Mosquitoes are the main vectors for many infectious human diseases such as malaria. During a ‘blood feed’ a female mosquito can infect multiple hosts, greatly accelerating disease propagation. The findings of a new study, published the 3rd of October in Nature Communications suggest the potential for an interesting way forward: Change human ‘flavor’ to be less appealing to mosquitoes. If this could eventually be implicated it could save over 430 000 lives a year.
It is known that mosquitoes use olfactory cues to locate humans and it is thought that they use olfactory cues to distinguish humans from other animals, with some mosquitoes such as A. Gambiae (the main carrier of the parasite that causes malaria), preferring to feed on humans. Olfactory cues include CO2 output from expiration and the smell of sweat. It is on these findings that is based the principle of ‘insect repellants’ that somewhat disguise the human smell.
However despite a certain understanding of the olfactory cues used by mosquitoes to locate their food there was little understanding of the actual mechanisms behind the olfactory system.
Malaria is a disease caused by infection by the plasmodium parasite. Many diverse symptoms are associated with the disease and an infection left without adequate medication can be fatal. In 2015, 95 countries had ongoing malaria transmission.
Despite an 18% decline in the number of malaria infections since 2000, over 214 million people were infected in 2015. Thanks to progress regarding malaria treatement there has been a 48% reduction in the number of deaths associated with malaria since 2000. However despite this 438 000 people died of a malarial infection in 2015.
Currently there is no vaccine against malaria, but antimalarial treatments, when available do greatly reduce the risk of becoming ill. (up to 90% less). However antimalarial treatments do not prevent infection with the malaria parasite, rather, they suppress the symptoms of the infection by killing the parasites either in the liver or as they leave the liver and enter the bloodstream.
Thus in addition to antimalarial medication, other physical prevention must be used. This includes using insecticide treated mosquito nets (ITNs) and the use of anti-mosquito sprays in both living areas and directly on the skin.
Most of the individuals affected by malaria in the world will not have access to regular antimalarial drugs.
The study focused on establishing a better understanding of the sensory signalling system of the mosquito brain. To do this they developed transgenic generations of Anopheles gambiae (one of the mosquito species known to carry malaria) in which the olfactory receptor neurons (neurons associated with ‘smell’) (expressing the co-receptor Orco gene) were labelled with GFP (Green Fluorescent Protein) thus making them ‘glow’ green.
Mapping the presence of CFP allowed the research team to identify the mechanics of mosquito olfactory and gustatory (associated with ‘taste’) signals. These appear to run from the antennae to the antennal lobe (area of the brain responsible for ‘smells’) and from the labella on the proboscis to the subesophageal zone (area of the brain responsible for ‘tastes’) or SEZ. Such findings suggest that integration of both the olfactory and gustatory signals occur in the brain region.
It is known that on the extremity of the labbellum there are two small areas that have both gustatory neurons, for picking up tastes, and olfactory neurons, for picking up smells. However this study, for the first time, took a look at where these neurons worked within the mosquito brain.
It was once thought that male mosquitoes, relying only on nectar to feed, had a rudimentary sense of smell compared to that of female mosquitoes who need to identify blood sources. This study proved however that both male and female mosquitoes have the same level of complexity in the area of the brain dedicated to smell. Males merely appear to have a less powerful sense of smell. Ie. they can smell just as well as females, but not as far
It seems that both of these neurons work within the SEZ. Thus it suggests that the SEZ could be a region of the brain that combines both olfactory and gustatory signals.
Combining these two signals is how humans perceive ‘flavor’.
So it could be thought that mosquitos also perceive flavours.
In which case this a new area that, if exploited, could prove invaluable in the fight against malaria. If researchers could find a way to modify human ‘flavor’, then the mosquitoes would perhaps no longer find us as such an attractive target.
The idea is said to be highly interesting but “remains to be explored” in more depth.
Such possible perception of ‘flavour’ had already been observed in the Hawk Moth, a tool thought to be used to evaluate the quality of flowering plants.
Riabinina, O. Task, D. Marr, E. Lin, C. C. Alford, R. O’Brochta. D. A. and Potter, C. J. (2016) ‘Organization of olfactory centres in the malaria mosquito Anopheles gambiae’ Nature Communications 10.1038/ncomms13010
Find the full article here: http://www.nature.com/articles/ncomms13010
World Health Organisation (2015) ‘World Malaria Report’ Find the full report here: http://www.who.int/malaria/publications/world-malaria-report-2015/report/en/
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