The War on Mosquitoes

By Kevin Tankersley

Baylor professor sets out to reduce mosquito population

It’s summertime in Texas, which means we have to deal with heat, humidity and mosquitoes.

Cheolho Sim is doing his best to help us deal with that last one.

Dr. Sim, an associate professor of biology at Baylor, and the graduate students who work in his lab are trying to shorten the life span of a certain variety of mosquito which, instead of living about a month like most mosquitoes, goes into a sort of hibernation during the winter and emerges in the spring to torment us once again.

Sim earned two degrees from Korea University and his Ph.D. at University of Notre Dame. He and his wife, Moon, came to the U.S. from South Korea to continue their education. They have one daughter, Hajin, who will be at Midway High School this fall, where she will play violin in the school orchestra.

Wacoan writer Kevin Tankersley talked with Sim recently in his office in the Baylor Sciences Building, just down the hall from Sim’s lab, where numerous kinds of mosquitoes under study are kept in mesh cages.

WACOAN: What led you to study mosquito genomics?

Sim: I was interested in genetics research, and my master’s degree adviser recommended a school that had a research program in the vector-borne diseases and mosquito research, which is relevant to many human infectious diseases. I was interested in studying vector biology and also the infectious diseases in a developing country, such as malaria, dengue and Zika virus.

WACOAN: When is mosquito season in Waco?

Sim: There are two seasons, usually spring and right after the summer. We’ve had an active surveillance program for the last 10 years in the Waco area. It depends on each year’s climate conditions, but usually in the springtime, right before the summer, and then right after summertime.

So probably late May and early June, and then September and October are the highest peak times of the mosquito populations. It goes down during the dry summer season.

WACOAN: And are we in the midst of an unusually wet season?

Sim: Yes. We found that there has been a certain change from last year because last year was very humid and it has continued on this year. It’s kind of interesting. For the last 10 years, the dominant mosquitoes were a Culex mosquito species, which transmits West Nile virus, but the dynamics of mosquito species are changing. We don’t know why exactly.

Each mosquito species has a unique pathogen that can infect humans. Aedes albopictus transmit the chikungunya virus. We are worried about the chikungunya virus. For the last five years, we have been searching for the mosquitoes carrying this virus to detect the virus in the mosquitoes, so that we can give this information to the public health district in McLennan County. But luckily, any virus-infected mosquitoes were not caught.

In the lab, we have active surveillance programs. We go out every other week and set up the traps at 10 sites to catch mosquito samples and bring them in to the lab. And then we try to understand what’s going on in the field. Last year, there was a certain shifting in the mosquito populations. There were not only Aedes albopictus, but also Aedes aegypti. We are catching almost equal numbers of these two species. Aedes albopictus transmits chikungunya, and Aedes aegypti transmits the dengue and Zika viruses.

It seems like they are competing against each other for the habitats in the Waco area. We found equal numbers of these two species since last year. I don’t know exactly the answer for that, but it seems like some humid conditions have changed the habitats and brought them to compete in the Waco area.

A month ago, we caught the Psorophora species for the first time — a mosquito that is a different species. It was the first time we caught those mosquitoes in Waco. It was surprising to us, so we immediately reported it to the McLennan County health district officials. This mosquito can transmit equine encephalitis virus.

It clearly shows that in this area, the mosquito population is diversifying in different species. And new mosquito species are expanding their habitats more toward the west side. Usually, the main areas were Florida, Alabama and Louisiana, but not much in Texas. But now we can catch the Psorophora mosquitoes here.

WACOAN: Are mosquitoes really bad right now, or are they dangerous?

Sim: What I can tell you is that there are high numbers of mosquitoes right now, so we are catching a lot more Aedes aegypti and Aedes albopictus. However, the Culex quinquefasciatus population, which transmits the West Nile virus, is very low, and this is unusual. Last year, the number of Aedes aegypti we were catching was modest. But this year, Aedes species populations are really high, but not much on Culex, which means we have less danger to West Nile virus but more danger of other viruses like dengue or Zika virus.

WACOAN: Who is most at risk of getting these viruses?

Sim: People who have their immune systems compromised are most at risk. Seniors and young children are in danger from these viruses. But with healthy people [who catch the virus], it acts like a cold and then it goes away. In the last couple of years, the Zika virus was a big issue, but it may seem like it’s not a big issue in the United States now. But it is more dangerous to pregnant women because the virus can infect the fetus, especially during the period when the baby is developing the neuron systems.

WACOAN: What’s the ideal breeding environment for mosquitoes around houses?

Sim: Small containers with water are perfect breeding sites for mosquitoes. A small pond can be a breeding site too. The most important thing is to take care of the backyard.

WACOAN: I think I read some of your research that said there are mosquitoes that can breed in dirt instead of water. Is that right?

Sim: Yes. That is a Psorophora mosquito species. It’s very unusual. They can breed in a small amount of dirt. That is unique. Maybe there are certain habitat changes or humidity conditions that allow them to expand their habitat into the Waco area because it’s very humid right now.

WACOAN: In your work, you also talk about diapause. Can you tell me what that is and how that’s important to mosquitoes and how that’s important to your study?

Sim: People think that mosquitoes die in the cold season, in wintertime. This is true of certain mosquitoes because those mosquitoes don’t have diapause.

Diapause is like hibernation. Another name for it is insect dormant stage. It’s an alternative development program. Diapause in insects, including mosquitoes, is a more-tightly, genetically-controlled development program, which means that when mosquitoes hatch, they are already programed to undergo dormant stage.

Only female mosquitoes feed on humans. The only reason that they feed on the blood from humans or animals is that they need the protein sources to produce eggs. So male mosquitoes don’t take blood from humans; it’s only the females. Then they lay eggs and survive for about one month in fields, and then they die. This happens to most mosquitoes.

During the late summer or fall, the days begin to shorten, and the [mosquito’s] brain understands that the daylight is shortening and winter is coming. This is an environmental cue.

They understand that winter is coming, so the mosquitoes start this genetic program called diapause. The female mosquitoes don’t take a blood meal because they know the eggs cannot survive in the wintertime. They look for a flower or rotten fruit, a sugar source [to hibernate in], and they preserve the sugar and increase the fat and glycogen in their abdomen. They’re really big, fat mosquitoes. Then, they expand their life span five times longer than nondiapause mosquitoes. They can survive more than five months. In that way, they can survive the winter season. So next is spring. They still survive, and then they are ready to take a blood meal and start a new season.

This diapause is one way that they survive during a winter season. In my lab, we try to understand which gene controls this diapause developmental pathway. If we can understand which gene controls the diapause phenotype, we can hopefully find a way to control and apply certain types of chemical or some gene drive system to stop or block the mosquitoes entering diapause, so they can die out. This is the idea that we now have.

WACOAN: What happens to the diapause mosquito in the spring?

Sim: After the winter season, they come back out into their normal developmental pathway. They look for blood and lay eggs.

WACOAN: So if you can figure out how to stop that diapause —

Sim: Then we have fewer mosquitoes next season.

WACOAN: You have 10 sites where you gather mosquitoes, right?

Sim: With Baylor’s program, yes, we have 10 different locations in the Waco area.

WACOAN: So how do you collect the mosquitoes?

Sim: There is a mosquito trap with certain chemicals that attract mosquitoes.

WACOAN: Are these traps very expensive?

Sim: They are about $300 to $500 for each trap, so not very expensive for research. But the important part is the laboratory. Someone has to go out and set the trap, and then he or she has to bring it back. Then, he or she separates all the different individual mosquitoes and identifies the species. It takes a lot of time. When we test a virus inside a mosquito, there is a certain molecular technique, so we need a highly trained graduate student to extract mosquito RNA, and then we need to convert it to DNA. Finally, we do an extra test called qRT-PCR, which is a quantitative real-time polymerase chain reaction. It is a labor-intensive process.

WACOAN: Here’s why I asked if the traps were expensive. If I could buy a trap for $300 and put it in my yard and it would trap all the mosquitoes, I would do that. Is that a viable option?

Sim: I believe not. A trap is for sampling mosquitoes in fields, for a huge area to cover. Traps are better for a sampling purpose rather than controlling mosquitoes. Unfortunately, you cannot catch all the mosquitoes in your yard using just one or two traps. The best way for your backyard is a spray, fogging the area with a pesticide.

WACOAN: What kind of pesticide is safe for pets and safe for plants but would get rid of mosquitoes?

Sim: That is a really tricky question. The city needs to get a license or the approval from [the Environmental Protection Agency] to apply pesticide in the open fields, and there is a lot of documentation process because certain pesticides can cause harmful side effects to nature and also to humans.

It is a Catch-22 situation right now. The best way right now is spreading a DEET repellent. The highest-biting time is at dusk and dawn. Avoid those times if you play golf or if you work outside. Wear long-sleeve shirts. Personal protection is the best way.

The city uses biopesticides. When we find a certain location that has a higher population of mosquitoes, the city uses biopesticides in that area.

WACOAN: When I was a kid in Bellmead, there was a city truck that came through my neighborhood every night fogging for mosquitoes. What was in that fog?

Sim: I don’t know. It would be a type of pesticides, I guess, but those pesticides are not allowed to be used any more. I grew up in South Korea, and during the summer they used a fogging truck, similar to the one you mentioned. When I was a kid, I was stupid and used to follow the truck.

WACOAN: That was my next question. Those of us who ran behind the trucks, was that a stupid thing to do?

Sim: I would not recommend. The pesticides and the chemicals could harm humans as well. If a city wants to fog an area, they need an approval by the EPA. It is a very difficult process and not cost-effective.

WACOAN: In your research, you’ve talked about mosquitoes and ecological suicide. What does that mean?

Sim: We are still developing those tools. I explained about diapause that mosquitoes successfully initiating the diapause program can survive the winter season. There are a couple of ideas we are testing. First, you can use a certain chemical, so the mosquitoes taking those chemicals cannot start the diapause program, and in the winter season, they automatically die.

Another method is called a gene drive system. We insert or remove certain genes in the mosquitoes, and those genes are important to initiate the diapause program. Mosquitoes missing those genes or blocking those genes cannot initiate diapause in the fall or winter. So what happens is the mosquitoes die. We call this ecological suicide.

WACOAN: So in your lab, you’re trying to eliminate diapause in mosquitoes.

Sim: Yes.

WACOAN: So mosquitoes are still going to be here. We can’t get rid of them completely, but instead of them living for four or five months, you’re just hoping to limit them to that one month.

Sim: Yes.

WACOAN: Are there other worries about mosquitoes besides disease? Can they do anything besides potentially spread disease, or are they just irritating?

Sim: I think people mostly complain about the irritation, but a bigger danger is transmitting a disease, like a virus. In African countries, malaria causes more than half a million deaths per year. It is a very serious issue in many developing countries. Also, the dengue virus in southeast Asian countries causes more than 20,000 deaths per year.

WACOAN: Have deaths from mosquito-borne viruses been an issue in Waco or Texas or the U.S.?

Sim: It is not a big issue yet in the United States, but we need to watch out. Developing countries do not have an established infrastructure in public health. When people are infected, limited access to health care causes a huge number of deaths in those countries. The public health system does not exist in many regions in those countries. Controlling mosquitoes is the best way to control vector-borne diseases in those countries, and curing these diseases is another issue.

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