The Zika virus is keeping Florida officials on their toes.
It’s now popped up in a third part of the state: in Pinellas County, the peninsula that is home to both the cities of Clearwater and St. Petersburg.
In all, 42 people have caught Zika in Florida in the three outbreaks in Miami Beach, Pinellas and the Wynwood neighborhood north of Miami. Travelers from New York, Texas and Taiwan have been infected in Florida, and more than 2,200 travelers have carried Zika to the U.S. from elsewhere.
“We wouldn’t be surprised if we saw more local cases and more clusters. It is the middle of mosquito season,” Centers for Disease Control and Prevention Director Dr. Thomas Frieden told NBC News.
Zika virus continues to spread across Latin America and the Caribbean, also. The latest travel warning from the U.S. Centers for Disease Control and Prevention came for the Bahamas. It’s causing a serious epidemic in Puerto Rico, a U.S. territory and experts fully expect small outbreaks in Texas, Louisiana and other Gulf coast states that are home to the Aedes aegypti mosquito that transmits the virus.
The virus, once believed to be harmless and fairly boring, is breaking all the rules. It’s causing small epidemics of terrible birth defects wherever it turns up. It’s also causing severe cases of the paralyzing Guillain-Barre syndrome and it has turned out to be transmitted by sex. It stays in semen for months and may cause invisible brain damage in babies and perhaps even adults.
The sexual transmission is adding another dimension of unpredictability to Zika’s spread, said Scott Weaver, chair of the Global Virus Network’s Zika task force and a virus expert at the University of Texas Medical Branch.
“It is looking like a lot of Zika transmission is sexual in South America,” Weaver said.
That’s hard to tease out when mosquitoes are infecting people who live in close quarters and who also have sex, but one way to do it is by looking at infection rates by age, Weaver said.
“Several groups are seeing quite a bit higher infection rates in women than in men after you reach the age of something like 12, when girls are of childbearing age and can be sexually active,” Weaver said. Zika spread explosively in northeastern Brazil, an area also known to have very high teen pregnancy rates.
“It looking like there is quite a bit of male-to-female sexual transmission in these areas,” Weaver added. “That may be helping the virus to spread more efficiently.”
The Aedes mosquitoes that carry Zika usually travel no further than about 500 to 600 yards in their lifetimes. People, of course, travel much further. Mosquito-borne viruses spread when infected people move around, get bitten and infect mosquitoes in new areas.
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Back in February, Frieden told Congress that if they wanted to predict how Zika would spread, they need only look at chikungunya.
The virus first arrived in the Western Hemisphere in December 2013, on the Caribbean island of St. Martin. By the fall of 2015 it had infected more than 1 million people. It arrived in Florida in a non-traveler in July of 2014.
Chikungunya is a cousin of Zika, carried by the same mosquitoes. It’s not usually deadly, but it can cause a very bad headache, joint pain, rash and fever. Its name in the Makonde language, spoken in Tanzania and Mozambique in Africa, refers to the racking pain it causes.
“I think it pretty much burnt through the Caribbean in one year,” Weaver said – although there have been more than 100,000 cases this year, according to the Pan American Health Organization.
If more people are infected the first year, there are fewer to become newly infected the next and thus to infect mosquitoes. If an entire population lacks immunity to a new infection, it can move very quickly, a phenomenon scientists call a “virgin soil” epidemic.
That happened with chikungunya in Puerto Rico, and it appears to be doing the same with Zika. It’s already infected thousands on every part of the island, Frieden says.
“Zika is very much following the playbook of chikungunya,” Frieden said.
"We projected, based on the chikungunya experience, that Zika might infect a quarter of the population in the first year and it is very much on track to do that."
But the sexual spread adds a dimension of uncertainty to predicting what Zika might do.
Frieden and other experts don’t expect the same explosive spread in the U.S. because people aren’t as exposed to the mosquitoes that spread it. In much of central and South America and the Caribbean, people live in open houses, often without screens and air conditioning. The American way of life can interfere with Aedes mosquitoes, which like to live in and around houses.
But people do spend more time outside in southern states with cool sea breezes – like Miami Beach and the Wynwood area, Frieden said.
“I went through every block in that area,” he said. “You’ve got construction sites, you’ve got retail sites, you’ve got vacant lots, you’ve got high-class housing and you’ve got housing without screens."
And, of course, "people are walking in the streets," he added.
That puts them at the mercy of the Aedes mosquitoes, which bite in daytime and breed in tiny containers of water.
“It bites stealthily. It sneaks up on people quietly and bites them on their ankles,” Frieden said.
And it’s hard to say how long it will take to eradicate the virus, once it’s been spreading from people to mosquitoes and back again for a while.
“It’s gotten into a place where it’s really hard to stop it,” Frieden said. Once dengue, another Zika relative, got into the Florida Keys, it took a year to eradicate its spread, he said.
“This is just a really, really tough mosquito to control,” Frieden said. “It truly is the cockroach of mosquitoes and we don’t get rid of cockroaches easily, especially in warm southern climates.”
“The big question is how much is it going to continue to spread in Florida and other parts of the southern U.S. and nobody knows for sure,” he said.
"We are getting close to the peak of rainy season in the Northern Hemisphere. We probably just passed the peak of temperatures," Weaver said. "We are probably going to see more cases and we are probably peaking in the risk of transmission."