Mosquitoes find people by combining carbon dioxide, body heat, infrared radiation and skin chemistry. Here is what peer-reviewed research shows about each cue.
Carbon dioxide starts the search
A mosquito on the hunt begins with a plume of carbon dioxide. Every breath we exhale releases it, and it travels downwind far enough to warn a female that a warm-blooded host is nearby. Sorrells and colleagues, writing in eLife in 2022, showed something important about that first signal. Working with Aedes aegypti, they delivered a single five-second pulse of carbon dioxide and watched the behaviour afterwards. Females stayed in a heightened hunting state for more than ten minutes, becoming far more responsive to body heat. Some would feed on blood meal substitutes up to fourteen minutes after the original exposure. The state appeared only in unfed females seeking blood; recently fed females and males did not show it. Carbon dioxide, in other words, does not merely attract. It switches the mosquito into a persistent search mode.
Body heat and infrared at close range
Once a mosquito is near, heat becomes a usable compass. Chandel and colleagues, writing in Nature in 2024, showed that female Aedes aegypti sense thermal infrared radiation coming off human skin. The authors describe infrared as an outstanding mid-range directional cue that works alongside other senses rather than alone. Females preferred an infrared source up to about 70 cm away; beyond that, preference fell sharply, reaching a preference index of 0.15 at 80 cm with none at 90 cm. Adding carbon dioxide, human odour and infrared at 34 degrees Celsius doubled host-seeking behaviour. The host-seeking index rose from 6.50% to 12.93%. Attraction peaked at 34 degrees Celsius, human skin temperature.
The molecular switch for heat detection
The infrared cue depends on a specific heat-activated channel. Chandel and colleagues state that detecting thermal infrared requires TRPA1, an ion channel expressed in neurons at the tip of the mosquito antenna. Two opsins sit alongside it in those neurons and increase sensitivity when infrared intensity is low. The system behaves like a thermometer tuned to skin. Preference tracked the temperature difference between the target and the surrounding air, so the cue fades as the environment approaches skin temperature. That detail explains why mosquitoes hunt less in the hottest part of a very warm day and why the combination of cues matters. Heat alone rarely completes the journey; it narrows the final approach once carbon dioxide and odour have already brought the insect close.
Skin odour is personal
Different people attract different numbers of bites, and De Obaldia and colleagues examined why. Writing in Cell in 2022, they tested 64 healthy subjects. Each wore nylon sleeves on the forearms for six hours after washing, and the worn fabric went into two-choice olfactometer assays with Aedes aegypti. The team ran more than 2,330 behavioural trials across 174 experimental days. Preferences proved remarkably stable over months and years, and held across two wild-type mosquito strains. The most attractive subject scored four times higher than the next most attractive, and over 100 times higher than the two least attractive. Chemical work linked that ranking to elevated skin carboxylic acids, particularly pentadecanoic, heptadecanoic and nonadecanoic acids.
No single receptor explains attraction
The De Obaldia team also tested whether one receptor pathway controls how attractive a person appears. Mutant mosquitoes lacking the ionotropic receptor co-receptors Ir8a, Ir25a or Ir76b showed severely reduced attraction to human odour overall, yet they still separated highly attractive subjects from weakly attractive ones. Orco mutants kept the wild-type preference ranking. The result means no single receptor pathway accounts for the difference between people. Odour blends, acid levels and other compounds all contribute. For a reader trying to understand personal bite rates, that is useful context. Attractiveness is not one chemical or one gene in the mosquito. It is a composite signal from human skin, and the insect reads it through several parallel systems.
Skin bacteria shape the blend
Skin microbes help produce the chemicals that mosquitoes smell. Verhulst and colleagues published a study in PLoS ONE in 2011 testing this with Anopheles gambiae sensu stricto, a malaria vector. Forty-eight adult male volunteers aged 20 to 64 took part, and emanations collected from their feet were tested against an ammonia standard in olfactometer bioassays. Highly attractive individuals carried significantly higher bacterial abundance but lower diversity than poorly attractive individuals. At a sequencing depth of 1000, phylogenetic diversity was 38% higher in the poorly attractive group. Staphylococcus species were 2.62 times more abundant in the highly attractive group, while Pseudomonas species were 3.11 times more abundant in the poorly attractive group. The authors conclude that skin microbiota composition directly influences host attraction.
Why this matters for prevention
Human-biting preference is partly a heritable trait of mosquito populations, not a fixed feature of every species. McBride and colleagues, writing in Nature in 2014, studied domestic and forest forms of Aedes aegypti in Kenya. Forest females preferred a guinea pig, while domestic females weakly preferred a human and responded about twice as often overall. The authors linked that shift to increased expression and ligand sensitivity of the odorant receptor Or4, which responds to a human odour fraction corresponding to 6-methyl-5-hepten-2-one, known as sulcatone. Seven major Or4 alleles segregated in the studied populations, and the two effects together explained 92% of the variation. This research explains why carbon dioxide, heat and skin chemistry combine into such an effective hunting system, and why physical barriers and correctly specified repellents remain practical protection.
Sources
- Thermal infrared directs host-seeking behaviour in Aedes aegypti mosquitoes: Nature
- Differential mosquito attraction to humans is associated with skin-derived carboxylic acid levels: Cell
- A persistent behavioral state enables sustained predation of humans by mosquitoes: eLife
- Composition of Human Skin Microbiota Affects Attractiveness to Malaria Mosquitoes: PLoS ONE
- Evolution of mosquito preference for humans linked to an odorant receptor: Nature