Jamey Keaten and Mike Stobbe

GENEVA: The Zika virus is “spreading explosively” in the Americas, which could see up to 4 million cases over the next year, international health officials said Thursday, announcing a special meeting next week to decide if they should declare an international health emergency.

The warning from the World Health Organization came amid a call to arms by officials on both sides of the Atlantic over the mosquito-borne virus, which has been linked to a spike in a rare birth defect in Brazil.

Brazil’s president — noting there is no medical defense against the infection — called for a crusade against the mosquitoes spreading it.

“As long as we don’t have a vaccine against Zika virus, the war must be focused on exterminating the mosquito’s breeding areas,” said President Dilma Rousseff.

The U.N. health agency called the special session in part to convey its concern about an illness that has sown fear among many would-be mothers. It may also have acted quickly because the agency was criticized for its slow response to the Ebola epidemic in West Africa.

Meanwhile, U.S. health officials said Thursday while they have not yet seen spread of the disease in the 50 states, the number of U.S. travelers infected over the last year in the Caribbean or Latin America has climbed to 31.

The Zika virus was first discovered in Africa in 1947. But until last year, when it was found in Brazil, it had never been a threat in the Western Hemisphere.

The virus causes no more than a mild illness in most people. But there is mounting evidence from Brazil suggesting infection in pregnant women is linked to abnormally small heads in their babies — a birth defect called microcephaly.

Earlier this month, U.S. health officials advised pregnant women to postpone visits to Brazil and other countries in the region with outbreaks.

“For the average American who’s not traveling, this is not something they need to worry about,” said Dr. Anne Schuchat, of the U.S. Centers for Disease Control and Prevention.

But “for people who are pregnant and considering travel to the affected areas, please take this seriously,” she added. “It’s very important for you to understand that we don’t know as much as we want to know about this yet.”

In Geneva, WHO Director-General Dr. Margaret Chan noted it had been less than a year since the virus arrived in the Americas, “where it is now spreading explosively.”

Although there is no definitive proof that the Zika virus is behind the spike in brain defects in Brazil, “the level of alarm is extremely high,” she added.

“The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions,” Chan said.

Researchers are also looking into a potential tie between Zika infections and cases of Guillain-Barre syndrome, which can cause temporary paralysis.

According to the CDC, the Zika virus is now in more than 20 countries, transmitted by the same mosquito that spreads other tropical illnesses such as dengue and yellow fever.

Sylvain Aldighieri, head of WHO’s epidemic response team in the Americas, estimated there could be 3 million to 4 million Zika infections in the region over the next year. The Aedes mosquitoes that spread Zika are widespread.