The outbreak of Zika virus in Brazil and other countries has raised concern that the pathogen could start spreading widely in the United States, as well. But federal health officials and other infectious disease specialists say so far that seems unlikely.
"Based on what we know right now, we don't think that widespread transmission in the United States is likely," says Dr. Beth Bell, director of the National Center for Emerging and Zoonotic Infectious Diseases at the Centers for Disease Control and Prevention.
There are several reasons for Bell's cautious optimism that isolated cases that show up in the U.S. could be contained. The first is that the two species of mosquitoes that could be capable of transmitting the virus, Aedes aegypti and Aedes albopictus, live mostly in the southern, more tropical parts of the U.S. That makes it likely that transmission would be limited primarily to these areas. And for various reasons, the chain of events and conditions the virus needs in order to spread is more easily disrupted in the U.S. than elsewhere.
For example, many people in the U.S. have air conditioning in the summer, so aren't as likely to leave windows open at the times of day when mosquitoes are especially active. Open windows also tend to have screens. And many counties and other municipalities spray to kill mosquitoes and are vigilant in trying to eliminate pools of standing water where the insects can breed.
"These are all conditions that make it less likely for ongoing, large-scale spread to occur," Bell says.
Still, travelers who have gotten infected with the Zika virus in other countries have already arrived in the U.S. and more are expected, raising the possibility that the virus could spread a bit beyond those cases, Bell says.
"There certainly is the possibility for transmission," Bell says. And since the virus is new to the U.S., most people have no immunity to it.
But even if that occurs, Bell and the CDC predict that any outbreaks would likely be very small. That's been the case so far with two similar viruses that have spread rapidly throughout Latin America and the Caribbean in recent years: dengue and chikungunya.
"What we saw with chikungunya, and what we've seen with dengue," Bell says, "is some small situations with localized spread in southern parts of the United States, but with very limited transmission."
Bell says she doubts Zika would fan out across the U.S. in the way that West Nile virus has spread. For one thing, West Nile is primarily transmitted by a different type of mosquito — one that is found throughout the country. Also, birds can be infected with West Nile and carry it from place to place. That doesn't happen with Zika virus, which has no known bird or animal reservoir.
Dr. Anthony Fauci, who heads the National Institutes of Allergy and Infectious Disease, says he agrees that it's unlikely that the U.S. will see a widespread outbreak of Zika.
"If you look at historically what we've seen, I think we can say that it's a remote possibility and unlikely to happen," Fauci says. Nevertheless, Bell acknowledged that experts can " 'never say never,' " and Fauci agrees anything is possible.
There are still many uncertainties, including exactly what Zika is doing to pregnant women and their babies, what other complications the virus may cause and why it has suddenly taken off in parts of Brazil and elsewhere.
Those unknowns mean "there is a risk" to some people, says Dr. Albert Ko of the Yale School of Public Health, who's been studying the outbreak in Brazil. "We don't know how big that risk is. It seems low. But we know not that much about this disease at this moment."
Ko and other experts say they are rushing to learn more about the Zika virus and are on the lookout for disturbing surprises.
ROBERT SIEGEL, HOST:
The Zika outbreak is raising questions about whether the infection could become a problem in the U.S. as well. NPR health correspond Rob Stein has been looking into that, and he says at the moment, it doesn't seem likely.
ROB STEIN, BYLINE: First, the bad news - people who got infected with the Zika virus in other countries have already started showing up in the United States. And Beth Bell of the Centers for Disease Control and Prevention says that's not all.
BETH BELL: There are parts of the United States, especially the Southern United States, where the mosquitoes that have the capacity to spread Zika live, and therefore, there certainly is the possibility for transmission.
STEIN: When these mosquitoes bite infected people and then bit other people. Since the virus is so new, we have no immunity to getting infected.
BELL: What we're all concerned about is the potential for Zika virus to spread within the United States.
STEIN: And we've seen mosquitoes quickly spread other new viruses across the country before, like West Nile. But now for the good news - Bell says Zika's no West Nile. It doesn't seem to be carried by birds like West Nile, and...
BELL: West Nile is carried by a completely different species of mosquito.
STEIN: A species that lives all over the United States. The Zika mosquitos live mostly only in the South, and Bell says chances are, Zika will be a lot more like two other viruses called dengue and chikungunya. They've also spread throughout the Caribbean and Latin America in recent years and spread a little in this country but not much.
BELL: What we saw with chikungunya and what we've seen with dengue is some small situations with localized spread in southern parts of the United States but with very limited transmission.
STEIN: Why? It's a combination of factors. Part of it is the mosquitoes that spread these viruses really only thrive in much more tropical climates, and the United States does a much better job protecting people from mosquitoes than other countries.
BELL: For the most part, there are screens on windows. Many houses in the southern part of the United States have air-conditioning. The density of housing may not be as tight as in some other parts of the world. These are all conditions that make it less likely for ongoing large-scale spread to occurred.
STEIN: Taken together, other experts agree that the chances of a wide-scale outbreak in the United States is remote. Anthony Fauci is the top infectious disease expert at the National Institutes of Health.
ANTHONY FAUCI: That doesn't mean that it's impossible that it would happen, but if you look at historically what we've seen, I think we can say that right now, it looks like this is a remote possibility and unlikely to happen.
STEIN: But everyone agrees there's still a lot we don't know about the Zika virus. We really don't know exactly what it's doing to pregnant women and their babies. We don't know what other complications it may cause or why it's suddenly taken off. Albert Ko at the Yale School of Public Health has been studying the outbreak in Brazil.
ALBERT KO: There is risk. We don't know how great that risk is. It seems low, but we know not that much about this disease at this moment.
STEIN: So Ko and other experts say they're rushing to learn more about the Zika virus and are watching closely for any disturbing surprises. Rob Stein, NPR News. Transcript provided by NPR, Copyright NPR.