Discovery of mosquito blood-feeding moderation: Chisako Sakuma contributes to infectious disease control measures through ecological understanding

Mosquitoes draw blood without permission, leaving only itchiness behind when they depart. It is doubtful than anyone loves these extremely bothersome creatures (only females feed on blood). The itchiness alone is inexcusable, but they are actually the organisms that kill the most humans. According to estimates released in 2014 by the World Health Organization (WHO) and other institutions, they cause the deaths of as many as 720,000 people annually as vectors for infectious diseases, including malaria.

Last year brought unique research findings about the blood-feeding behavior of these detestable mosquitoes: they stop feeding at "moderate fullness." August 20, is "World Mosquito Day," established to commemorate the day when British bacteriologist Ronald Ross discovered malaria parasites inside mosquitoes. In honor of Chisako Sakuma, Senior Research Scientist at the RIKEN Center for Biosystems Dynamics Research, who cheerfully responded to Science Portal's interview, we ask you to look at mosquito ecology with a slightly broader perspective.

Questions and interest in risky blood-feeding behavior

— First, I absolutely must ask... Do you like mosquitoes, Dr. Sakuma?

Not at all! I'm no different from everyone else. I swat them away when I find them, and I can't stand it when they fly around my ears while I'm sleeping. Of course, I hate having my blood sucked too.

— That's reassuring. So why did you begin researching these detestable mosquitoes?

Originally, I was researching how nerves are formed, using fruit flies as subjects. It was research using complex genetics to change just one neural cell in the brain into a mutant cell and elucidate the mechanisms behind shape abnormalities. While it was interesting research because you could visually see changes in cell shape, it was also a time when many people were beginning behavioral research.

During this period, my then-boss encouraged me, saying "blood-feeding organisms are interesting." The mosquito genome sequence had just been revealed, and few people were researching it, so I became interested. When I looked into it, it became increasingly fascinating. "Why do they deliberately take risks when there's a danger of being attacked by the host?" It was due to such questions and interest that I shifted course toward mosquito research.

Blood coagulation components identified through trial-and-error experiments

— Why do mosquitoes stop blood-feeding at "moderate fullness," something that's difficult even for humans?

I believe it's probably a response to the risks I mentioned earlier. My hypothesis is that they stop blood-feeding before reaching a full (distended) state to avoid encountering host avoidance behaviors (swatting, hitting, etc.).

— What is the mechanism for "knowing moderate fullness" that you revealed in this study?

We found that a component called fibrinopeptide A (FPA) contained in the host's blood is involved. When you get injured, the blood at the wound clots and bleeding stops, right? This is called blood coagulation, and FPA is a substance produced in its early stages. As blood coagulation progresses, FPA is cleaved from the blood and becomes unnecessary for the host, but we discovered that mosquitoes respond to this cleaved FPA and stop blood-feeding.

— What kind of experiments led to this discovery?

Adenosine triphosphate (ATP), also contained in blood, had been previously known as a substance that promotes blood-feeding. When we had mosquitoes feed on an ATP solution, they continued feeding until they reached a distended state. In other words, unlike with blood, they didn't stop at moderate fullness.

Next, we tried adding serum (the supernatant liquid remaining when blood coagulates) to the ATP solution. It was previously known that mosquitoes won't feed on serum alone. When it was mixed with ATP solution, the number of mosquitoes that continued feeding until distension was greatly reduced.

In other words, some component contained in serum must have the effect of stopping blood-feeding. That said, serum contains an enormous number of components. Like elementary school science experiments, we tried possibilities at random. We tried applying heat to serum, or changing the blood itself to that of frogs or bichir (lungfish).

Success in verification with new information and technology, but research is only halfway there

— Why did you expect the secret to lie in the host's blood? It seems natural to think the reason would be in the mosquito's body, like satiety in humans.

While this is also a result of the "trial-and-error" approach I mentioned... Originally, research from the 1960s suggested the existence of a mechanism by which mosquitoes physically sense distension and control blood-feeding. It appears there are nerves in the abdomen for transmitting satiety to the brain.

However, for example, when the proboscis doesn't penetrate well into a patch of thick fur on a dog and blood-feeding doesn't proceed smoothly, they stop partway through even without reaching moderate fullness. In other words, I thought there must be a separate mechanism that controls feeding not only based on the "amount" consumed but also the "timing" when blood begins to coagulate. From the perspective of avoiding the risk of encountering avoidance behaviors, stopping blood-feeding at a certain timing makes sense.

— More than 50 years have passed since the existence of nerves that sense satiety was suggested. Why were you able to make this new discovery now?

There have been many studies that treated blood-feeding behavior phenomenologically, such as "how many minutes it took to withdraw the proboscis." However, in the past 10 years or so, the accuracy of mosquito genetic information has improved, and technologies like CRISPR-Cas9 (the current mainstream genome editing tool) have emerged, enabling us to successfully verify the mechanism.

However, while we now know that host FPA has blood-feeding inhibitory effects, there should still be sensors (reception mechanisms) in the mosquito's body as well. Since we haven't yet clarified their existence, the research is still only halfway complete.

The terror of how a single mosquito can spread infectious diseases to a great extent

— Mosquitoes also feed on blood from creatures other than humans, don't they? Is FPA contained in all living things?

Yes, definitely in mammalian blood. It's called a "highly conserved" gene, FPA is incorporated into sequences across species and exists in blood. When we examine blood that mosquitoes have fed on, we can also see that they repeatedly stop blood-feeding partway through. Not only does the mosquito contain blood from multiple people, but we also find combinations like human, horse, and dog blood. Until they reach moderate fullness, they basically attempt blood-feeding as many times as necessary.

— The way mosquitoes transmit infectious diseases in various forms—from person to person, and from animals to people—makes sense.

What makes mosquitoes terrifying is how they can spread infectious diseases to a great extent. As I just mentioned, it's not uncommon for a single mosquito to feed on blood from multiple people in a short space of time, and conversely, one person can be bitten by multiple mosquitoes. In Japan in 2014, 160 people, mainly those who visited Yoyogi Park in Tokyo, contracted dengue fever. When patient blood was examined, almost all infected individuals had the same viral strain. In other words, the infection source was just one person, and it spread through a chain of mosquito blood-feeding.

— Considering that we're being bitten without knowing it on a regular basis, how can we prevent this...?

What was excellent about the Yoyogi Park case was that doctors suspected dengue fever. Since the early symptoms of dengue fever share many similarities with the common cold, diagnosis is difficult. By identifying it as dengue fever early on, they were able to prevent the spread of infection by promptly closing the park and taking other measures.

As this case shows, preventing mosquito-borne infectious diseases is not easy and requires integrated responses from people in various positions. The veterinary medical association to which I belong also includes public health experts, doctors, and local government and administrative officials.

Metabolic inhibition and oviposition mode induction... Various efforts to reduce blood-feeding

— What can researchers like yourself do?

This also cannot be addressed by a single method. I'm thinking about whether we can make mosquitoes lose interest in humans without killing them, and recently I've been focusing on "metabolism." Mosquito metabolic mechanisms are actually almost the same as those in humans. For example, mosquitoes that feed on blood from people taking metabolic inhibitors like anticancer drugs have their metabolism inhibited in the same way.

So, I'm considering whether we could artificially promote mosquito metabolism to induce a state close to moderate fullness. If we could do that, I think we could also apply the technology to human health.

Furthermore, for three days after blood-feeding, mosquitoes don't approach hosts. This is to concentrate nutrients into their eggs. If we could artificially induce this oviposition mode, we might be able to reduce blood-feeding.

Additionally, various efforts are being made, including researchers studying mixing poison into bait (since mosquitoes also feed on flower nectar) and people researching ways to weaken the muscles needed for flight. However, the reality is that there are many challenges to be addressed, such as the pros and cons of releasing genetically modified mosquitoes into the natural world and their impact on ecosystems.

Rising risks from global warming, a desire to expand the circle of researchers

— It seems difficult to escape from mosquitoes immediately.

Unfortunately, we'll need the same kinds of countermeasures as before for a while. Not creating water sources that larvae prefer, wearing long sleeves when entering bushes, and properly using insect repellent—not neglecting these is the first step in preventing not only uncomfortable living but also infectious diseases.

However, concerning changes are occurring with global warming. This became a topic last year: extreme heat has led to situations where mosquitoes are inactive during midsummer but continue their normal activity until around the end of the year. Moreover, the habitat of Aedes albopictus, whose northern limit was said to be the Kanto region, is gradually expanding northward, and cases of overwintering in Aomori Prefecture were confirmed several years ago.

A. aegypti, which is my research subject, is not currently established in Japan, but some researchers predict it will definitely enter the country again. Since its ability as a vector is extremely high, this would also increase the risk of infectious diseases. Incidentally, Anopheles mosquitoes, which transmit malaria (accounting for more than half of the deaths from mosquito-borne infectious diseases), do inhabit Japan, but so far, no increased risk due to global warming has been indicated.

— Please tell us about your future aspirations.

Since mosquitoes are organisms we encounter daily, easily implementable countermeasures are needed. To that end, I'd like to expand the circle of researchers who can work together in the same direction.

Furthermore, Japanese mosquitoes are said to be phylogenetically unique. There must be things that can only be done in Japan. I want to contribute as a researcher so we're prepared for whatever might happen at any time.