Where Zika virus might spread next

Flight records and global ecological data provide clues as to which regions could be affected.

The WHO has declared Zika virus a global health emergency. Just how global is it? Infectious disease researcher Isaac Bogoch and his colleagues analyzed flight records, climate conditions, and mosquito populations to anticipate how Zika virus will spread internationally. We ask him what areas are at risk and what governments there can do to protect people.  

ResearchGate: How did you estimate the potential for Zika virus to spread internationally?

Isaac Bogoch: We created a “suitability map” for the Americas, looking at climate and environmental conditions as well as presence of the appropriate mosquito vectors. Over that, we superimposed passenger level flight data looking at international travel patterns from affected areas. Combined, this data suggests the probability that a person with a virus in their blood will travel to a distant part of the world, leading to local transmission there. If somebody infected with Zika got on a plane and landed in Canada, there would be no further transmission. There just aren’t the right mosquito vectors in Canada. But if they landed in a part of Colombia that has potential for ongoing Zika virus transmission, that would cause the virus to spread to that area. And in fact, we saw a pretty large number of travelers leaving Brazil and going to parts of South America, Central America, the Caribbean, all areas with environmental conditions and mosquito populations that support local transmission.

RG: Your map focuses on the Americas. Might other areas be at risk?

Bogoch: We did look at global transmission as well, focusing on Europe, Africa, Asia, even places where the virus originated and could be re-introduced given the extent of the current outbreak. We’re working on that in more detail now and hope to publish that data in the near future.

Image: The Lacet.
Image: The Lacet.

RG: Were you able to make any predictions about the timeframe for Zika spreading internationally?

Bogoch: We didn’t make predictions regarding a timeline. I can say it’s happening rather quickly. One reason for that is there’s simply a lot of travel and a high degree of interconnectivity between infected areas and previously unaffected ones. This year is also an El Niño year, so warmer and wetter environments may be further facilitating this rapid transmission. Another important factor is that we’re dealing with a completely non-immune population. The Americas have never seen Zika virus before, and there is likely little to no immunity to it in this part of the world. So, you’ve got the right temperature, the right mosquitoes, a non-immune population, and a high rate of travel between affected and unaffected areas. That’s a recipe for the rapid spread of this virus, and that’s what we’re seeing.

RG: The WHO has declared Zika a global emergency. Do you agree with that assessment?

Bogoch: This is the fourth time that the World Health Organization has declared a public health emergency of international concern. The most recent one previously was of course the Ebola virus outbreak in West Africa. Now, no one is saying that Zika virus causes the same burden of illness as the Ebola virus. What the WHO is saying here is that, because this is such a wide-spread infection—the outbreak has spread to over twenty countries over a relatively short period of time—learning more about the virus and getting it under control will require international cooperation and coordination. The WHO declaration means that there will be better communication among affected countries and more resources available for diagnostic testing, consolidated disease surveillance, and mosquito control efforts. Basically, we’ll work together as a unified team rather than have every country fend for themselves during this outbreak. I think that’s a really good idea, not just to create a coordinated response, but also to accelerate research into what exactly the association is between Zika virus infection during pregnancy and birth defects. The real concern now is understanding that relationship, and quite frankly, the WHO wouldn’t likely have called this a public health emergency if it weren’t for that aspect.

RG: The 2016 Olympic games will likely increase travel to and from Brazil. Do you think that could make the spread of Zika worse?

Bogoch: I think it’s already spreading relatively quickly. Of course when there could be hundreds of thousands—even millions—of people congregated in Brazil for the Olympics, we certainly have to be aware that these individuals are going to return to their home regions and home countries, introducing Zika virus to areas that were previously unaffected. So public health officials, clinicians, and governments should be aware and be prepared to do the appropriate surveillance to ensure that if the virus is introduced, they can respond quickly.

RG: Your study was based on previous years’ rates of travel from affected airports. Do you think that because of concerns about Zika there will be less travel in and out of Brazil and other affected areas this year?

Bogoch: It’s tough to say right now how travel patterns will be affected, and we’re certainly following that very closely. In previous outbreaks, we have seen travel patterns change. In West Africa, for example, travel during the Ebola epidemic was significantly reduced for a period of time. But every outbreak is unique, so it’s tough to say what will happen with travel patterns in and out of Zika virus affected areas. Certainly there may be the potential for decreased tourism to some of these areas. Of course, many national health agencies have cautioned women who are pregnant or considering becoming pregnant to consider postponing travel to Zika virus affected areas, which will obviously include Brazil during the Olympics. Pregnant women and women considering becoming pregnant should abide by those guidelines and consider postponing travel if they can.

RG:  Can the spreading of the virus be stopped? Should authorities check passengers for symptoms at airports like they did with Ebola?

Bogoch: No, I don’t think they should do that. 80 percent of people who are infected won’t have any symptoms, and those who do have symptoms will typically have a very, very mild course. Areas with the potential for local transmission will need to monitor their mosquito populations closely to see what viruses are circulating and see if Zika virus is present. The focus needs to be on the mosquitoes, because human infections might not come to attention quickly enough. Those who are asymptomatic—and even most people with symptoms—will not seek medical care. Patients who do really represent the tip of the iceberg of all the people who are infected with the virus, so it’s important to have good surveillance of mosquito populations in areas where local transmission is possible. Then, obviously mosquito control efforts, reducing breeding grounds for mosquitoes and spraying activities, are very important as a preventative measure in at-risk areas.

RG: Your assessment assumes mosquito transmission. There have been some indications that Zika can be transmitted sexually. How might that affect the virus’s global impact?

Bogoch: There have only been very few cases of sexual transmission reported, so the primary problem by far is still going to be mosquito vectors. We still cannot discount this as a mode of transmission and have to ensure the safety of pregnant women and women considering becoming pregnant. We will see guidelines emerging in the coming days surrounding sexual transmission of the virus and condom use. But I still think we should focus our attention on controlling mosquito populations. That is very high yield approach.

ResearchGate has put together a comprehensive collection of emerging Zika virus research, including scientific papers, interviews with researchers, and discussions among them. 

Featured image courtesy of IAEA Imagebank.