Travellers to FIFA's World Cup in Brazil now have a guide to which host cities are at highest risk for dengue fever.

Scientists in Spain have found a way to pinpoint the host cities most likely to be dengue hot spots: Natal, Fortaleza and Recife.

They used climate data and combined it with historical disease trends and population patterns to predict that these three northeastern venues will see over 300 cases per 100,000 people.

Dengue fever is on the World Health Organization's "10 most deadly viruses list." Each year, more than 50 million people are affected worldwide.

The fever is caused by a virus transmitted by mosquitoes called Aeges aegypti. The insects prefer areas of high population density.

The mosquito-borne virus is more common after the rainy season, which in Brazil is March to May. It reproduces in standing water, like malaria, and prefers biting times in the early morning and late afternoon.

Symptoms include fever, headache, rash muscle aches and fatigue.

The 2014 FIFA World Cup in June will have hundreds of thousands of people in attendance. Concerns about a dengue outbreak have been voiced — Brazil has seen seven million cases of dengue over the last 13 years, more than anywhere else in the world.

Study author David Harley says this information on high- and medium-risk areas can help local health authorities prepare appropriately and will tell Canadians in which cities they need to be most careful.

And it’s not just Brazil. According to the WHO, cases of dengue have increased worldwide from 122,174 between 1970 to 1979 to 968,564 from 2000 to 2007.

Dr. Jay Keystone, an infectious disease expert at the University of Toronto and author of a recent paper surveying returned travellers from Brazil, says dengue is the most frequent, known cause of fever in returned travellers from Brazil and seen in 45 out of 1,000 ill, returned travellers.

"Dengue is more common than malaria in Brazil. While we should be aware that there may be increasing rates of infection during FIFA, the endemic [already present at a low rate in the population] rate of infection is already a concern."

Keystone says the overall risk of transmission is still low and that the virus is rarely deadly the first time. But if you’ve already had it once, it can make you very sick the second time.

"Only one-third of infected patients get sick. The other two thirds won’t actually have symptoms, so it’s important to take precautions."

While there is ongoing research on vaccines for dengue fever there is nothing definitive yet and prevention is the best treatment.

Canada's Public Health Agency recently issued a travel health notice about the FIFA World Cup. The agency warned   people to protect themselves from mosquitoes. The suggestions include:

  • Use mosquito repellent.
  • Wear loose-fitting clothes.
  • Stay in air conditioned places.

For people who return from the high-risk cities with symptoms, health care authorities may be able to more easily identify what what otherwise look like a regular flu-like illness.