An Aedes aegypti mosquito seen on a human hand in a lab of the International Training and Medical Research Training Center in Colombia. (Jaime Saldarriaga/Reuters)

The dozens of countries now battling the Zika virus should try an array of tactics — from traditional spraying to genetic engineering — to control the massive mosquito population responsible for spreading the disease, the World Health Organization said Tuesday.

“Given the magnitude of the Zika crisis, WHO encourages affected countries and their partners to boost the use of both old and new approaches to mosquito control as the most immediate line of defense,” the agency said in a new assessment.

In advocating for more aggressive mosquito control, the WHO noted that there currently is no treatment or vaccine available for Zika, and that it could take at least 18 months before large-scale vaccine trials begin. In addition, diagnostic testing for the disease remains unreliable and not widely available. Given those shortcomings, the agency said that combating the source of the virus's spread remains the most promising way to mitigate the the potentially devastating illnesses linked to Zika — including serious birth defects and a paralyzing autoimmune disorder known as Guillain-Barre Syndrome.

"If these presumed associations are confirmed, the human and social consequences for the over 30 countries with recently detected Zika outbreaks will be staggering," the agency wrote.

WHO endorsed several methods Tuesday that countries might experiment with to kill the Aedes mosquitoes that spreads Zika, dengue, chikungunya, yellow fever and other viruses.

At the top of that list: Using genetically-modified mosquitoes that are designed to reduce overall pest populations. One British company called Oxitec has already been testing its genetically-engineered mosquito in Brazil and Panama, and the WHO noted that a previously conducted trial in the Cayman Islands showed significant reductions in the Aedes population there. Oxitec's mosquitoes are altered males that are released into the wild to mate with females. The offspring never reach adulthood, blocking the next generation of disease carriers. Another trial is awaiting approval in the Florida Keys, where it has met with resistance from environmental activists. The WHO on Tuesday recommended "further field trials and risk assessment to evaluate the impact of this new tool on disease transmission."

Another technique under development involves mass release of male insects that have been sterilized by low doses of radiation. When they mate with females, the resulting eggs are not viable, leading the insect population to die out. The approach has proven effective in controlling agriculturally important pests, the WHO said. In addition, another method includes using male mosquitoes carrying the Wolbachia bacteria, which are found in roughly 60 percent of insects. The bacteria do not infect humans or other mammals, but when female mosquitoes mate with males carrying the bacteria, their eggs do not hatch.

Some countries are using different techniques altogether. In El Salvador, for instance, the government is introducing larvae-devouring fish into water storage containers to combat mosquito breeding, the WHO said.

The WHO repeatedly has said that a well-executed mosquito control plan can reduce the transmission of mosquito-borne viruses. But the agency acknowledged Tuesday that such efforts are "complex, costly, and blunted by the spread of insecticide resistance" among mosquitoes. In addition, few countries have dedicated, well-funded programs for mosquito control.

WHO officials said that while the use of fogging to kill adult mosquitoes provides the most visible evidence of governmental action, the difficult work of eliminating mosquito breeding sites is more effective for protecting people from bites. The Aedes mosquito can breed in minute amounts of water, such as a bottle cap, a plant container, a pet water bowl or a bird bath. It's eggs can survive for long periods in a dry state, only to hatch once submerged in water. All of that makes the Aedes difficult to control.

Health officials have said that while government actions are essential in combating mosquito populations, individuals in Zika-affected areas also must take measures to avoid being bitten. That includes using mosquito repellent, wearing long sleeves and pants and placing screens on windows and doors.


Read more:

Why the United States is so vulnerable to the alarming spread of Zika virus

Beyond Zika: The map of things that keep NIH’s infectious diseases director up at night

Zika and sex: Seven key things you need to know about the case in Dallas

FAQ: What is Zika, and what are the risks as it spreads?

‘Zika isn’t important': The infuriating case of a scientist’s search for funding.

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