MALARIA: Smelly chemicals confuse mosquitoes

1 June 2011 By James Gallagher

 Female mosquitoes follow a trail of carbon dioxide
Chemicals which interfere with a mosquito's ability to sniff out humans have been developed by US scientists, according to research in Nature.

It is hoped they could be used to develop the next generation of mosquito traps and repellents.
A UK expert said the discovery could be a "major step forward" if the chemicals were safe and cheap.
Blood sucking female mosquitoes use carbon dioxide in exhaled breath to track down humans.
They can detect minute changes in the concentration of the gas.
This knowledge is already used in carbon dioxide traps, but requires dry ice or gas cylinders - which mean they are rarely used in developing countries.
Researchers have been looking for chemicals which can disrupt or confuse a mosquito's carbon dioxide sense.

Deception
Scientists at the University of California, Riverside, tested smelly chemicals on three species of mosquito: Anopheles gambiae, which spreads malaria; Culex quinquefasciatus, which spreads filariasis and West Nile virus; and Aedes aegypti which spreads dengue and yellow fever.
The researchers say that these insects combined infect half a billion people each year, some in at risk countries will be infected multiple times in their lifetime, and cause millions of deaths.
The researchers identified three groups of chemicals, which disrupt a mosquito's carbon dioxide receptors.
One mimicked carbon dioxide and could be used as bait in insect traps, another prevented the mosquito from detecting carbon dioxide and the last group tricked the mosquito's brain into thinking it was surrounded by huge quantities of the gas - so it could not pick which way to go.

 

Anopheles gambiae is responsible for the vast majority of malaria cases in Africa Professor Anandasankar Ray, from the University of California, Riverside, said: "These chemicals offer powerful advantages as potential tools for reducing mosquito-human contact, and can lead to the development of new generations of insect repellents and lures.

"The identification of such odour molecules, which can work even at low concentrations, and are therefore economical, could be enormously effective in compromising the ability of mosquitoes to seek humans, thus helping control the spread of mosquito-borne diseases."
Carbon dioxide is not the only way mosquitoes can find their dinner however, as the smell of human sweat and skin can also be used.
Dr James Logan, from the London School of Hygiene and Tropical Medicine, said: "Whilst this is an exciting study, the authors are yet to show that the chemicals are capable of protecting a human being from being bitten.
"Although carbon dioxide is an important cue for mosquitoes, we know that mosquitoes respond differently to a trap releasing carbon dioxide than to a real human being, which releases a complex mixture of many attractive chemicals, heat, visual cues and moisture.
"The key question is - do the 'response modifying odours' actually protect a human being?"
The chemicals also need to be used at high concentrations, which could be hazardous to human health. The researchers say their next step is to develop safer chemicals.
Dr Nikolai Windbichler, from Imperial College London, said work needed to be done to ensure they were safe and could be produced at low cost.
He added: "These compounds have novel and desirable properties because they can confuse the mosquitoes' host seeking behaviour even when the substances are no longer present or the mosquitoes have left the area of application.
"This, if realised, could be a major step forward and could protect large groups of people or large areas, something that is not currently feasible with existing repellents."
Mark Stopfer, from the US National Institutes of Health, said the study offered "a promising line of defence."
http://www.bbc.co.uk/news/health-13614781

MALARIA: New Mosquito Subgroup is Highly Susceptible to Malaria-Causing Parasites

Most studies of malaria vectors, such as Anopheles gambiae, have focused on mosquitoes that rest indoors, since they are more likely to have contact with humans and transmit the malaria parasite. Larval stage and outdoor-resting mosquitoes are not as well-studied because they are more difficult to find and collect.
Differences Between Outdoor- and Indoor-Resting Mosquitoes
Because outdoor- and indoor-resting mosquito subgroups live in such separate environments, they do not interbreed and have grown to differ genetically. For example, multiple recent studies have documented the beginnings of speciation between A. gambiae subgroups M and S, but this emerging speciation is taking place only among indoor-resting mosquitoes. In an NIAID-funded study published in February 2010 in the journal Science, researchers collected and genetically analyzed A. gambiae larvae from three village sites in Burkina Faso. The investigators were led by Kenneth Vernick, Ph.D., of the Institut Pasteur in Paris.

What These Differences Could Mean for Malaria Control
Analyzing the genomes of the larvae, they found two A. gambiae subgroups. One was genetically indistinguishable from indoor-resting adult mosquitoes in the Burkina Faso area, but the other did not match any subgroup of adult mosquitoes that had been described so far. The researchers found that mosquitoes belonging to this second group, named GOUNDRY after the area in which they were found, were much more susceptible to the parasite that causes malaria; the prevalence of Plasmodium falciparum infection among GOUNDRY mosquitoes was 58 percent, compared to 35 percent among the indoor-resting mosquitoes.
Although there is no evidence of GOUNDRY mosquitoes biting humans, any human feeding by these mosquitoes is of concern. Also, it should be noted that current malaria control measures such as bed nets and insecticide spraying focus on indoor-resting mosquitoes and are less effective against outdoor-resting subgroups.
http://www.niaid.nih.gov/topics/Malaria/research/Pages/newMosquitoSubgroup.aspx

MALARIA: Survival prospects boosted for antimalarial mosquitoes

Barbara Axt : 21 April 2011
 Finding a way to allow GM mosquitoes to thrive in wild populations has proven difficult: Wellcome Images

A major stumbling block to using GM mosquitoes engineered to stop transmission of malaria may have been solved with a new genetic technique to ensure that they survive and propagate in natural environments.
The first genetically modified (GM) mosquito was produced in 2000 by scientists at Imperial College London, United Kingdom. Subsequent studies have shown that such modifications could be used to create mosquitoes with a reduced ability to transmit the deadly Plasmodium parasite responsible for malaria.
But finding a way to allow GM mosquitoes to thrive in wild populations has proven difficult. Without a suitable mechanism to ensure their survival, the mosquitoes would simply be out-competed by their native counterparts and die out, together with their disease-proof genes.
"Up to now — almost ten years later — no-one has really found out how to do that," Marcelo Jacobs-Lorena, a professor at the John Hopkins Malaria Institute, United States, told SciDev.Net last year.
Now, scientists at Imperial College have demonstrated a method to spread the beneficial genes through large populations, starting with just a small number of GM mosquitoes. The laboratory-based study was published in Nature yesterday (20 April).
The team bred Anopheles gambiae mosquitoes, the most important carriers of malaria, to contain a green fluorescence gene that makes them glow in the dark.
They then introduced the homing endonuclease gene (HEG) — which makes a copy of itself, ensuring all offspring end up with a copy of it as well — into around one per cent of the population.
The HEG gene, which is found in fungi, plants and bacteria, was designed to replace the glow-in-the-dark genes so that, if it spread through the population over time, less and less mosquitoes in each subsequent generation would glow in the dark.
The researchers found that in just 12 generations, the gene had spread through half of the population.
"We believe [that] in three to four years we will be able to apply this technique in native mosquito populations," Andrea Crisanti, lead researcher and a professor at Imperial College London, United Kingdom, told SciDev.Net.
Crisanti said the next step is to engineer the HEG to displace mosquito genes important for the transmission of malaria, as well as assessing the safety of the technique in human and animal populations.
The researchers plan to perform the first tests in two African countries, to be selected from a list of six that are being assessed at the moment.
"We need countries that not only have a serious malaria problem, but also clear safety and environmental legislation concerning GM organisms, and where we can work with a work with local team of scientists," said Crisanti.
Safety tests with larger populations of mosquitoes in more realistic environmental conditions will also take place, at the new EU INFRAVEC Mosquito Confined Release Facility, in Italy.
"This paper is a significant step forward," said Jacobs-Lorena. "For ten years we've known it is possible to engineer mosquitoes to make them poor transmitters of the disease, but we needed to give them some advantage over the native population. This research is a proof of principle demonstration in this direction."
http://www.scidev.net/en/news/survival-prospects-boosted-for-antimalarial-mosquitoes-.html

MALARIA: New mosquito type could undermine malaria control

Joanna Carpenter : 4 February 2011

 Bednet use could drive mosquitoes to increasingly bite humans out of doors
Flickr/Georgina Goodwin/Vestergaard Frandsen

Scientists have discovered a new mosquito sub-type that could become a key malaria transmitter out of doors, thus avoiding the mainly indoor control measures, according to a paper published in Science 4th February.
Malaria rates have decreased in many African countries following the widespread use of insecticide-treated bednets and indoor pesticide spraying. But research in Burkina Faso now suggests that up to half of mosquitoes may never enter households and therefore cannot be controlled by nets or sprays.
The finding comes at a time when other researchers are expressing fears that increasing bednet use could drive those mosquitoes that bite indoors into outdoor biting behaviours.
In the Burkina Faso research, scientists took mosquito larvae from ponds near houses and used genetic analysis to identify a recently evolved, sizeable genetic subgroup of Anopheles gambiae s.s. — the most effective transmitter of malaria across Africa. This group has not been detected before, indoors or outdoors.
They found lab-grown adults of this subgroup were highly susceptible to infection by the malaria parasite.
The group does not yet know whether these mosquitoes bite humans: "We're trying to catch them outdoors to see whether we can establish the extent of human feeding," co-author Michelle Riehle, a researcher at the University of Minnesota in the United States, told SciDev.Net.
The research adds to a growing literature that suggests current mosquito control measures may be inadequate. For example, George Christophides, reader in infection and immunity at UK-based Imperial College London, recently co-authored two papers in Science suggesting that the indoor-resting strain of A. gambiae s.s. is diverging into two separate species.
He told SciDev.Net: "Malaria mosquitoes are evolving fast … possibly due to man-made pressures. We know this is happening with insecticides and it may happen with bednets. The more bednets we apply in Africa, the more we may push the mosquitoes to bite outdoors."
Steve Lindsay, professor of public health entomology at the UK's London School of Hygiene and Tropical Medicine, said: "We have very effective controls against A. gambiae s.s., but they're all directed against mosquitoes coming indoors. We're not very good at controlling outdoor biting ... Compliance with repellents is a problem."
Christophides said, "It's very likely we will need a suite of methods depending on what type of mosquitoes we find in each place. Maybe bednets will be functional in one place but not in another."
But transmission-blocking interventions [which interrupt the life cycle of the parasite, which passes from mosquito to human and back again] could be "a universal solution to stop malaria transmission", he said.
Transmission-blocking vaccines or drugs would be given to people to ensure that a mosquito that had bitten a treated person would no longer be able to transmit the disease.
http://www.scidev.net/en/news/new-mosquito-type-could-undermine-malaria-control-1.html

MALARIA: New mosquito type raises concern

3 February 2011 : By Jonathan Amos : Science correspondent, BBC News

 Anopheles gambiae is responsible for the vast majority of malaria cases in Africa

 

Scientists have identified a new class of mosquito.

It is a subtype of Anopheles gambiae, the insect species responsible for most of the malaria transmission in Africa.
Researchers tell Science magazine that this new mosquito appears to be very susceptible to the parasite that causes the disease - which raises concern.
The type may have evaded classification until now because it lives away from human dwellings where most scientific collections tend to be made.
Michelle Riehle, from the Pasteur Institute in Paris, and colleagues made their discovery in Burkina Faso, where they gathered mosquitoes from ponds near villages over a period of four years.
When they examined these insects in the lab, they found many to be genetically distinct from any A. gambiae insects previously recorded.
The team grew generations of the unique subtype in the lab to assess their susceptibility to the malaria parasite and revealed them to be especially vulnerable, more so than indoor insect types.
But the scientists caution that these mosquitoes' significance for malaria transmission is not yet established.
Team-member Dr Ken Vernick, from the Pasteur Institute in Paris, said further study was needed: "We are in a zone where we need to do some footwork in the field to identify a means to capture the wild adults of the outdoor resting sub-group; so then we can test them and measure the level of infection with malaria, and then we can put a number on how much - if any - of the actual malaria transmission this outdoor resting subgroup is responsible for."
The researchers tell Science magazine the new subgroup could be quite a recent development in mosquito evolution and urge further study to understand better its consequences for malaria control.

 Larvae are collected from pools of water for study

 

Commenting on the study, Dr Gareth Lycett, a malaria researcher from the Liverpool School of Tropical Medicine in the UK, said it was an interesting advance that might have important implications for tackling malaria.

"To control malaria in an area you need to know what mosquitoes are passing on the disease in that district, and to do that you need sampling methods that record all significant disease vectors," he told BBC News.
"You need to determine what they feed on, when and where, and whether they are infectious. And where non-house-resting mosquitoes are contributing to disease transmission, devise effective control methods that will complement bed-net usage and house spraying. A recent 12m-euro multinational project (AvecNET), funded by the European Union, and led by the Liverpool School of Tropical Medicine has the specific aims of doing just this."
According to the World Health Organization (WHO), there are more than 200 million cases of malaria worldwide each year, resulting in hundreds of thousands of deaths, most of them in Africa.
Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected female Anopheles mosquitoes
http://www.bbc.co.uk/news/science-environment-12352565

MALARIA: emerging speciation between two forms of Anopheles gambiae mosquitoes

A pair of studies conducted by NIAID grantees at the University of Notre Dame recently found evidence of emerging speciation between two forms of Anopheles gambiae mosquitoes, M and S. NIAID-funded researchers examined the relationship between M and S and the TEP1 gene in a new study published last month.
In a genome-wide comparison of the emerging species, researchers found one allele of TEP1 that exists almost exclusively in M and not in S mosquitoes, even in areas where M and S co-exist, and showed that this allele confers resistance to rodent and human malaria parasites. Although the resistance conferred by TEP1 variants is not specific to malaria parasites, and probably did not evolve in response to malaria infection in the adult mosquito, it may still affect malaria transmission and control.
http://www.niaid.nih.gov/topics/Malaria/research/Pages/anophelesGambiae.aspx.

MALARIA: Evolutionary forces on Anopheles: what makes a malaria vector?

Cohuet, A., et al.Institut de Recherche pour le Développement, Montpellier, France
In human malaria, transmission intensity is highly dependent on the vectorial capacity and competence of local mosquitoes. Most mosquitoes are dead ends for the parasite, and only limited ranges of Anopheles are able to transmit Plasmodium to humans. Research to understand the determinants of vectorial capacity and competence has greatly progressed in recent years; however, some aspects have been overlooked and the evolutionary pressures that affect them often neglected. Here, we review key factors of vectorial capacity and competence in Anopheles, with a particular focus on the most important malaria vector Anopheles gambiae. We aim to point out selection pressures exerted by Plasmodium on Anopheles to improve its own transmission and discuss how the parasite might shape the vector to its benefit. © 2009 Elsevier Ltd. All rights reserved.
http://www.scopus.com/record/display.url?eid=2-s2.0-77049109244&origin=inward&txGid=F43pX9mfrGMCwIyvjybA0R1%3a12