CHICAGO/GENEVA (Reuters) - The World Health Organization will hold a special session on Thursday on the Zika virus as the U.N. agency comes under pressure for quick action against the infection linked to thousands of birth defects in Brazil that is spreading through Latin America and the Caribbean.

WHO Director-General Margaret Chan was set to address the agency’s executive board in Geneva as countries took new steps on Wednesday to try to stop the mosquito-transmitted virus linked to the dangerous birth defect called microcephaly.

The United States said it will block people who have visited regions impacted by the virus from donating blood in a bid to fight its spread.

The U.S. Food and Drug Administration said it is working with other federal agencies, blood collection establishments and industry organizations to quickly implement “donor deferral measures for travelers who have visited affected regions in order to protect the blood supply in the United States.”

Brazilian President Dilma Rousseff said her country must wage war against the Aedes aegypti mosquito that spreads the virus, focusing on getting rid of the insect’s breeding grounds. The mosquito thrives in dense tropical cities, and Rousseff called for the elimination of stagnant water spots where it lives and reproduces.

U.S. researchers called on the WHO to take swift action. Georgetown University researchers urged Chan to heed the lessons of Ebola and called on the WHO to convene a special emergency session of health and infectious disease experts to consider declaring Zika a serious health crisis that endangers international public health.

Just convening the meeting would focus attention on funding and research, they said in an article published in the Journal of the American Medical Association.

The WHO’s leadership admitted last April to serious missteps in its handling of the Ebola crisis, which was focused mostly on three West African countries and killed more than 10,000 people. Some critics have said the WHO’s slow response played a major role in allowing the epidemic to balloon into the worst Ebola outbreak on record.

AIRLINES OFFER REFUNDS

Airlines are reacting to concern among pregnant women about travel to affected countries.

Chile-based LATAM Airlines LAN.SNLFL.N, Latin America's largest carrier, said it would offer refunds or the opportunity to change destination to pregnant women and their traveling companions with international flights booked to Brazil, Colombia, Mexico, and other affected countries.

U.S. airline United Airlines UAL.N expanded its program allowing customers with reserved tickets for travel to impacted regions to postpone their trips or obtain refunds with no penalty.

A tropical climate, dense cities, poor sanitation and slipshod construction provided ideal conditions for mosquito breeding grounds and the spread of the Zika virus in Brazil’s northeast, across the country and to more than 20 others throughout the Americas.

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There is no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya and causes mild fever, rash and red eyes. An estimated 80 percent of people infected have no symptoms, making it difficult for pregnant women to know whether they have been infected.

Recent models for how the disease is spreading predict "significant international spread by travelers from Brazil to the rest of the Americas, Europe, and Asia," Dr. Daniel Lucey, an infectious disease expert, and Lawrence Gostin, a global health law expert, wrote in the viewpoint article in the Journal of the American Medical Association. (bit.ly/1lSlgXa)

Lucey said only Chan had the authority to convene an emergency meeting of top experts on the International Health Regulations’ Emergency Committee to consider declaring Zika a “Public Health Emergency of International Concern.”

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“That in my view clearly needs to happen, and should have happened already,” Lucey said. Convening the meeting would allow for global coordination of travel advisories, research priorities and infection control measures, he said.

There was word of more cases outside the affected region among travelers who had been to those countries. Portugal said five people tested positive after recent trips to Brazil.

Four similar cases were reported in New York, as well as single cases in California, Minnesota, Virginia and Arkansas among people who had traveled to the affected region.