Scientists, attempting to halt Dengue fever, for the first time released mosquitoes into the wild that had been genetically modified to pass on deadly DNA that kills their offspring.

About 19,000 lab-altered insects were released into 25 acres on Grand Cayman Island in 2009, according to a study, published yesterday in the journal Nature Biotechnology. Later tests showed they made up about 16 percent of the mosquito population and that the fatal gene was carried by about 10 percent of larvae. Scientists estimated the modified insects — all males — were about half as successful in mating as normal.

There are as many as 100 million cases of Dengue reported each year worldwide, making it one of the most medically significant viruses carried by mosquitoes, the report said. There’s no vaccine, boosting the need to limit the insects that carry it, the researchers said. The experiment, by scientists from closely held Oxitec Ltd., a biotechnology company based in Oxford, England, has spurred concern that there may be unintended
environmental consequences.

“These data also allow us tentatively to estimate how many mosquitoes might need to be released in this area to suppress the target population,” the researchers said.

The use of genetically enhanced mosquitoes was discussed in a series of articles in the magazine Scientific American this month. In those articles, Helen Wallace, the director of GeneWatch UK, said she was concerned that the new form of insect would become part of a complex system involving predators and prey that scientists have no control over.