Keeping mosquitoes healthy for humans' sake.

Efforts to stop the spread of mosquito-transmitted diseases usually focus on killing the insects. Although researchers are developing novel techniques for reducing mosquito populations (SN: 4/27/96, p. 270), the bugs and the diseases they carry continue to spread.

So some scientists are taking a new approach-trying to keep mosquitoes free of disease. They hope eventually to genetically engineer the insects to resist viral infections and pass that trait on to their offspring. A Colorado State University team in Fort Collins took the first step toward this goal by using a piece of RNA to prevent the dengue virus from replicating in mosquito saliva. Mosquitoes treated in this way cannot then transmit the virus.

The dengue virus, carried by Aedes aegypti mosquitoes, poses an increasing threat to people living in warm regions from Texas to Asia (SN: 4/6/96, p.218). Dengue fever causes flulike symptoms in humans, and dengue hemorrhagic fever can kill infants and young children.

In their study, Kenneth E. Olson and his coworkers attached a small piece of dengue virus RNA to Sindbis, a common virus that has little effect on mosquitoes, they report in the May 10 Science. They then infected mosquitoes with the engineered Sindbis virus, as well as with normal dengue virus. The engineered virus replicated in almost all of the mosquitoes' tissues, including their salivary glands. While replicating, the dengue RNA molecules from the engineered virus jammed the replication machinery of the normal dengue, although how they did so remains unclear.

The researchers found no dengue virus in the mosquitoes' saliva. Moreover, when the scientists injected the saliva into uninfected mosquitoes, no signs of the virus appeared.

This is the first successful attempt to confer "intracellular immunity" on an important human pathogen by introducing a foreign gene into an organism, Anthony A. James of the University of California, Irvine asserts in a commentary accompanying the report.

The method appears to have had some success with other mosquitoborne viruses as well. In a study in the April 30 Proceedings of the National Academy of Sciences, Ann M. Powers and other members of the Colorado team report that they halted replication of the LaCrosse virus-which causes encephalitis in children-in A. triseriatus mosquitoes. However, the Sindbis virus failed to infect the salivary glands of A. triseriatus, so the mosquitoes could still transmit the disease.

The scientists hope to use dengue and LaCrosse RNA that interfere with viral replication to meet their larger goal of ensuring that mosquitoes can pass disease resistance on to their offspring. But "that's quite a big step," notes coauthor Barry J. Beaty.

Researchers have created transgenic mosquitoes using marker genes, but these genes don't alter insects' ability to transmit diseases. They fashioned these transgenic bugs by inserting DNA into mosquito eggs. A new technique now being tested involves inserting DNA into retroviruses, RNA-containing viruses that may integrate the DNA directly into the mosquito genome. - T. Adler