The transgenic mosquitoes have generated opposition, with complaints ranging from doubts about their effectiveness to concern about whether the impact on the ecosystem has been adequately studied.
“There are scenarios in which the dengue could worsen,” said Dr. Helen Wallace, a mathematician who’s the executive director of GeneWatch UK, a British group that monitors genetic science. Wallace said that if the program succeeded in reducing the population of the Aedes aegypti mosquito, there was nothing that prevented some other type of mosquito from adapting to fill its niche and carry the virus.
“It could be harder to get rid of than the targeted mosquito,” she said.
Another critic, Camilo Rodriguez-Beltran, a French-trained biosecurity expert working in Chile, said the gene-altered mosquitoes could cross international boundaries, violating international treaties on biosafety.
“All consequences that could occur are unforeseen,” Rodriguez said. “It’s been developed very rapidly.”
A Panamanian environmental lawyer, Olmedo Carrasquilla, said his nation should use better techniques to educate the public on mosquito control.
“Why invest millions in methods and technology when there are no guarantees? When we know there are rudimentary methods that work?” he asked.
Sosa, the health official, dismissed some of the criticism, especially about the transgenic mosquito’s potential impact on the ecosystem.
“The mosquito dies in a few days. So it’s very improbable that it will go into the environment or into another organism,” Sosa said. “It’s not that we are doing something that is environmentally unfriendly.”
Mosquito control officials in the Florida Keys announced last year that they were considering testing the Oxitec mosquito. In response, a Key West businesswoman gathered more than 120,000 names on a petition, halting the plan temporarily.
Hadyn Parry, the chief executive of Oxitec, said in a telephone interview that his company thought its transgenic mosquito was safer than using insecticides, which he asserted “affect all insects in a given area” and can filter through the ecosystem and persist.
By using dead-end mosquitoes, only one species is affected, he said.
“It’s a highly targeted sniper’s rifle instead of a blunderbuss that takes out everything it finds,” Parry said.
Mosquitoes generally spend their three-week life spans in an area 200 yards from where they were born. The Oxitec mosquitoes, he said, can always be detected.
“You can actually look at any of our insects under a fluorescent light, and you’ll see a red color. This is so important when it comes to monitoring. We can tell how far our insects fly and where they are going,” Parry said.
Oxitec mosquitoes have been tested in the Cayman Islands, Malaysia and Brazil, Parry said, and he expects tests in India and in the Florida Keys, if the U.S. Food and Drug Administration and Florida authorities give final approval.
Officials in the Cayman Islands kept the 2009 and 2010 tests secret for more than a year, angering some residents even as Oxitec reported that it had reduced the mosquito population in a test area by 80 percent.
In tests earlier this year in the village of Mandacaru, in Brazil’s northeastern Bahia state, Oxitec reported 96 percent suppression of the dengue mosquito.
Health experts say they need additional tools to combat dengue, and they encourage the global pharmaceutical companies that are racing to create a vaccine. But research is costly and slow, partly because there are four virus types for dengue, each different. A French company, Sanofi Pasteur, announced in July that its trials in Thailand found that a potential vaccine was effective against several types of dengue but not Type 2, a more disease-producing strain.
The most severe cases, once called hemorrhagic dengue because of the bleeding they provoke, often are caused when a person who has contracted one type of the dengue virus is later sickened by a different type.