SOURCES: March 10, 2016, media briefing with Anthony Fauci, M.D., director, U.S. National Institute of Allergy and Infectious Diseases, and Tom Frieden, M.D., M.P.H., director, U.S. Centers for Disease Control and Prevention; March 11, 2016, The New York Times
FRIDAY, March 11, 2016 (HealthDay News) -- U.S. health officials on Friday gave tentative approval to a field test in the Florida Keys of mosquitoes genetically tweaked to help curb the spread of the Zika virus.
Officials at the U.S. Food and Drug Administration said they made the preliminary determination that the test of the genetically engineered insects poses little harm to people, animals or the environment, The New York Times reported.
But, final approval for the trial won't come until the FDA considers comments from the public, which is likely to take months, the newspaper said.
The mosquitoes -- which have already been the subject of controversy among Florida residents -- are being developed by a British company, Oxitec. The company says male mosquitoes can pass along a gene during mating with wild females that causes premature death in offspring -- potentially lowering mosquito populations.
It is thought that introducing the modified mosquitoes into the wild might curb the spread of serious mosquito-borne illnesses, such as the Zika and dengue viruses. Tests similar to the one proposed in Florida have already shown success in reducing mosquito numbers in Brazil and other countries, the Times reported.
However, introduction of the gene-modified insects has met with opposition from some residents in the Key West area, who worry about unexpected consequences, the Times noted.
The FDA recently expedited the approval process for Oxitec's mosquito, due to the expected arrival of Zika-carrrying mosquitoes in Florida as the weather warms. The agency's decision on Friday is based on a 300-page draft environmental assessment submitted to the FDA by Oxitec, the Times said.
The decision comes one day after U.S. health officials issued a worrisome assessment on the continued spread of the Zika virus. The pathogen is already suspected of causing thousands of birth defects in Brazil and has made inroads into Puerto Rico.
Federal researchers said Thursday that they learning much about the virus. But the more they learn, the more they realize how much they don't know, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said during a morning media briefing.
"Unfortunately, the more we learn the worse things seem to get," Fauci said.
Researchers believe -- but haven't proven -- that the mosquito-transmitted Zika virus is linked to microcephaly, a birth defect that causes babies to be born with unusually small heads and underdeveloped brains. The virus is suspected of causing an epidemic that started last spring in Brazil, where there have been more than 5,600 suspected or confirmed cases of microcephaly.
The virus has also been linked to Guillain-Barre syndrome, an immune system disorder that can occasionally lead to a fatal form of paralysis.
But officials said Thursday that it's not known if Zika causes other birth defects besides microcephaly, or other neurological disorders.
Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said: "We are learning more about Zika every day. The link with microcephaly and other possibly serious birth defects is growing stronger every day. The link to Guillain-Barre syndrome is likely to be proven in the near future, and the documentation that sexual transmission is possible is now proven."
First discovered in Uganda in 1947, the Zika virus wasn't thought to pose major health risks until last year, when it became clear that it posed potentially devastating threats to pregnant women.
But, for most other people the virus offers little threat -- approximately 80 percent of people who become infected never experience symptoms.
Meanwhile, the virus continues to spread in Latin America and the Caribbean.
It is not expected to pose a significant threat to the United States mainland, federal health officials have said in the past.
In Puerto Rico, however, the situation is "of great concern," Frieden said.
"Puerto Rico is on the frontline of the battle against Zika," said Frieden, who just returned from the island. "And it's an uphill battle."
By next year, Frieden said, there could be hundreds of thousands of cases of Zika in the territory, and "thousands of infected pregnant women."
The CDC is trying to help stem the tide of Zika in Puerto Rico, he said. The agency has sped up production of kits to test for the virus and has started to distribute prevention kits. The prevention kits contain mosquito repellent and condoms, and information for women on how they can reduce their risk of infection while pregnant or planning to become pregnant, he added.
The CDC is also trying to find an effective insecticide because "we are finding a wide resistance to some insecticides," Frieden said. In addition, the CDC and local officials in Puerto Rico are looking to get window screens and air conditioners installed in homes to thwart the mosquitoes.
"We have never had a mosquito-borne infection that could cause serious birth defects on a large scale," he said. "There is no silver bullet to control the mosquito or dramatically reduce the risk of Zika infection on a population-wide basis."
All these efforts take money, Frieden said. "It's worth trying anything that might protect women," he said. "We know we won't be able to protect 100 percent of women, but for every single case of Zika infection in pregnancy we prevent, we are preventing a potential personal and family tragedy," he said.
Last month President Barack Obama asked Congress for $1.9 billion to fight the Zika virus. To date Congress has not approved the funding and both Frieden and Fauci expressed concern that efforts to fight Zika are in jeopardy if the funds aren't forthcoming.
For instance, Fauci said efforts to develop and test a vaccine could be imperiled if the money were held up. A vaccine would take several years to test and gain U.S. Food and Drug Administration approval. Although a vaccine could be in phase 1 trials by fall, that is just the first step, and the vaccine might not be available for general use until 2018, he said.
One goal is to create a vaccine that can be given to children before they reach puberty to prevent Zika, Fauci said. "We cannot do what needs to be done in a sustained way without those resources," he said.
The CDC has this advice for pregnant women:
The Zika virus has now spread to over 33 countries and territories in Latin America and the Caribbean. The World Health Organization estimates there could be up to 4 million cases of Zika in the Americas in the next year.
For more on Zika virus, visit the U.S. Centers for Disease Control and Prevention.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.