GENEVA (AP) — The World Health Organization says that the traditional insecticide spraying methods have had no significant impact in slowing dengue, raising significant questions about how officials might stop the spread of the Zika virus, also spread by mosquitoes.

At the conclusion of a Zika research and development meeting on Wednesday, WHO's Marie-Paule Kieny said that "evidence is missing" that the classical ways of fighting dengue have made any substantial dent in reducing cases. She says the same might apply to Zika.

Officials also considered whether innovative ways like using genetically modified mosquitoes might be necessary to stop the outbreak, but noted that "extreme rigor" must be used in evaluating such new tools.

Last month, WHO declared the explosive spread of Zika in the Americas to be a global emergency.