Signs of Winter 2: Mosquitoes, Skin Oils, the Cutaneous Microbiome and Cats!

Aedes albopictus Photo by J. Gathany CDC Wikimedia Commons

(Click on the following link to listen to an audio version of this blog … Mosquitoes, Skin Oils and Cats

Winter seems like a funny time to talk about mosquitoes. The cold weather has either killed off all of the adults of our local species leaving behind their tough, seasonal-resistant eggs, or it has driven the remaining adults of our few overwintering species into their inactive torpors. There have been, though, quite a few research papers about mosquitoes published in last few months!

We’ve talked about mosquitoes before. Very notably back in May, 2016 (Signs of Spring 13) when we highlighted research by Dr. Nora Besansky on the many positive aspects and ecological benefits of mosquitoes.  Besansky points out that of the four thousand or so species of mosquitoes less than one hundred actually transmit human diseases. The rest occupy important places in many food chains (for mammals, birds, fish, amphibians, reptiles, and wide range of insects and other arthropods) and also function as pollinators for many species pf plants (all adult mosquitoes, Dr. Besansky stated, drink flower nectars). Blood feeding by mosquitoes is only carried out by females and is needed to provide the energy to make viable eggs. The female mosquitoes take their blood meals from a wide variety of potential hosts.

Aedes aegypti Photo by J. Gathany CDC Wikimedia Commons

Another perfectly reasonable point of view, though, concerning mosquitoes is to look at the nearly one hundred mosquito species that can transmit diseases to and in between people. There are some serious diseases on the mosquito transmitted list: malaria, yellow fever, dengue fever, West Nile virus, Zika virus and a whole slew of encephalitis syndromes! Each year, according to EB Medicine (an on-line medical reference site), seven hundred million people come down with a mosquito transmitted illness. That number is double the number of yearly cases of COVID-19 at the peak of our recent (and on-going) SARS-CoV2 pandemic!  Most of these mosquito-transmitted cases occur in human populations residing in tropical and subtropical climate zones, and about half of all of these cases involve malaria.

So, we need to keep both an ecological and a medical perspective on mosquitoes!

An observation that many of us have made during mosquito season is that some people attract mosquitoes more vigorously (i.e. get more mosquito bites) than others. A recent paper (October, 2022) published in journal Cell explores a compelling reason for this uneven host selection by mosquitoes.

Secretions from sebaceous (“oil”) glands in the skin are a complex mix of moisturizing and waterproofing lipids that are designed to keep the skin both flexible and water-tight. Each individual has a unique mixture of lipids that accomplishes these important tasks. This unique array of chemicals also generates an individual’s particular scent which has a wide range of subtle influences in chemical communication and, possibly, even interpersonal interactions.

Pentadecanoic acid, Figure by Krakatit,Wikimedia Commons

Some of the lipids in these sebaceous gland secretions are long-chain, carboxylic acids (“fatty acids”) that include pentadecanoic acid, heptadecanoic acid and nonadecanoic acid. Some people are genetically predisposed to produce large quantities of these particular carboxylic acids in their sebaceous gland secretions. The research behind the October, 2022 Cell paper clearly showed that these particular fatty acids in a person’s skin secretions makes them extremely attractive to mosquitoes!

1-Decanol. Figure by Edgar, Wikimedia Commons

These carboxylic acids, though, are not the only “mosquito magnets” being synthesized by the skin. In a paper published in Nature (May, 2022) researchers showed that long chain aldehydes (like decanol and undecanol) which are also synthesized by cutaneous sebaceous glands, are also powerful mosquito attractants. Decanol and undecanol are important “scent” molecules in a wide range of flowers and fruits and act to attract pollinating insects to these plants. Considering the ecological role of many mosquitoes in pollination cycles and the importance of flower nectars to mosquitoes as a food, this chemical attraction is very logical. It is unfortunate, though, especially for those individuals predisposed to synthesize high levels of these long-chain aldehydes in their skin oils, that these flowery secretions draw such crowds of hungry mosquitoes!

Another observation that has been made repeatedly in the medical literature concerning mosquitoes and humans is that individuals with mosquito-transmissible viral infections seem to attract mosquitoes (and, thus, get more mosquito bites) than individuals who do not have one of these mosquito-transmissible viral illnesses. This phenomenon would serve to more rapidly spread the infectious virus and lead to more and more replication of the viral nucleic acids in new hosts (thus satisfying Richard Dawkins’ “selfish gene” hypothesis as the engine of viral evolution (see Signs of Fall 8, October 20, 2022 for more discussion of this hypothesis).

Photo by Rama, Wikimedia Commons

Researchers at Tsinghua University in China explored this observation with regard to flaviviruses (the viruses that cause dengue fever and Zika) and published their results in Cell (June, 2022). Using mice infected with these viruses, they clearly showed that mosquitoes (in this case Aedes aegypti, the mosquito that spreads these viruses among humans and other hosts) were strongly attracted to the infected mice. Further, they showed that infected mice produced a specific chemical that attracted the mosquitoes. This chemical was acetophenone.

This acetophenone, though, was not synthesized by the sebaceous glands of the mice. Instead, it was synthesized by bacteria in the cutaneous microbiomes of the mice. These microbiome bacteria were, apparently, being manipulated by the infectious virus in the mice! The bacterial derived acetophenone, then, facilitated the spread of the viruses between mouse hosts.

To finish up this winter digression into mosquitoes, I do want to talk about some very interesting observations concerning one of my favorite animals and one of their favorite intoxicants: cats and catnip! Amazingly, all of these treads do come together into a neat package!

Cat and catnip. Photo by D. Sipler, Wikimedia Commons

Anyone who has ever been close to a cat has inevitably marveled at the effect of catnip on their affects and behaviors. The obvious “high” that the cat experiences from rolling around in fresh or dried leaves of catnip (or several other, mostly unrelated, plant species) is even more remarkable in that no other animal species seems to experience any reaction at all to these plants. House cats, mountain lions, lynx, bobcats, tigers, jungle lions  and jaguars, though, all get high on catnip or one of the other cat-intoxicating plants.  A research paper published in the journal iScience (June, 2022) may explain the how and why of this behavior.

Many plants synthesize chemicals called “iridoids.” Iridoids make the plant less palatable to herbivores and thus reduce the damage grazing animals might do to a plant’s biomass. Catnip (and the other, cat-intoxicating plants) all synthesize an iridoid called “nepetalactone.” Nepetalactone binds with a cell surface protein receptor called “TRPA1.” TRPA1 receptors are found in many species of both invertebrate and vertebrate animals, and they are found on many types of cells but are especially abundant on nerve endings in the skin, the respiratory system and the gastrointestinal tract.

TRPA1 receptor. Figure by Landini et al. Int J. Molec. Sci.

In cats, the activated TRPA1 receptors stimulate the production of opiates in the their brains. Rising levels of these opiates cause the “high” observed in the cats. In mosquitoes, though, and in many other blood sucking insects, the activated TRPA1 causes pain and distress.  In other words the nepetalactone acts as an insect  repellant which has been compared in effectiveness to DEET.

Cats rolling on a catnip plant not only cover themselves with nepetalactone but also cause the catnip plant to release 20X more nepetalactone than it was releasing at rest! The rolling cat, then, gets covered with insect repellant and gains an edge against blood loss and insect transmitted disease. It gets an immediate reward (the “high”) and a long-term reward of protection against disease and debilitation!

Not all cats, though, react to the chemicals in catnip! It is estimated that only 60 to 70% of house cats are sensitive to nepetalactone. The other 30 to 40% must have genes that make their TRPA1 receptors into slightly different, non-reactive protein configurations. Natural selection, then, has not been complete when it comes to rewarding a cat’s roll in protective insecticide.

Years ago (June 3, 2014, Signs of Spring 13), I wrote about a catnip-like reaction my house cats had to a pile of crushed ants that I was sweeping off of my deck and also to some spilled olive oil. The 18-carbon fatty acid called “oleic acid” (a potent insect pheromone) was present in both materials was thought to be the chemical stimulus. I now know that the oleic acid (and maybe its associated phenylethanoid called oleocanthal) were stimulating my cats’ TRPA1 receptors causing their ridiculous, rolling and gyrating behaviors!