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 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. The open windows of most homes also 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."
So Ko and other experts say they are rushing to learn more about the Zika virus, and are on the lookout for disturbing surprises.