The risk will be greatest in countries where the mosquitoes spreading the disease, Aedes aegypti, are found naturally.
In the first risk assessment of Zika transmission in Europe, published Wednesday,
WHO investigated the risk of an outbreak in the region's 53 member states and identified the highest likelihood of transmission to be on the island of Madeira and the north-eastern coast of the Black Sea.
Another 18 countries (33% of the region) were determined to have a moderate likelihood of an outbreak. These countries, in order of likelihood, are France, Italy, Malta, Croatia, Israel, Spain, Monaco, San Marino, Turkey, Greece, Switzerland, Bulgaria, Romania, Slovenia, Georgia, Albania, Bosnia and Herzegovina and Montenegro.
"In [these] countries in the European region, especially in the Mediterranean basin, there is a moderate likelihood of local Zika virus transmission," said Dr. Nedret Emiroglu, director of the Communicable Diseases and Health Security Division, WHO Regional Office for Europe
The remaining 36 countries were decide to be at low, very low, or no likelihood of a Zika outbreak due to the absence of the mosquito vectors and unsuitable climates for them to survive if the mosquitoes were to spread there.
Overall levels of risk across the region were determined using this likelihood of the virus to spread within countries, but also on their existing services and capacity to prevent, or contain, an outbreak if transmission was to occur. Population density, urbanization and any history of transmission of Dengue fever -- which spreads via the same mosquitoes -- were also taken into account.
On combining these factors, the risk was determined to be low to moderate during late spring and summer across the European region.
"The new evidence published today tells us that there is a risk of spread of Zika virus disease in the European region and that this risk varies from country to country," said Dr. Zsuzsanna Jakab, WHO regional director for Europe, in a statement
. "With this risk assessment, we at WHO want to inform and target preparedness work in each European country based on its level of risk."
The organization is now calling on countries at highest risk to strengthen their national capacity and prioritize the activities needed to prevent a large Zika outbreak from occurring.
Their recommendations include strengthening mosquito control, equipping health teams with the ability to diagnose cases accurately, detect cases early and to spot when transmission of the disease is occurring locally -- meaning infections are spreading from person-to-person and not only occurring in travelers returning from outbreak hotspots. WHO also highlighted the need to inform people to remove any potential breeding sites for mosquitoes, such as shallow water.
"We stand ready to support European countries on the ground in case of Zika virus outbreaks," Emiroglu said. "Our support to countries in the region to prepare for and respond to health risks such as Zika is a key aspect of the reform of WHO's work in emergencies."
The Zika virus poses the greatest risk to pregnant women because an infection can cause microcephaly
and other birth defects in babies. The world health body is continuing to recommend they, and others at risk, protect themselves from infections, including through sexual transmission.
The virus has also been linked to Guillain-Barré Syndrome
, a rare disease in which a person's own immune system damages the nerve cells, causing muscular weakness and, in some cases, paralysis.
"Summer is approaching fast and the risk of Zika spreading to Europe is real," said Jimmy Whitworth, professor of International Public Health at the London School of Hygiene and Tropical Medicine. "WHO reports that most countries in the European region are well prepared...however, countries in Southern Europe, including France and Italy, need to be especially vigilant and it's important that holidaymakers follow public health advice while abroad, "
As of May 11 2016,
58 countries and territories are reporting continuing mosquito transmission of the Zika virus.
The organization will convene in June to re-examine the risk of an outbreak and identify any gaps in a country's ability to control the Zika virus.