Dengue, Zika and other mosquito-borne flaviviruses in the Aedes are a major public health issue, sickening hundreds of millions of people and killing tens of thousands each year. These arboviruses are expected to spread better and to more people geographically in the future in our warming climate. For millennia, people have recognized that during outbreaks of mosquito-borne diseases, patients with these diseases should be protected from further mosquito bites (e.g. with bed nets) while recovering to limit the spread. of these diseases to healthy people. The reason for this is partly that recovering patients might be less able to protect themselves from mosquito bites because of their symptoms, and partly because mosquitoes might be more attracted to people with mosquito-borne diseases. Such a strategy would be evolutionarily advantageous for mosquito-borne pathogens to maximize their transmission. There is evidence of increased attraction of people infected with mosquito-borne diseases to blood-seeking female mosquitoes, via manipulation of their host. For example, we know that Plasmodium parasites produce an isoprenoid precursor that increases the release of volatile mixtures from human red blood cells, making malaria-infected mice much more attractive to Anopheles vector mosquitoes. However, until now it was not known whether arboviruses, transmitted by Aedes mosquitoes would similarly manipulate their host to maximize disease transmission.
In a tour de force of a study, Hong Zhang, Yibin Zhu and their colleagues at Tsinghua University demonstrated that mice infected with flaviviruses were significantly more attractive to blood searchers. Aedes mosquitoes. Mosquitoes were allowed to choose to move to cages closer to healthy or infected mice in a three-cage olfactometer as well as in a two-cage olfactometer. Mosquito preference was related to volatiles emitted by hosts, as evidenced by elimination of preference by elimination of volatiles.
By analyzing how the antennae of female mosquitoes reacted to the volatiles of healthy and infected mice, the researchers found that the mosquitoes reacted significantly to acetophenone, decanal and styrene, but not to the other volatiles. Acetophenone attracted more mosquitoes when placed on healthy mice or on a human hand, compared to solvent alone. Using GC-MS, ten to one hundred times more acetophenone was released from Zika virus and dengue infection compared to healthy mice.
Volatiles collected from the armpits of dengue patients attracted significantly more mosquitoes than either simple solvent or volatiles collected from the armpits of healthy donors. Volatiles collected from the armpits of dengue patients also contained ten to one hundred times more acetophenone than volatiles collected from the armpits of healthy donors.
When skin bacteria from mice infected with Zika or dengue virus were eliminated, mosquito preference for infected mice was eliminated, but not when gut bacteria were eliminated, indicating that the release of acetophenone is bound to skin bacteria of mice. Acetophenone release was also reduced to a level similar to that of healthy mice when the flavivirus-infected mice’s skin bacteria were removed, but not when their gut bacteria were removed.
Mice infected with dengue and ZIKV had ten times more bacteria, with an increased relative abundance of Bacillus and Staphylococcus, but a reduced relative abundance of Lactobacillus, compared to healthy mice. Bacillus bacteria produced large amounts of acetophenone when cultured on the skin of germ-free mice and elicited increased attraction of female mosquitoes looking for blood in the olfactometer at high enough concentrations.
Resistin-like molecule-α, an antimicrobial protein specifically expressed by epidermal keratinocytes and sebocytes, was expressed at significantly reduced levels in the skin of Zika virus and dengue infected mice, compared to healthy controls, on the basis of RNA-seq, qPCR and Western blot. This protein, along with its human analogue, RETN, significantly reduced the viability of the acetophenone-producing bacterium Bacillus, compared to other bacteria. Based on previous research, RELM-α can be induced by dietary derivatives of vitamin A, such as isotretinoin.
RELM-alpha production was restored to healthy levels in Zika virus and dengue infected mice that received isotretinoin by oral gavage, and the relative abundance of Bacillus bacteria on the skin of these mice n was also not significantly different from that of healthy mice. Moreover, blood-seeking female mosquitoes were not preferentially attracted to mice given isotretinoin, despite being infected with Zika virus and dengue fever. About half the number of mosquitoes were infected among the mosquitoes allowed to feed on mice infected with dengue virus and Zika virus when they received isotrenioine, when their Zika virus titers were not not significantly different.
In summary, the authors found that Zika and dengue viruses suppress the production of RELM-alpha, allowing Bacillus bacteria to proliferate, thereby increasing the production of acetophenone, which acts as a powerful lure for bloodthirsty mosquitoes. Additionally, this process can be impeded by the administration of isotretinoin, increased RELM-alpha, reduced Bacillus bacteria, and acetophenone production.
One of the strengths of this study is that it provides an in-depth analysis of this important topic, and it combines a large number of studies using a variety of techniques ranging from mosquito choice tests to electroantennograms to GS- MS, RNA-seq and microbiological studies in the quest to find the mechanism by which mosquitoes prefer infected mice and people. The study provides strong evidence that flaviviruses modify their host’s microbiome to increase the production of volatile compounds that attract mosquito vectors, ensuring their transmission to other hosts. This is similar to how Plasmodium parasites increase the release of volatile mixtures from human red blood cells to attract more Anopheles mosquitoes. There are also similar examples of insect-borne plant pathogens, such as when cucumber mosaic virus causes infected squash plants to produce a volatile mixture that attracts aphids despite the poor quality of the infected plant. . These results suggest that we have a whole other reason to protect people infected with flaviviruses from mosquito bites, because even all other things being equal, they will attract more mosquitoes.
Another exciting part of this study was the demonstration that a dietary vitamin A derivative, isotretinoin, was able to reverse the attractiveness of infected animals, with only about half of the mosquitoes feeding on the host. infected. This is particularly exciting because it suggests that in addition to protecting recovering people from mosquitoes with bed nets and mosquito nets, we can counteract their increased attraction to mosquitoes by using a drug that is safe, inexpensive and easy to administer. Isotretinoin is already used to treat acne under the brand name Accutane, so it could be repurposed for this new indication, which would be much easier and faster to approve compared to a brand new drug. However, we must remember that this study was conducted primarily on mice, with the exception of taking volatiles from people’s armpits and testing compounds on their hands. A possible next step might be to conduct a randomized, double-blind study with dengue or Zika patients to see if administering isotretinoin orally would also increase RETN production in their skin, reduce the relative abundance of Bacillus and the production of acetophenone, and ultimately their attractiveness to female blood. look for mosquitoes. If the results remain comparable in human studies, we could halve the number of mosquitoes picking up Zika or dengue viruses from infected people, thus contributing significantly to the control of these flaviviruses.
Finally, the volatile responsible in the current study for the increased attractiveness of infected mice, acetophenone, is widely used in perfumes and perfumes, due to its floral and sweet scent to people. So the next time you travel to areas where dengue, Zika or other flaviviruses are endemic, or where Aedes populations (such as eastern North America), which will increase in the future, be sure to check that the perfume you wear does not contain acetophenone!