Zika infections are expected to continue rising in the Asia-Pacific region, where authorities are increasing surveillance, preparing responses to complications and collaborating on information about the disease, the World Health Organization said Monday.

Complicating the fight against the virus, spread by mosquitoes, is the lack of a "foolproof" approach to mosquito control, as shown by decades of efforts to contain dengue virus, WHO Director General Margaret Chan said in her address to a Western Pacific regional meeting of the world health body.

In this Jan. 18, 2016, file photo, a female Aedes aegypti mosquito, known to be a carrier of the Zika virus, acquires a blood meal on the arm of a researcher at the Biomedical Sciences Institute of Sao Paulo University in Sao Paulo, Brazil. Andre Penner / AP

She said other questions included why the first signs of the virus's existence in the Asia-Pacific region came from travelers whose infections were confirmed once they returned home.

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"Is this weak surveillance an indication of population-wide immunity, or proof that the virus has somehow acquired greater epidemic potential?" she asked.

Zika symptoms are mild and no deaths have been reported globally, said Dr. Li Ailan, director for health security and emergencies at WHO's Western Pacific regional office. But she said based on WHO's risk assessment, Zika viral infection will continue to spread in the region and authorities are preparing for complications.

The complications include like microcephaly and Guillain-Barre syndrome. Babies born to Zika-infected mothers have been found to have microcephaly, or a birth defect where the head is abnormally small and brains might not have developed properly. Guillain-Barre syndrome is a disorder in which the body's immune system attacks part of the peripheral nervous system.

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The Western Pacific region is the second most Zika-affected region in the world, Li said. Nineteen of its 27 countries have reported Zika cases since 2007 and 13 of them this year.

Dr. Shin Young-soo, WHO's Western Pacific regional head, said they are working very hard to increase surveillance and detection of Zika, and long-term response to the disease are among the topics to be discussed at the five-day confer