Areas most at risk are those where Aedes mosquitoes may spread the virus, like the Black Sea coast of Russia and Georgia and the island of Madeira.
Countries with a moderate risk include France, Spain, Italy and Greece, while the risk in the UK is low.
The UN agency is not issuing any new travel advice at this time.
The WHO is calling on countries to eliminate mosquito breeding sites and to make sure that people - particularly pregnant women - have information on the potential harmful consequences of the disease.
The agency says most countries that could be affected are well prepared to pick up any new cases and deal with them quickly - but others must bolster their ability to diagnose the virus.
Zika has been linked to a rise in brain defects in babies and the virus has been seen in more than 50 countries during this outbreak.
Researchers have deliberately infected developing mice pups and tiny clusters of lab-grown brain cells so they could observe and measure the damage that ensued.
The results of these experiments nail down the causal link between Zika and the rash of birth defects seen over the past year in Brazil. And they begin to show how a virus thought to be little more than a pest crosses the placental barrier and makes a beeline for a fetus' brain.
The new studies, published Wednesday in the journals Nature, Cell and Cell Stem Cell, suggest that when the Zika virus takes hold in the first trimester, it makes its way quickly to the uterus and to cells that line and normally help protect the placenta.
While that firewall between mother and child is still immature, Zika attacks those cells and penetrates the placental barrier. As it does so, the virus also disrupts the growth of placental blood vessels, limiting blood flow to the baby and stunting fetal development.
If the fetus survives the onslaught, the virus will make its way to its developing brain. There it will wreak destruction on the stem cells that are meant to develop into a mature organ.
As Zika replicates madly, these stem cells and immature neurons die off in droves, the researchers found. The differentiation of tissues that normally produces a healthy brain goes awry — sometimes subtly, sometimes horribly.
In humans, scientists have observed microcephaly, diffuse calcium deposits and brain structures that are either abnormally large or small. In surviving mouse pups, researchers found that brains infected in utero were abnormal in a variety of ways, not all of them identical to effects seen in humans.
"Zika alone is enough" to cause these effects, Diamond said.
That collective finding puts to rest many scientists' suspicions that Zika might need to interact with some other factor — say, a past infection with dengue fever — for brain damage to result in a fetus.
"That’s not to say there aren't other factors that may predispose people to get Zika," Diamond said. "But you don’t need all those things" for brain damage to occur, he added.
Nature - The Brazilian Zika virus strain causes birth defects in experimental models
Calls to postpone the Olympic games in Brazil
Harvard Public Health Review is calling for the Olympic games in Brazil to be delayed
- Rio de Janeiro is more affected by Zika than anyone expected, rendering earlier assumptions of safety obsolete. When in January the International Olympic Committee declared Rio a “safe environment” for the Games, it was speculating, because Brazil’s Ministry of Health temporized until February to declare Zika a notifiable disease and begin counting cases. Now with those data finally available, the situation seems not so safe: Rio de Janeiro’s suspected Zika cases are the highest of any state in in Brazil (26,000), and its Zika incidence rate is the fourth worst (157 per 100,000). Or in other words: according to the Brazil’s official data, Rio is not on the fringes of the outbreak, but inside its heart.
- Although Zika virus was discovered nearly seventy years ago, the viral strain that recently entered Brazil is clearly new, different, and vastly more dangerous than “old” Zika. Phylogenetic mapping demonstrates that this particular virus arrived in Brazil from French Polynesia in 2013. Although the danger went unnoticed in French Polynesia at first, retrospective analyses now show that the risk of microcephaly increased by 23 to 53 fold.
- Brazil’s Zika inevitably will spread globally — given enough time, viruses always do — it helps nobody to speed that up. In particular, it cannot possibly help when an estimated 500,000 foreign tourists flock into Rio for the Games, potentially becoming infected, and returning to their homes where both local Aedes mosquitoes and sexual transmission can establish new outbreaks
- When (not if) the Games speed up Zika’s spread, the already-urgent job of inventing new technologies to stop it becomes harder. Basic Zika research is already on the fast track, and with time, the odds are excellent that scientists can develop, test and prove an effective Zika vaccine, antiviral drug, insecticide, or genetically-engineered mosquito. But by spreading the virus faster and farther, the Games steal away the very thing – time – that scientists and public health professionals need to build such defenses.
- Proceeding with the Games violates what the Olympics stand for. The International Olympic Committee writes that “Olympism seeks to create … social responsibility and respect for universal fundamental ethical principles”. But how socially responsible or ethical is it to spread disease?
SOURCES- BBC News, CBC, Harvard public health review, LA Times, Nature