Of the nearly 3,000 species of mosquito, a handful are choosy and will only target people for their blood. Some of these, for instance, Aedes Aegypti, Anopheles and Culex are well known as vectors for diseases such as dengue, malaria and West Nile fever. How these species are specifically able to detect humans — and often from as many as 50 metres away — has intrigued scientists.
Principle of obfuscating
A science question apart, understanding these behavioural patterns of the insect influences methods to trap and eliminate them. Mosquito repellents work on the principle of obfuscating the insect’s sense of smell. The most common chemical in repellents, diethyltoluamide (DEET) for instance, confuses the mosquito’s antennae that’s sensitive to sweat and carbon dioxide from the human body.
However, this and several other ingredients such as picaridin, IR3535 and eucalyptus oil are not foolproof. The reason for this, a study this month in the journal Cell reports, is that mosquitoes have evolved resilient back-ups in their olfactory system that make sure they can always smell our scents.
“Mosquitoes are breaking all of our favourite rules of how animals smell things,” Margo Herre, a scientist at Rockefeller University and one of the lead authors of the paper, said in a statement.
Olfactory neurons, that are designed to transmit sensations of smell to receptors in the brain, are only responsible for detecting one type of odour in animals. Thus, if a single odorant receptor is damaged in a person, all of the neurons that express that receptor will lose the ability to sense that smell. However, the researcher-team found that mosquito’s olfaction didn’t work like that.
“You need to work harder to break mosquitoes because getting rid of a single receptor has no effect,” said co-author Leslie Vosshall of the Howard Hughes Medical Institute, Maryland, in a statement. “Any future attempts to control mosquitoes by repellents or anything else has to take into account how unbreakable their attraction is to us.”
Detection by neurons
1-octen-3-ol, is a chemical secreted in human sweat that can be detected only by specific set of neurons in people and mosquitoes. In these insects, however, these neurons are also stimulated by amines, another type of chemical mosquitoes use to look for humans. This is unusual since according to all existing rules of how animals smell, neurons encode odour with narrow specificity, suggesting that 1-octen-3-ol neurons should not detect amines.
Thus, the neurons for detecting humans through 1-octen-3-ol and amine receptors were not separate populations and could allow all human-related odours to activate the part of the mosquito brain that senses the presence of humans even if some of the receptors were lost, acting as a fail-safe.
Carolyn McBride, assistant professor of ecology and evolutionary biology at Princeton University, works at deciphering how animal brains interpret complex aromas. Human odour consisted of more than 100 different compounds and those same compounds, in slightly different ratios, are present in most mammals.
Testing by guinea pigs
To test the combinations mosquitoes prefer, McBride’s lab used guinea pigs, small mammals with a different blend of many of the same 100 odour compounds of humans.
Researchers collected their odour by blowing air over their bodies, and they then presented mosquitoes with a choice between the smell of guinea pig and from a human arm. Human-specialised “domestic” A. aegypti mosquitoes will go toward the arm 90-95% of the time, said McBride.
A related study in her lab on human volunteers found that some people are more attractive to the insects than others though each human was more similar to each other than to the guinea pigs.
“There’s nothing really unique about any animal odour,” said McBride in a statement “There’s no one compound that characterises a guinea pig species. To recognise a species, you have to recognise blends.”
The McBride lab aims to collect hair, fur, feather and wool samples from 50 animal species, extract odour and analyse them by breaking down the chemical composition of each odour and the difference in proportions of their constituent odours. These values could be fed into a computational model to understand how exactly mosquitoes may have evolved to distinguish humans from non-human animals.
- A science question apart, understanding these behavioural patterns of the insect influences methods to trap and eliminate them.
- Olfactory neurons, that are designed to transmit sensations of smell to receptors in the brain, are only responsible for detecting one type of odour in animals.
- Carolyn McBride, assistant professor of ecology and evolutionary biology at Princeton University, works at deciphering how animal brains interpret complex aromas.