Scientist's Goal: Control Mosquito-Borne Diseases

As Christensen stops to describe different species in the room, collected from native ranges in Egypt, India, Iowa and Colorado, a single free-range mosquito darts overhead. The mind anticipates that next step in the dance: a sudden stinging pain and an awkward slap at some hard-to-reach patch of exposed skin.

The nuisance factor of Wisconsin's unofficial state bird is trivial compared to the life-and-death issues that concern Christensen's lab. He is exploring the mosquito's role as a carrier of serious diseases such as malaria, yellow fever and encephalitis.

Christensen, a professor of animal health and biomedical sciences, has been tracking mosquito-borne diseases for the past two decades. The work begins with field collections of mosquitoes and the parasites that cause human disease, primarily in sub-Saharan Africa and other tropical regions.

In the lab, Christensen's team investigates the genetic rules that determine whether mosquitoes can kill disease-causing parasites, or pass them on to humans.

This is a high-stakes field, and getting more so every year, Christensen says. In a recent address, President Clinton identified malaria along with tuberculosis and HIV as the three greatest public health threats facing the world.

"Most people don't realize how catastrophic these things are," he says. "When you're dealing with malaria, you're dealing with about a half-billion people infected at any one time.

"We're talking about one to three million people dying each year. The vast majority are in sub-Saharan Africa and the vast majority are children under the age of five.

"As an analogy, think of anywhere from 10 to 18 jumbo jets full of children crashing every day. Can you imagine the amount of emphasis malaria would have if this was a disease that was ravaging the North American continent or Europe?"

What's worse, Christensen says, is that traditional methods of controlling mosquitoes are losing their effectiveness. And potentially useful controls such as DDT are so toxic they potentially create more problems than they solve. DDT is targeted for a worldwide ban on its usage.

That means new controls are needed and some ideas are coming from genetic research.

"Our major question is, what genetic functions allow a mosquito to kill parasites?" Christensen says. All mosquitoes have an innate ability to kill a foreign invader, but serious parasites like malaria somehow elude these natural defenses.

By fully understanding the genetics involved, Christensen hopes to identify the anti-parasitic genes in mosquitoes. The question is not only finding a potential genetic silver bullet, but transferring it into a wild population without monstrous consequences.

Although it sounds far-fetched, Christensen says this line of work could lead to a "mosquito vaccination strategy," by introducing molecules into the environment that produce an immune response in mosquitoes. It could be passed on in wild mosquitoes from regions with high infectious disease rates.

"We're not trying to produce a "super-mosquito' with a foreign gene that would make it radically different from its counterpart," he says. "We're not changing genes as much as helping mosquitoes express genes they already have."

Christensen's group has been focusing special attention on a mosquito from the Nile River Delta which carries a disease called lymphatic filariasis, caused by a parasite whose only host is humans. They are finding some strains of the mosquito that are remarkably adept at killing this parasite, which gives the researchers a comparison group to other mosquitoes.

Infectious diseases are beginning to take center stage in this country as fears are fueled by disease outbreaks in new territory, such as the West Nile Fever outbreak recently in New York City. And as Americans travel more and pursue ever-more exotic locales, the likelihood of new outbreaks increases.

On a recent international plane trip, Christensen was headed to Bombay, India, via airport stops in Chicago and England. On the way, he became aware of a fly buzzing around in the cabin. He had no idea whether it came from Chicago or London. "But it's definitely headed to Bombay," he says.

Christensen sees an encouraging trend on the UW-Madison campus today. Many students are interested in studying infectious diseases, and nearly 200 undergraduates take his general parasitology course each semester. Some of them go on to do stints in the Peace Corps.

"For infectious diseases, it's a very scary time right now," he says. "People are starting to feel more vulnerable."