugs in many regions of the world. For Dengue, yellow fever or Zika, there are no specific medications. The most important thing is to stop mosquitoes biting people. Insecticides are still important in this, but are not so useful.

The insecticides of today have been used for decades. They came from fighting insects in agriculture. But transferring technology from farms to public health doesn’t work so well anymore because the two areas need different things. The Head of BASF’s Public Health division explains that in agriculture, pests usually eat the toxin, but mosquitoes must be killed by contact only. On the field, chemicals need to work fast and then break down and leave nothing. But in mosquito control you want the chemicals to stay active for at least six months.

New insecticides against mosquitoes would have to be developed specifically for public health, but they cannot make enough money from them. Developing an insecticide costs about €250 million ($266 million). Public health authorities and NGOs can pay less than €1 billion ($1.06 billion) per year, but farmers spend about €45 billion ($48 billion) a year on agricultural pesticides.

So scientists from the Liverpool School of Tropical Medicine started the International Vector Control Consortium (IVCC) 11 years ago. They get money from foundations and development aid organizations and they work with six large agrochemical companies to find new active ingredients. There has been some progress but nothing very new.


A field worker from Ifakara Health Institute in Tanzania identifies mosquitoes. © Eliza Powell

They need new larvicides. They have found indigenous plants that kill larvae. But none of these are being developed by transnational companies or by local entrepreneurs. Why? There are controversial opinions about it.


Dead mosquitoes. © Camille Kirnidis

Rajander Sharma, who was director of the Indian National vector-borne disease control programme, says it’s not enough to just show how well the plant extract works. They also need to show effects on other organisms in the water and on humans. Also, a larvicide must be effective at least for seven days – and botanical pesticides never are. So nature is a good model, but only a synthetically produced natural compound may be good enough to produce.

Marcello Nicoletti, Professor of Pharmaceutical Biology at the University of Rome (working with scientists from Southern India on indigenous plants) doesn’t agree. He says that one leaf can have 400 active components. So mosquitoes might take longer to develop resistance against them. But this kind of work takes much longer than two to three years; and scientific projects don’t usually get money for longer than this. So the researchers, often PhD students, move to something different.


Dead mosquitoes. © Camille Kirnidis

Nicoletti, together with about 60 Italian and international scientists, has studied Neem for more than 15 years. But this project is now ending. They need to make a product that will make money. But they can’t find sponsors or companies that will provide the money. Nicoletti now wants to try one more time. He has asked a private foundation for €1 million ($1.06 million). He says that’s enough to make a product ready for the market. If this fails, he will give up, and all the information he has will stay in a file on his shelf.

The European Journalism Centre (EJC) has supported this project with its Innovation in Development Reporting Grant Programme (, run with financial support from the Bill & Melinda Gates Foundation.