World Mosquito Day – a Wellcome Perspective
Today is World Mosquito Day and we commemorate the discovery back in 1897 of the link between mosquitoes and malaria. Since this discovery major progress has been achieved in malaria control with insecticide treated bednets, but the growing threat of insecticide resistance threatens these gains. Marta Tufet, International Activities Adviser at the Trust, explains more.
I lay on my bed in a Ugandan hotel at 3am for the fourth night in a row, with the incessant sound of a mosquito buzzing in my ear, clearly not fully satisfied despite having feasted on much of my body already. My sleep-deprived brain kept flashing images of “how to” guides for making bed-net out of hotel curtains juxtaposed with visions of mosquito armies lining up by my bed to take revenge for the years I spent dissecting their relatives at the Imperial College insectary in a stubborn quest to understand the intricate details of malaria transmission.
Suddenly I had an epiphany. This was not some sort of conspiracy between the mosquitoes and the poor hotel manager who, also for the fourth night in a row, attempted to reassure me that no bed-nets were required as rooms were sprayed daily with residual insecticide. What if these mosquitoes were the insecticide-resistant mosquitoes which researchers have long been warning us about…?
From the back of my mind came a more worrying thought: were these mosquitoes carrying malaria?
It is no secret that to avoid getting malaria you should avoid being bitten by mosquitoes carrying the causative parasites. British Doctor Ronald Ross made this first observation in India back in 1897, receiving a Nobel Prize for his discovery and laying the foundations for our relentless fight against this disease. So why haven’t we won the battle if we have known this for over 100 years? Surely bed-nets and insecticides should’ve done the trick by now?
Indeed, these mosquito control measures have been the most effective, as spectacularly demonstrated in the 1940-50s by mass spraying of the insecticide DDT, before it fell into disrepute for its environmental impact and fears for human health. More recently, vector control measures based on pyrethroid insecticides coupled to impressive efforts to provide their universal access have also led to huge reductions in malaria (up to 50%!) in many sub-Saharan African countries, momentarily reviving the hope that one day we will eliminate malaria.
Regrettably insecticide resistance is standing in the way of future hope. The mass scale-up of vector control tools highly reliant on a single class of insecticide has put enormous selection pressure on mosquitoes. They have outsmarted us by developing insecticide resistance, which is now reported in about two thirds of countries with ongoing malaria transmission. Darwinian evolution in real time!
In Tanzania, PhD students, Bilali Kabula, Jovin Kitau and Johnson Matowo, supported through the Wellcome Trust-funded Malaria Capacity Development Consortium (MCDC) and working at the Kilimanjaro Christian Medical Centre (KCMC) have provided some of the supporting evidence of the occurrence of pyrethroid resistance as well as insights into the behavioural responses of the mosquitoes to existing insecticides and repellents. In Cameroon and Benin, Charles Wondji (Wellcome Trust (WT) senior fellow), Jean Rousseau Djouaka (WT intermediate fellow) and Christophe Antonio-Nkondjio (WT training fellow) are also beginning to understand the evolution of resistance to the key insecticides and its geographical distribution.
In response to the increasing accounts of widespread insecticide resistance, the WHO has developed a “Global Plan for Insecticide Resistance Management in Malaria Vectors” with measures to reduce selection of resistance and to incentivise the chemical industry to produce new mosquito control products. But there is no market incentive for the manufacturing industry to develop new insecticides for a disease intimately connected with poverty, and most of the new classes of insecticide being routinely developed are intended for agricultural use and are too toxic or too short-lived to be used in malaria control.
Nonetheless, industry, supported by academic research consortia, has responded with innovative approaches such as nets combining pyrethroids with a chemical synergist that can knock-out the resistance mechanism so that the net continues to protect, or by producing long-lasting non-pyrethoids that can be sprayed on walls to remain effective for over a year.
Mark Rowland from LSHTM (a recipient of a Global Health Trials Initiative award jointly funded by DFID, the MRC and the Wellcome Trust) understands the urgency to deploy this new generation of tools before this growing resistance sets back control and undermines our confidence to eliminate malaria. In collaboration with his African partners at KCMC and National Institute for Medical Research (NIMR), he is therefore preparing to conduct a trial to test different combinations of current and new tools and to determine which are the most effective in controlling resistant mosquitoes.
The work of all these researchers is going to be essential in determining the impact of resistance on control interventions, informing resistance management strategies and deploying new preventative measures. For the sake of the health of millions (and my sanity on sleepless nights in Uganda), I say “Keep up the good work!”
Marta Tufet, Wellcome Trust, International Activities Adviser
The 20th of August commemorates the discovery or the link between mosquitoes and malaria. You can find out more about the work that is being done at the MCDC in Tanzania here.