The Science Behind Why Mosquitoes Bite Certain People More Than Others

We’re all probably familiar with the itchy red bumps that develop after we’re bitten by mosquitoes. Most of the time, they’re a minor annoyance that goes away over time.

Mosquito bites are annoying, unpleasant, sometimes painful, often the cause of allergic reactions – but also potentially deadly. These insects kill more people than any other animal on Earth.

But do you ever feel like mosquitoes bite you more than other people? There may be a scientific reason for that!

Mosquitoes transmit pathogens that cause serious tropical diseases such as malaria, dengue, Zika, West Nile virus and Japanese encephalitis. Global warming is causing mosquito habitats to expand northward, threatening an increased mosquito-borne disease (MBD) burden in the future.

People who strongly attract mosquitoes are obviously at higher risk for MBD. What is the science behind the differences in mosquito-human attractiveness?

The Mosquito’s Sense Of Smell:

Mosquitoes track down potential hosts with visual cues as well as with combination of odorants called “kairomones”. The final short-range phase of the mosquito’s fhuman- sensing flight is directed by the host’s body heat. Heat and odor suffice for the versatile mosquito if CO2 sensing is impaired.

Mosquitoes have multiple ‘noses’ or olfactory organs – the antennae, the proboscis, and the maxillary palps. These are rich in olfactory receptors. The signals are carried back to olfactory sensory neurons (OSNs). Mosquitoes have up to 80 types of OSNs, depending on the species.

OSN axons that carry the same signals, irrespective of location, end in the same glomerulus, an organ in the brain mapping the odor source. These signals are processed to tell the mosquito where the host is.

Kairomones are chemical signals that attract one species to another for the benefit of the former. They play a crucial role in long-range mosquito-human attraction. Kairomones are detected by three types of olfactory or smell receptors: the odorant, gustatory, and ionotropic receptors (OR, GR, and IR, respectively).

The human scent is made up of hundreds of volatile substances forming individual signatures. These include carbon dioxide (CO2), lactic acid, ammonia, organic fatty acids, ketones, aldehydes, alcohols, and esters. Combinations of human kairomones are stronger than single attractants.

In many animal species, each OSN detects only one type of odor molecule, but a recent study in the mosquito species Aedes aegypti revealed that a single OSN tracks multiple olfactory receptors. This may explain how they sense the right blend of odorants that signals ‘humanness.’

Mosquitoes sense CO2 in exhaled air from about 30 feet, using GRs on cpA neurons, making it the main long-range attractant. Body odors and CO2 are sensed by ORs on cpB and cpC neurons via the olfactory receptor co-receptor (Orco).

The antennal receptors selectively recognize human odors in the presence of CO2. Deficiencies in the IR coreceptor genes Ir8a, Ir25a, or Ir76b weaken the response to human odors while preserving the preference for the strong attractors.

Genetic Factor:

Mosquito-human attraction depends on body odor intensity and composition. The human scent depends on the number of skin glands, the skin pH, the metabolic rate, body mass, and the rate and intensity of respiration, as well as the skin microbiota.

Mosquito attractiveness is thus determined by genetic, dietary, and environmental factors.

The HLA system in humans encodes human scent kairomones like sulcatone, geranylacetone, decanal, undecanal, 2-methylbutanoic acid, tetradecanoic acid and octanal. Interestingly, sulcatone can become a repellent or masking chemical at high concentrations. Multiple other gene variants could result in different combinations and concentrations of kairomones.

Additionally, some alleles or gene variants affect the risk of mosquito-borne infection, of symptomatic disease, and thus the risk of transmission as well. West Nile virus-linked symptomatic infection is more likely in people bearing the rs333 gene variant, while symptomatic dengue is more common among those with the TNF-α-308G/A SNP. Japanese encephalitis risk is higher with the ICAM-1 K469E gene variant.

Environmental Factors:

Mosquito-human attractions change with age, body size, and physiological changes, as well as the distance from the host. Bananas and perhaps alcohol may enhance mosquito attractiveness, but not garlic or vitamin B supplements.

Pregnant women attract more mosquitoes, due to the greater body heat and metabolic rate, and perhaps a distinctive scent.

Malaria also attracts mosquitoes due to a distinct “malaria smell” enriched for aldehydes and thioethers, during the gametocyte stage of Plasmodium which can develop further only in the mosquito host.

Mosquito-borne Diseases

Female mosquitoes bite warm-blooded mammals to obtain protein for egg-laying. During these bites, they suck up pathogens and become infected, to later deposit them in the host tissues, transmitting the infection to other hosts.

MBDs contribute about 17% of infectious diseases worldwide, associated with encephalitis, kidney failure, meningitis, and fetal defects if acquired during pregnancy. About 50% of the world’s population is exposed to malaria and 40% to dengue.

Anopheline mosquitoes carry Plasmodium, the malarial parasite that causes over 200 million cases of malaria a year and over 400,000 deaths, mostly in sub-Saharan Africa. Dengue causes 400 million infections and 96 million cases annually.

Zika virus infection in pregnancy is associated with a rise in congenital disabilities of the central nervous system as well as Guillain-Barre syndrome. West Nile virus is the leading domestic insect-borne neuroinvasive virus in the mainland USA.

Culicine mosquitoes such as Ae. Aegypti carry dengue and Zika virus, yellow fever, and chikungunya viruses. The impact of such diseases includes acute and chronic illness, death, loss of economic opportunities, and huge healthcare expenditures.

Two mosquito bites at least are necessary for malaria transmission, first to suck in the infectious gametocyte stage of Plasmodium, and then to inject it at the sporozoite stage (about two weeks later) into a new host.

Conclusion

People who attract mosquitoes have specific scents with high carboxylic acid content. They could probably be identifiable by chemical or genetic testing. People with MBDs are also at higher risk and should also receive targeted advice on how to avoid mosquito bites. Personal protection and environmental hygiene remain key measures.

Culled from:https://www.news-medical.net/health