Please could you give a brief introduction to the types of malaria-transmitting mosquitoes that were previously known about?
In the area in which we carried out the study, which was in the western Kenyan highlands, the main mosquitoes that have been confirmed to transmit malaria are the females of Anopheles gambiae sensu stricto, Anopheles arabiensis and Anopheles funestus.
Where was the new type of mosquito found?
We carried out the study where we found the mosquito in a small village in Kisii Central in the highlands of western Kenya over a period of 4 months. This area has low transmission of malaria, but is prone to epidemics. As yet, we are not sure how widely distributed this mosquito may be. The mosquitoes we caught which are of the Anopheles genus differ to Anopheles gambiae s.s., Anopheles arabiensis and Anopheles funestus but as yet we do not know if this is a mosquito that has previously been described but never implicated in malaria transmission or whether it is a new species altogether.
How was this new type of malaria-transmitting mosquito discovered and what is different about it?
We had been carrying out some collections of mosquitoes within the area as part of our larger project. The manual that we use to identify the species, which is the standard manual used in sub-Saharan Africa, is a little outdated. It dates back to the 80’s.
We were finding it quite difficult to actually definitively identify these species. They had subtle differences in their physical appearance compared with the known malaria-transmitting mosquitoes from the area.
So we then sent these samples to the University of Notre Dame in the United States to do some more detailed molecular analysis looking at the DNA. The DNA of these mosquitoes was shown to be different to Anopheles gambiae s.s., Anopheles arabiensis and Anopheles funestus mosquitoes from the area, i.e. those that had been implicated in malaria-transmission before.
Why does this new species of mosquito pose a threat to humans?
I think the term threat at the moment is probably a bit strong. What I would say is that it raises our awareness to mosquitoes that have different behaviors.
We set some traps indoors and outdoors to catch these mosquitoes. Interestingly, 80% of the mosquitoes that we caught of these “unidentified species” were caught outdoors, and of those, 70% were caught before 10.30pm.
We carried out some questionnaires in the area about what time people entered their houses and what time they would go underneath a bed net. The results indicated that the majority of people would actually not yet be underneath their bed net before 10.30 at night.
This means that there is the potential for people to be bitten by these mosquitoes prior to being underneath a bed net.
What impact do you think this discovery will have on current malaria control techniques, such as insecticides in homes and bed nets? And what control techniques are necessary to protect people from the threat of the new type of mosquito?
We have no evidence from the study area of an increase in malaria cases. At the moment the use of insecticide-treated bed nets and spraying the insides of houses with insecticides is still extremely effective against malaria. We still advocate universal access and use of treated bed nets.
However, not only our study, but other studies from East and West Africa, have shown that some malaria-transmitting mosquitoes are displaying outdoor behavior, i.e. they are resting or they are feeding outdoors. So, if outdoor transmission in shown to be of significance, it may be advisable to use a mix of interventions. These combinations could include the proven indoor methods combined with for example, measures such as treatment of water bodies where mosquitoes breed, or a more logistically feasible method may be to target the parasite rather than the mosquito that transmits the disease. Intermittent treatment of pregnant women (IPTp) with anti malarial drugs is policy in Kenya whereby pregnant women who attend antenatal clinics are given a dose of prophylactic treatment against malaria. This would prevent them developing malaria during the course of their pregnancy. More recently, there have been advances made in the development of a malaria vaccine. By targeting the parasite, indoor and outdoor transmission may be interrupted.
Are there plans in place to implement these control techniques?
The application of safe chemicals to water bodies to kill immature stages of mosquitoes, which is called larviciding, has shown some success in areas of East Africa where water bodies are easy to locate and treat. It is, however, very labor intensive and is, as yet, not on the agenda for wide-scale roll out in Kenya.
Although there have been some promising results on the development of malaria vaccines, it is still some way off before it may be licensed and rolled out.
Intermittent preventative treatment for pregnant women was first rolled out in 1998, but coverage and uptake since then has been low unfortunately.
There are now other studies on regularly testing and treating sectors of the community that might have the highest infection rate. These include the regular screening and treatment of children at schools, or communities that are at a higher risk of malaria. Again, these are very much in the research stage at the moment.
Would you like to make any further comments?
I think at this stage it is important to emphasize that the research that we’ve carried out was very much preliminary and what we would like to do now is see how widespread this mosquito might be. We want to confirm where else within other districts of the highlands, we may find this mosquito. Is it still showing outdoor behavior in these areas? And are we finding malaria parasites within the mosquito?
At this point in time we don’t know whether this is a really novel species or whether it is a species that has been described before but has never been associated with malaria transmission. So there is still quite some work to do. We are looking for funding to try and be able to carry out molecular analyses on all the mosquitoes that we’ve caught over the past 2 years to really examine the spatial distribution of these mosquitoes.
I also want to acknowledge the consortium we fall under, the Malaria Transmission Consortium, the collaborators we have been working with, the Kenyan Medical Research Institute, and our funders, the Bill and Melinda Gates Foundation.
Where can readers find more information?
Our paper is available here: http://wwwnc.cdc.gov/eid/article/18/9/12-0283_article.htm
They can also find more information on the London School of Hygiene and Tropical Medicine here: http://www.lshtm.ac.uk/
About Jennifer Stevenson
Jennifer Stevenson is Research fellow at the London School of Hygiene and Tropical Medicine, specializing in entomology. She is based in Kenya working with the Kenya Medical Research Institute, Kisumu on the Malaria Transmission Consortium, a Bill and Melinda Gates Foundation. She completed her Masters and PhD at LSHTM in control of disease vectors.