Declaring that the Zika virus is "spreading explosively," the chief of the World Health Organization announced that it will hold an emergency meeting of experts on Monday to decide if the virus outbreak should be declared an international health emergency.

At a special meeting Thursday in Geneva, WHO director general Dr. Margaret Chan said the virus — which has been linked to birth defects and neurological problems — is becoming more of a threat.

The level of alarm is extremely high.

— Dr. Margaret Chan, WHO director general

Chan said although there was no definitive proof that the Zika virus was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, "the level of alarm is extremely high." She also noted a possible relationship between Zika infection and Guillain-Barré syndrome, which can cause temporary paralysis.

"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions. The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities," Chan said.

Four reasons the WHO is concerned:

- The possible association of infection with birth malformations and neurological syndromes

- The potential for further international spread given the wide geographical distribution of the mosquito vector

- The lack of population immunity in newly affected areas

- The absence of vaccines, specific treatments, and rapid diagnostic tests.

The Zika virus was first detected in 1947 but was believed only to cause mild disease. Chan noted that "the situation today is dramatically different." According to the U.S. Centers for Disease Control and Prevention, active transmission of the Zika virus is occurring now in more than 20 countries, mostly in Central and South America. It is spread by the Aedes mosquito, which also spreads dengue and yellow fever.

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"Last year, the virus was detected in the Americas, where it is now spreading explosively. As of today, cases have been reported in 23 countries and territories in the region," Chan said in a statement. "The level of alarm is extremely high."

WHO called the special session in part to convey its concern about an illness that has sown fear among many would-be mothers, who have responded by dousing themselves in insect repellent or covering themselves head-to-toe in clothing in largely tropical areas.

Why WHO is 'deeply concerned'

Chan cited four main reasons why WHO is "deeply concerned" about Zika: The possible link to birth defects and brain syndromes, the prospect of further spread, a lack of immunity among people living in the newly-affected areas and the absence of vaccines, treatments or quick diagnostic tests.

Declaring a global emergency is akin to an international SOS signal and usually leads to more money and action to address an outbreak. The last such emergency was the devastating 2014 Ebola outbreak in West Africa, which killed more than 11,000 people.

Still, convening an emergency committee does not guarantee that a global emergency will be declared — WHO has held 10 such meetings to assess the Middle Eastern respiratory syndrome, coronavirus, and no emergency has followed.

One reason the U.N. health agency may be examining the Zika virus is because WHO was criticized for its slow response to Ebola; nearly 1,000 people died before the agency declared it to an international emergency. The Associated Press found that senior agency officials resisted the Ebola declaration for two months, citing political and economic reasons.

Marcos Espinal, WHO's director of infectious diseases in the Americas, said Brazil is conducting studies to determine if there is enough scientific evidence to declare that the Zika virus causes birth defects and neurological problems. He said they are hopeful Brazil may have data to share in a couple of months.

Brazil's Zika outbreak and the spike in microcephaly cases among babies have been concentrated in the poor and underdeveloped northeastern region of the country. The prosperous southeast, where São Paulo and Rio de Janeiro are located, is the second hardest hit area.

Rio is of special concern, since it will host the Olympic Games in August, which are expected to draw millions of people from around the world to the country.

Earlier this week, officials in the city ramped up the fight against the mosquitoes that spread Zika, dispatching a team of fumigators to the Sambadrome, where the city's Carnival parades will take place next month.

There is no specific treatment or vaccine for Zika. The virus is related to dengue, which scientists struggled for decades to develop a vaccine for. The first such shot made by Sanofi Pasteur was licensed last year in Brazil.

Threat to the United States?

One American health official says he doubts the U.S. is vulnerable to a widespread outbreak of a virus linked to a wave of birth defects in Brazil.

Dr. Anthony Fauci, who heads the National Institute of Allergy and Infectious Disease, appeared on "CBS This Morning." He said administration officials do not believe there are major ways of spreading the virus "other than by mosquito bites."

Hopefully, he added, it can be kept at bay with "mosquito vector control."

President Barack Obama hosted a meeting of federal health specialists on the issue earlier this week.

Putin: Zika a 'nasty thing'

Russian President Vladimir Putin instructed his health minister to prevent the spread of Zika virus in Russia, saying the country needs to pay "close attention to this."

"There's some nasty thing spreading from Latin America, some sort of a virus. Mosquitoes spread it. They, of course, will not fly across the ocean, but the infected people can fly, and they fly," Putin reportedly said at a meeting with the government members.

Only isolated cases in of the disease have been found in Europe so far.

Reporting based on the Associated Press.

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