While Europe handled hundreds of imported cases during the virus outbreak three years ago, it never had a native case – where local mosquitoes developed and spread the virus – before now.
All three patients developed symptoms within a few days of each other, ECDC said, which meant they were probably part of the same transmission cycle. They have recovered, and the risk to residents and travelers to the region is low, the organization added.
But the global warming climate and increased travel between continents make tropical diseases including Zika more likely to flourish in Europe, experts have said.
The results are "the first time locally acquired Zika cases have been identified, posing new challenges for the control of these diseases," Moritz Kraemer, a researcher on infectious diseases at the University of Oxford, told CNN.
Researchers had expected some insect bite infections to occur in Europe, as previously seen with dengue fever, Kraemer added. But it was "unexpected" to see the Zika transmission, as it is usually associated with Aedes aegypti mosquito.
While that mosquito is not usually found in southern Europe, another – Aedes albopictus – is.
The mosquito "has become common in parts of southern France, where it has probably also been responsible for transmitting dengue. It has also been detected throughout southern Europe and sporadically further north," Anna Checkley, a consultant in tropical medicine at the hospital for tropical diseases (HTD), said.
"Warmer temperatures favor its survival, and as we enter into the winter it is much less likely that we will see further new cases. (But) if global temperatures increase, this mosquito can spread further north in Europe and we can see small clusters of cases further north. "
The UK's Ministry of Health warned earlier this year that climate change could lead to tropical diseases reaching the UK – and a study by the Kraemers team at the University of Oxford has found that climate change can expose half of the world's population for disease-spreading mosquitoes in 2050.
Risks to pregnant women
Zika virus was first identified 70 years ago in the Zika forest in Uganda. Isolated small-scale outbreaks have occurred in various parts of the world, including Africa, Southeast Asia and the Pacific Islands, but no major complications had been linked to the virus until it arrived in Brazil, which has reported a major outbreak to the World Health Organization since March 2015 and soon extended to Central and South America.
As of March 2018, the US Centers for Disease Control and Prevention estimated that there were at least 90 countries and territories with active Zika virus transmissions.
Most people infected with Zika virus will not have symptoms. Of those who do, fever, rash, joint pain and conjunctivitis (red eyes) are the most common symptoms of Zika virus. Some patients may also experience muscle pain or headaches.
The main concern is women who are pregnant or may become pregnant, as the virus can cause microcephaly – a neurological disorder that results in children being born with abnormally small heads, which in turn can cause serious developmental problems and sometimes death.
A Zika infection can cause other birth defects, including eye problems, hearing loss, and impaired growth. Miscarriage can also occur.
"The risk of travelers acquiring Zika in southern Europe remains low," Checkley reminded tourists. "However, all travelers who spend time outdoors should be aware of precautions to avoid biting as there are many infections that can be transmitted by biting mosquitoes and ticks."