By mapping the destinations of airline travelers and combining that with disease transmission and climate data, researchers show how infections can quickly spread around the world.
Zika virus is transmitted by the same Aedes mosquito that spreads dengue and chikungunya. Typically, it causes mild illness and a rash, but reports from Brazil suggest it may be causing birth defects in newborn babies of women who have had Zika.
The virus is native to parts of Africa and Asia, but it is now spreading locally outside of its native regions in people who have not traveled abroad. There is no vaccine or antiviral treatment for the virus.
In a research letter in The Lancet, a team of experts who track the spread of infectious diseases says:
"The presence of Aedes mosquitoes across Latin America, coupled with suitable climatic conditions, have triggered a Zika virus epidemic in Brazil, currently estimated at 440,000-1,300,000 cases."
In November 2015, the Brazilian Health Ministry reported a surge in cases of babies born with small heads and underdeveloped brains - a condition known as microcephaly - linked to Zika infection during pregnancy. There has been a 20-fold increase in the number of babies born with the condition since Zika first appeared in Brazil in May 2015.
The Brazilian authorities are currently investigating 2,401 microcephaly cases, with 134 cases confirmed as linked to Zika virus infection. They are also investigating a possible link between Zika and Guillain-Barré syndrome and other neurological events.
60% of US population in areas favorable to seasonal Zika spread
Dr. Kamran Khan, an infectious diseases specialist at the University of Toronto and St. Michael's Hospital, Canada, notes:
"The summer Olympic Games in Brazil in August heighten the need for awareness of this emerging virus."
Infected travelers have spread the virus to other countries in the Americas. According to the latest information from the Pan American Health Organization, Zika virus is now spreading locally - that is, not imported via travelers - in 17 countries and territories in the region: Brazil, Colombia, Ecuador, El Salvador, French Guiana, Guyana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Puerto Rico, Paraguay, Saint Martin, Suriname and Venezuela.
To predict where the virus might spread, Dr. Khan and colleagues mapped the final destinations of passengers leaving airports in Brazil between September 2014 and August 2015.
Of the 9.9 million passengers, 65% were traveling to the Americas, 27% to Europe and 5% to Asia. The biggest traveler volumes were in the US, followed by Argentina, Chile, Italy, Portugal and France. In Asia, the biggest traveler volume was in China, and in Africa, it was in Angola.
The experts also mapped the geography of Aedes mosquitoes that can carry Zika and combined it with a model of the climate conditions needed for the virus to spread from mosquito to human.
The result suggests over 60% of the populations of the US, Argentina and Italy live in areas favorable to the seasonal spread of Zika virus.
In Mexico, there are 30.5 million, in Colombia, 23.2 million, and in the US, there are 22.7 million people living in areas where the virus could spread all year round.
Lessons from Ebola
Meanwhile, infectious disease experts are looking to see what can be learned from the recent Ebola outbreak in West Africa that can be applied to help local health professionals and researchers get ahead of the new pathogen.
A top priority is the need to move as quickly as possible into obtaining data, which means integrating research into public health response. For example, there is a need to share documentation, which requires new protocols and procedures.
Trudie Lang, professor of global health research at Oxford University in the UK and head of the Global Health Network, explains:
"Medical research is vital to understand, manage and hopefully stop the outbreak. However, if samples and patient information are to be used for research purposes then protocols are needed that have been given ethical approval by regulatory authorities and patients need to be informed and asked for their consent."
She says studies need to be planned, which takes time, but the lesson from Ebola is that it can be done faster.
Pregnant women advised to consider postponing travel
The US Centers for Disease Control and Prevention (CDC) have issued interim travel guidance for people intending to travel to areas affected by Zika.
Until more is known about the link between microcephaly and the virus, the federal authority advises "out of an abundance of caution" that pregnant women and women trying to become pregnant consider postponing travel to areas where Zika virus transmission is ongoing.
If pregnant women and women trying to get pregnant must travel to areas where Zika virus spreading locally, they should talk to their doctor first and strictly follow steps to avoid being bitten by mosquitoes during their trip, says the federal agency.
Travelers are urged to protect themselves from Zika infection by avoiding mosquito bites. Tips from the Pan American Health Organization include:
- Wear long sleeves, long pants and hats
- Use mosquito repellent as indicated by health authorities and according to label instructions
- If you sleep during the day - the Zika mosquito bites during the day - protect yourself with insecticide-treated mosquito netting
- Use permethrin-treated clothing and gear (such as boots, pants, socks and tents)
- Stay and sleep in screened-in or air-conditioned rooms
- Identify and eliminate possible mosquito breeding sites.
As the mosquito that spreads Zika is the same as the one that spreads dengue, advice for eliminating breeding sites for dengue mosquitoes apply to Zika - see, for example, this detailed list of breeding sites given out by the Singapore National Environment Agency.
Dr. Isaac Bogoch, a tropical infectious disease specialist at the Toronto General Hospital, who took part in the mapping research, concludes:
"The world we live in is very interconnected now. Things don't happen in isolation anymore. Infections from the farthest corners of the world can quickly arrive on our doorstep."
Worldwide, there are around 390 million cases of dengue a year. There is currently no vaccine, but a Science study that Medical News Today reported in July 2015 describes a discovery that could lead to a vaccine and treatments for dengue.