A year after the first outbreak of Zika in Miami, the mosquito-borne virus’s once-menacing march has slowed to a crawl, with Florida health officials reporting fewer infections in 2017 than last year and no local cases so far.
Zika has waned in the Caribbean and South America, too, which is key to the drop in Florida, experts say. But they warn that the virus could return to Miami — it might even be in Florida undetected, they say — and that other infectious diseases spread by the same species of mosquitoes could present new public health threats for the region.
“It’s very encouraging where we are now compared to last year,” said Dr. Henry Walke, the Centers for Disease Control and Prevention’s incident manager for Zika response. “We’re almost into the second week in August and we only have one [locally transmitted] case” in Texas.
At this time last year, Florida had reported 470 Zika cases statewide, including more than two dozen infections spread by mosquitoes in Miami-Dade. The infections caused the CDC to issue an unprecedented domestic travel advisory warning pregnant women to avoid the county because the virus can cause severe birth defects.
By the end of 2016, state health officials had confirmed 1,456 Zika infections in Florida, including 285 cases spread by mosquitoes in four South Florida zones. There is no vaccine or treatment for the virus, which can also spread through sexual contact.
This year has been different.
The Florida Department of Health has reported a total of 135 Zika cases as of Friday. All but one were found in people who had traveled to one of the 75 countries where the World Health Organization says mosquitoes are spreading the virus.
It’s very encouraging where we are now compared to last year.
Dr. Henry Walke, incident manager for CDC Zika response
The last confirmed case spread by mosquitoes in Florida occurred in December last year, said Mara Gambineri, a state health department spokeswoman. Only one state, Texas, has reported a single mosquito-borne infection this year.
There’s no longer a travel warning for pregnant women to avoid Miami-Dade county — it was lifted in June. And on Aug. 3, the CDC lowered its emergency response level to Zika due to the low number of cases. The only Zika travel warning left in the country is for pregnant women to avoid Brownsville, Texas.
What happened to Zika? And will the virus reemerge in Florida?
Experts say the answers may depend on the same factor that brought the virus to Florida in the first place: Infection rates in other countries, mostly in the Caribbean and South America, where the explosive outbreak of Zika that began in Brazil in 2015 and raced across the Americas last year has lost steam in 2017, according to the Pan American Health Organization.
Among the hardest hit places in the Caribbean was Puerto Rico, where the CDC reported about 35,000 cases in 2016, nearly all spread by local mosquitoes. As of Aug. 9, the CDC has reported 474 Zika infections in Puerto Rico.
Walke said Zika’s decline in the Caribbean and South America likely is due to the effects of what scientists call herd immunity, which occurs when enough people in an area are infected with a virus and develop resistance to it, reducing the chances of an outbreak.
A mosquito gets infected with Zika when it bites a person who already has the virus. That mosquito can then spread Zika by biting more people. Zika can also spread during sex, and from a pregnant woman to her fetus during pregnancy or around the time of birth.
“People that were infected before can’t be infected again. That’s our understanding,” Walke said. “So you don’t have as much of the virus circulating. That’s true not only in Puerto Rico but throughout the Caribbean and throughout South America.”
But while herd immunity elsewhere helps reduce new introductions of Zika into Florida, it won’t stop the virus from reemerging. In fact, scientists and state health officials feel certain that the disease will be back.
“As we’ve seen with [diseases such as] chikungunya and dengue, it is not unlikely that we will experience small outbreaks of Zika in the future,” Mara Gambineri, a spokeswoman for the Florida Department of Health, said in a written statement.
One reason Zika could return is that the virus hasn’t infected enough people in Florida to create herd immunity, which means that an outbreak elsewhere, such as in India, could lead to a re-introduction of the virus in the state.
Also, herd immunity eventually wears off, said Derek Cummings, an epidemiologist with the University of Florida’s Emerging Pathogens Institute.
“As years go by, more people are born into a population and they haven’t encountered Zika, and so they’re going to introduce susceptibility into the population,” Cummings said. “Some number of years from now, those susceptible populations will rise to where you’d have sufficient numbers and then maybe we’d be doing this all over again.”
Previous outbreaks of other viruses in the same genetic family as Zika, such as chikungunya, have followed a similar pattern, said Walke, who tempered his optimism about the low number of cases with a caution against complacency.
In Texas, where 219 pregnant women have contracted Zika since January 2016, health officials have reported 15 Zika-related birth defects, including microcephaly. Florida health officials have reported 384 pregnant women who tested positive for Zika in the same period, with nine delivering children with Zika-related birth defects.
“The threat is still there,” Walke said of Zika. “It hasn’t gone away. It will not go away any time soon.”
Though the CDC says Zika cases in the U.S. last year peaked in July, some scientists say it’s too soon to predict the likelihood of local infections this year.
“It’s early in the season. We’re not into the thick of it yet,” said Bill Petrie, the newly hired director of Miami-Dade mosquito control, who starts Monday.
Though Miami-Dade has ramped up mosquito control efforts, including more surveillance traps and year-round fumigating, Petrie said other factors, like rainfall patterns, could have an effect. The Aedes aegypti mosquito species that most effectively spreads Zika depends on frequent rainfall — and not so much on the amount — to breed, he said.
Another complicating factor, Petrie said, is that about four of five people who get Zika do not feel symptoms, which can include fever, joint pain, red eyes and a rash.
“So it could be circulating and we don’t know it,” he said.
The CDC’s official classification for Florida’s Zika outbreak is “interrupted,” which means that no new mosquito-borne cases have been reported for three months since the last confirmed case. But that doesn’t mean the virus won’t return.
Petrie, who led the Cayman Islands’ mosquito control agency for nearly 30 years before taking the job in Miami-Dade, said the island nation saw a drop in chikungunya infections after an outbreak that lasted almost two years.
But he believes that “natural causes” played a bigger role than any government agency in the reduction of cases.
“My take on it is that there’s no jurisdiction that can stop these viruses coming in,” he said. “It’s not the mosquitoes that carry the disease. It’s humans that transport it.”
Still, Petrie said, reducing the Aedes aegypti population is key to stopping the spread of Zika. And to do the job effectively, he said, local mosquito programs will have to work together, and also partner with the CDC, the Florida health department and researchers at local universities.
Collaboration is key, Petrie said, because it’s impossible to predict the next mosquito-borne disease outbreak, and not all viruses, even those with genetic similarities to Zika, behave the same. Dengue, for instance, has at least four distinct strains, which means people can get infected more than once.