Even if the mosquito isn’t carrying any dangerous pathogens, its saliva causes a reaction that can be detected for weeks after the bite.

Mosquito spit may affect your immune system for days. Image credits: Rico-Hesse et al, 2018.

Summer is just around the corner and that means that everyone’s least favorite insect is making a comeback — the mosquitoes are coming soon, to a moist area near you. But mosquitoes are more than just a nuisance. Around the world, 750,000 people a year die of mosquito-transmitted diseases, including malaria, dengue, West Nile, Zika, and chikungunya fever. Recent studies have shown that mosquitoes do more than “just” transmit the infection — the mosquito saliva exacerbates some of these diseases, and infections caused by a mosquito bite are often more severe than similar infections caused by a needle, for instance.

Even when it doesn’t carry an infection, the mosquito saliva contains hundreds of proteins that can affect our immune system. For starters, it causes inflammation that isn’t just itchy, but also helps potential viruses multiply and quickly spread to other parts of your body. Now, in a new study, scientists have shown that the impact of the mosquitoes’ saliva lasts much longer than expected.

Rebecca Rico-Hesse, of Baylor College of Medicine, USA, and colleagues wanted to study the effect of mosquito bites on human immune cells. But they didn’t want to have subjects bitten by mosquitoes (and no ethics committee would be happy to hear that idea) — so they came up with a different solution. They engrafted mice with human hematopoietic stem cells (stem cells that give rise to other blood cells) — leading the animals to have components of a human immune system. They then had each mouse bitten by four mosquitoes and monitored the behavior of these immune cells for several days.

“Understanding how mosquito saliva interacts with the human immune system not only helps us understand mechanisms of disease pathogenesis but also could provide possibilities for treatments,” the researchers say.

The team found that the immune responses lasted up to 7 days post-bite. Furthermore, they were seen in multiple tissue types, including the blood, skin and bone marrow. The fact that the effects lasted for so long and managed to elicit such a strong immune response is a reason for concern.

While it’s not clear exactly how this effect manifests itself, this is the first time measurements of this kind were ever carried out and, undoubtedly, the results will have major implications for the future study of mosquito saliva and its effect on the human body. In the long run, this could also enable researchers to develop better vaccines, by indicating which proteins to target for the long-term effect.

Journal Reference: Vogt MB, Lahon A, Arya RP, Kneubehl AR, Spencer Clinton JL, et al. (2018) Mosquito saliva alone has profound effects on the human immune system. PLOS Neglected Tropical Diseases 12(5): e0006439. https://doi.org/10.1371/journal.pntd.0006439