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What causes fleas, ticks, and mosquitoes to prefer certain individuals?Tirumalai Kamala:
Biting insects like mosquitoes, fleas, flies, ticks, and mites identify their hosts through chemical signals they release. These signals are volatile organic compounds (VOC) that are generated by skin microbes after breaking down the sweat and secretions of human glands. Essentially, an individual's VOC profile is shaped by their skin flora. Consequently, an insect's preference for a host depends on how its odor receptors detect the unique VOCs of each individual.
Skin glands include apocrine, eccrine sweat glands, and sebaceous glands (see below from 1).
Skin glands are unevenly spread across the body, and the abundance of human skin microbes correlates with their distribution (see below from 1).
The human odor profile is made up of over 400 distinct compounds (2). Research into which of these play the biggest role in attracting biting insects is still in the early stages.
In a small study (n = 48 adult male volunteers) focused on the African malaria mosquito Anopheles gambiae sensu stricto, it was found that individuals who were highly attractive to mosquitoes had distinct skin bacteria compared to those who were less attractive, specifically a higher abundance but lower diversity of skin-associated bacteria (see below from 3).
In another small study (n = 48 adult male volunteers), Anopheles gambiae sensu stricto mosquitoes were more drawn to individuals carrying the human leukocyte antigen gene Cw*07 (4). Since people carry different HLA haplotypes,
- Each person's unique HLA system produces different peptides, meaning the substances their skin microbes metabolize and convert into VOCs are distinctive.
- Each person's individual HLA plays a role in immune responses that shape their unique microbial profile by determining which microbes are accepted or rejected.
Genetics also play a role in determining skin temperature, humidity levels, and metabolic rate, all of which contribute to why some individuals are more attractive to biting insects. Metabolic rate impacts local carbon dioxide levels, which, alongside ammonia, lactic acid, and other fatty acids, affect the landing behavior of insects like mosquitoes (5).
Each person has a largely unique VOC profile, shaped by their genetics and skin microbiome. In turn, different biting insects possess their own specific odorant receptors. The combination of these factors likely makes certain humans more appealing to specific insects. While research on this is still in its early stages, there is more data available for mosquitoes that carry diseases than for other biting insects.
Since human lifestyle choices, particularly diet, influence skin microbiota, future studies will likely uncover how dietary habits can alter a person’s VOC profile and, in turn, affect the preference of biting insects for that individual.
A similar process likely explains why some dogs get ticks while others don’t. However, with ticks, immune status is also crucial as it determines whether or not an infection can be established. Healthier dogs tend to fend off ticks more effectively, preventing stable colonization compared to less healthy dogs.
Bibliography
1. Verhulst, Niels O., et al. "Chemical ecology of interactions between human skin microbiota and mosquitoes." FEMS microbiology ecology 74.1 (2010): 1-9.
2. Verhulst, Niels O., and Willem Takken. "Skin Microbiota and Attractiveness to Mosquitoes." Encyclopedia of Metagenomics. Springer US, 2015. 591-595.
3. Verhulst, Niels O., et al. "Composition of human skin microbiota affects attractiveness to malaria mosquitoes." PloS one 6.12 (2011): e28991.
4. Verhulst, Niels O., et al. "Relation between HLA genes, human skin volatiles and attractiveness of humans to malaria mosquitoes." Infection, Genetics and Evolution 18 (2013): 87-93.
5. Smallegange, Renate C., Niels O. Verhulst, and Willem Takken. "Sweaty skin: an invitation to bite?." Trends in parasitology 27.4 (2011): 143-148.
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