WASHINGTON -- Homegrown mosquitoes apparently have infected four people in the Miami area with Zika and now it's up to tried-and-true public health measures to stamp out the birth defect-causing virus before it spreads.
Florida Gov. Rick Scott made the long-dreaded announcement Friday of the first likely Zika transmission by mosquitoes on the U.S. mainland.
Scientists don't expect big outbreaks in U.S. states like those in hard-hit parts of Latin America and the Caribbean, including Puerto Rico. Containing community clusters will require good mosquito control and aggressive testing to track Zika, something Florida has done before in stopping a similar virus - dengue - that's spread by the same mosquitoes.
But Zika can cause severe brain birth defects, and even limited local spread could put nearby women who are pregnant or trying to conceive at risk. Officials are working to make sure those women have access to insect repellent and good health advice.
"Our top priority is to protect pregnant women," said Dr. Lyle Petersen of the Centers for Disease Control and Prevention.
Here are some questions and answers about Zika's evolving threat:
Q: How could Zika reach mosquitoes on the U.S. mainland?
A: The cycle starts with a person infected with Zika - presumably a traveler who recently returned home with the virus still lurking in his or her bloodstream. Then a mosquito bites that person, acquiring Zika by drinking in blood, and spreads the virus by biting someone else.
More than 1,650 cases of Zika have been reported in U.S. states that were linked either with travel or having sex with a returned traveler, another way the virus can spread.
Q: Who's at risk from Zika?
A: For most people, it causes only mild symptoms at worst. But during pregnancy, the virus can be devastating. Zika can spread from an infected mother into her fetus' developing brain and kill its cells, resulting in babies being born with unusually small heads, called microcephaly. Even if the brain appears to be developing normally, studies also have linked Zika to stillbirths, poor fetal growth and other problems.
Q: What's the advice?
A: Avoid mosquito bites, and pregnant women also should avoid sex with anyone exposed to Zika. Wear insect repellent; remove standing water around your home where mosquitoes can lay eggs; make sure windows and doors are properly screened; when outside in mosquito season, wear long-sleeved shirts and long pants.
Q: What about travel?
A: The CDC has long been advising pregnant women not to travel to Zika-affected parts of Latin America and the Caribbean. But CDC said Friday there are no plans to advise limiting travel to Florida. The infections occurred in a small area where officials are aggressively targeting mosquitoes.
Q: If a mother-to-be becomes infected, how likely is her baby to be harmed?
A: The largest study to try to tell found the risk of microcephaly can range from 1 percent to 14 percent. Studies better designed to answer that question are under way now.
Q: Does it matter when during pregnancy the mother is infected?
A: Specialists think the first trimester is especially vulnerable, because that's when organs develop. But the brain continues to grow throughout pregnancy and some studies have found problems even if infection occurred much later.
Q: Can the fetus be harmed even if the mother didn't know she was infected?
A: Again, that's not clear. Most adults report either mild or no symptoms from Zika, but it could just be that they didn't notice.
Q: Then how would pregnant women know if they'd been infected?
A: Women who are concerned should tell their doctors, who can order the appropriate diagnostic testing. There is no treatment for Zika, but those who were infected may need ultrasound exams to check fetal development.
Q: Are there other ways Zika can spread?
A: Aside from sex, federal authorities have told blood centers in two Florida counties to suspend collections until they're able to screen donated blood for the Zika virus using authorized tests. Visitors to South Florida in the last month are being urged to defer donations as well.
Health officials say there's no evidence that Zika can be spread through coughing or sneezing or routine touching.
Separately, there is one unexplained case under investigation. A Utah man became infected while caring for his dying father, who contracted Zika while abroad and had unusually high levels of the virus in his blood.
Q: Are there risks beyond to a developing fetus?
A: Zika also can trigger a rare disorder called Guillain-Barre syndrome that causes muscle weakness and paralysis. Guillain-Barre can occur after a number of bacterial and viral infections, when people recover only to have their immune system attack certain nerve cells.
Q: When will we see a vaccine?
A: Not for a while. One company has received permission for small safety tests of a candidate, and the government expects to start first-stage testing of its own version by September.
