Regular summer travel to and from Zika-affected areas will place some parts of Europe at risk of local spread of the virus in the coming months, a new study suggests.
Barcelona, Milan, and Rome are the most likely to be affected, with the risk highest in July and August, according to the research, published in the journal EBioMedicine. Also at risk is Madeira, an archipelago of islands that’s part of Portugal but is located in the Atlantic, west of Morocco.
The findings will be of interest to an expert panel that will meet Tuesday to consider whether the World Health Organization should urge the International Olympic Committee to postpone the Rio Olympics. At least 227 academics have signed a petition arguing that it’s unsafe to hold the Olympics and Paralympics in Rio this summer.
Olympic organizers have estimated the games will draw 500,000 to 600,000 visitors to Brazil. Some scientists worry that the travel will speed global spread of the Zika virus, projecting it to new places in the bloodstreams of athletes, officials, and spectators who get infected in Rio.
But travel between the Zika-affected countries and Europe is already substantial, the study reported. Paris, for instance, normally gets between 120,000 and 200,000 travelers a month from Zika-affected countries during July and August. London gets as many as 130,000, and Madrid isn’t far behind.
Zika is spread mainly through the bite of infected mosquitoes. The primary species involved in its spread is Aedes aegypti, which isn’t found much in Europe. Another species, Aedes albopictus, is more commonly found, especially along the Mediterranean. While there is evidence that these mosquitos may be able to transmit Zika, it’s uncertain if they will play a major role in spreading the virus.
“It is quite possible that we go through the year and we don’t actually see anything in southern Mediterranean Europe,” senior author Dr. Kamran Khan, an infectious diseases specialist at St. Michael’s Hospital in Toronto, told STAT.
“The risk is potentially there. But a lot of this hinges on how Aedes albopictus will actually act as a vector for Zika virus. And that’s still a bit of an unknown,” he said.
(The question of Aedes albopictus’s role has huge implications for the United States as well, broadening dramatically the expanse over which local spread of Zika might be able to occur.)
Aedes aegypti prefer to feed on people, so they breed in and close to homes. Aedes albopictus feed on a variety of animals and therefore don’t need to be in as close proximity to people.
Madeira does have Aedes aegypti mosquitoes and may face a higher risk of seeing Zika spread take off there, the authors suggested.
It gets far fewer travelers from countries affected by Zika — between 500 and 2,500 a month in July. But Madeira had a large dengue epidemic in 2012, the authors noted. It is generally assumed places that have a history of outbreaks of dengue or chikungunya could suffer Zika outbreaks. All three viruses are spread by the same mosquitoes.
Khan cautioned that the research doesn’t address how probable it is that local spread will take place. The right mosquitoes and an influx of infected travelers are key ingredients for local spread, but other factors like population density and housing conditions could also influence the likelihood of an outbreak.
“We’re saying if an outbreak is going to happen, these are the places we would expect it to be more likely to occur,” said Khan, whose research focuses on projecting how infectious diseases spread using international travel data.
Knowing where the risk is highest should help public health officials target mosquito control and public education efforts, he said.
The study doesn’t address how holding the Olympics in Rio might influence the spread of the Zika virus. The travel data — drawn from worldwide airline ticket sales compiled by the International Air Transport Association — reflects only regular, non-Olympic travel.
Khan said at this point it would be difficult to factor in Olympic travel, because concern about Zika may lead some unknown portion of people who were thinking of going to the games to reconsider.
Even if it doesn’t, Olympic travel would always displace some ordinary travel to a host city. People who might ordinarily go to Rio in August for business or to visit friends or family may go at another time, to avoid premium prices for airfares and hotel rooms during the Olympics, for instance.