By Terri Darr McLean
Ah, summertime in Kentucky—time for baseball games and backyard picnics, quiet conversation on the front-porch swing and …
… mosquitoes in search of an evening snack.
Yes, Kentucky's warm, humid summers are practically synonymous with the small, two-winged insects known for their ability to turn an idyllic outdoor evening into a blood-feeding frenzy.
While mosquitoes' annoying—and potentially dangerous—habits send others running for cover, College entomologists Stephen Dobson, Grayson Brown, and Lee Townsend eagerly seek out the pesky bloodsuckers. They are scientists in search of answers they hope will provide relief from the daily irritation and reduce the worldwide burden of disease caused by these important insect pests.
When someone asks Stephen Dobson why he spends so much time studying an insect that most Americans consider little more than a nuisance, he might answer with "Do you know the impact of mosquito-borne diseases on humans worldwide?"
Consider malaria. Though now eliminated from the United States, malaria affects more than 40 percent of the world's population and is responsible for up to 2.7 million deaths a year, mostly in Africa.
Then there's yellow fever, a disease that was considered the American plague until control efforts wiped it out 1900. At present, there are some 200,000 cases of yellow fever and 30,000 deaths a year in the rest of the world. It is most common in the tropical and subtropical regions of Africa and the Americas.
Dengue fever, human filariasis, West Nile virus, and Rift Valley fever are other diseases borne by mosquitoes, and all are maladies transmitted to humans by the amazingly efficient female mosquito, the one that bites. Females apparently must have at least one blood meal before their eggs can develop properly.
"So," said Dobson, "those little insects that people don't think much about unless they're having a picnic in their backyard are having a global impact on human health."
Of particular interest to Dobson is dengue fever, a rapidly expanding disease in most tropical and subtropical areas of the world. Dengue-related illnesses range from a severe, flu-like sickness to the deadly dengue hemorrhagic fever.
Dobson was one of 11 international researchers chosen last year to share a $10 million Grand Challenges in Global Health grant from the Bill and Melinda Gates Foundation. The researchers will try to improve methods for controlling the species that carry dengue fever. With what he calls his "slice of the pie," Dobson and his students are researching the use of the naturally occurring bacterium Wolbachia to reduce mosquito populations.
"Wolbachia occurs naturally in over 20 percent of insect species, from beetles to butterflies," Dobson said. "However, the primary mosquito that transmits dengue doesn't have this bacterium—not yet. Our goal is to infect the dengue-transmitting mosquitoes with the bacterium. The hope is that the artificially infected mosquitoes will die before they have the opportunity to transmit dengue."
Another mosquito-borne disease that has captured Dobson's imagination and dominates a large part of his research is human filariasis, which he calls a "close relative" of dog heartworm. Severe cases are characterized by the enormous enlargement of the limbs, known as elephantiasis. This disease, which has put more than a billion people in 80 countries at risk, is the impetus behind another of Dobson's research projects using the Wolbachia bacterium.
"The idea is … we release males that have been sterilized by Wolbachia," Dobson explained. "When they mate with female mosquitoes in the field, the females lay eggs, but those eggs don't hatch. By continuing to release male mosquitoes, we eliminate the population. Instead of spraying pesticide, we're releasing sterile males."
Dobson emphasized that the presence of male mosquitoes, which do not bite or transmit disease, can even be beneficial.
"The males feed on nectar, like butterflies, and can serve as pollinators," he said.
Although dengue and human filariasis affect few people in the United States, Dobson said that "the dynamics and the exposure that we have to these diseases are changing. With increasing international travel, the world is becoming a smaller place all the time."
In addition, he said, "there is a bit of a risk because our habitat is changing; our ecosystem is changing. Some swamps that were drained in the 1920s to 1940s to get rid of malaria and other diseases are now being recovered as wetlands, limited insecticides are available to fight mosquitoes, and resistance is developing to those insecticides. Overlay onto this the recent introduction of two exotic mosquitoes and West Nile virus into the U.S. "
While much of Dobson's work focuses on global concerns and takes him to distant locales, he finds it easy to relate his work to Kentucky.
"Even that work which has a global focus will benefit people in Kentucky," he said. "Controlling mosquitoes in far-off places is not a whole lot different from controlling mosquitoes right here in our backyards."
Grayson Brown knows a thing or two about mosquito management in backyards. He has spent several years researching methods to help homeowners keep mosquitoes at bay.
Brown, along with his graduate students, is currently assessing the effects of insecticide sprays on daytime resting sites of the adult female mosquitoes around suburban homes.
"As the sun begins to come up in the morning, the mosquitoes begin to go to try to find a place to spend the day and fly into those areas and get killed," Brown explained.
Brown and his students have seen about an 87 percent reduction of mosquitoes in the first week after a single insecticide barrier was applied and a 60 percent reduction over six weeks.
While Dobson's work involves a biological approach—using a living organism to "attack" a pest—Brown concentrates on insecticidal approaches.
"In residential areas, where you're dealing with the small spatial scale of the backyard, our emphasis is on the insecticide. … If you don't have some type of (insecticide) barrier, then mosquitoes are going to fly in from the adjoining property," Brown said.
"They can't fly very well. So the way they fly—distance-wise—is they hippy-hop from plant to plant. And as they hippy-hop through the bushes that have been sprayed with insecticide, you get them there," he said.
Along with his residential projects, Brown also works extensively with municipal governments, mostly the Louisville Metro Health Department, on projects to control disease-carrying mosquitoes.
"One of the things we want to do is … be able to work with public health agencies so if we have a serious health threat we'll be able to marshal a serious response," Brown said.
The importance of such work became evident after West Nile virus was discovered in the United States in 1999. It is a potentially serious illness that is now considered a seasonal endemic in North America. West Nile also affects livestock, including horses.
Much like his colleagues, Lee Townsend, an extension entomologist, spends a great deal of time focusing on mosquitoes and is equally concerned about being prepared for a mosquito-borne health threat. The bulk of his work involves surveying mosquito species in woodlot and horse farm settings to determine which species are present and when they are most active.
"Several species in the area are capable of transmitting West Nile and other viruses, so learning more about their habits and how to monitor their numbers will be very useful in developing management strategies for the future," Townsend said.
Of the nearly 60 mosquito species found in Kentucky, Townsend is especially intrigued with a species called Eastern tree hole mosquito, which breeds in water that collects temporarily in tree cavities and occasionally in containers such as discarded cans and bottles. This mosquito can transfer a rare but emerging virus called LaCrosse encephalitis to humans. It also can pass it along to offspring through the eggs the female lays. A few cases have been reported in the Appalachian region of Kentucky in recent years, increasing awareness of the disease.
Part of the College's mosquito research has focused on evaluating the surfaces in artificial tree holes. They are used to monitor the mosquito's egg-laying patterns and the virus infections that are being passed through to the eggs. Such research will also be valuable in detecting and monitoring other species as well, Townsend said.
"There is always one more bug," he said. "Two species (of mosquitoes) have been accidentally introduced into the United States in recent years. Both have become established in the Commonwealth, and both are potential public and animal health impacts. Investigations with artificial egg-laying sites can help develop techniques that can provide better detection and monitoring of other important species."
Townsend and colleagues Dobson and Brown may approach their mosquito management research from different angles, but they share a common purpose: to improve the quality of life for people in Kentucky and beyond. Finding new ways to manage mosquitoes is the ultimate goal.
"If we can build a better mousetrap to control mosquitoes … that would be important to a lot of people," Dobson said.