Four people infected with the Zika virus by mosquitoes in the Miami area are the nation's first such cases of local transmission unrelated to foreign travel, the Florida Department of Health reported Friday.
Health officials in Florida and elsewhere around the country expected such cases would occur during the summer months when mosquitoes are active.
In Maryland, where there have been 48 cases of travel-related Zika, six of them reported in the past week, the state Department of Health and Mental Hygiene has not reported any cases that were transmitted locally.
Florida officials said clinicians brought the Miami cases to the attention of the state Health Department, which is trying to determine now if there are more people affected by canvassing a portion of Miami-Dade County. They also are taking measures to control the mosquitoes, as are health officials in Baltimore and Maryland.
Once considered benign, the Zika virus has raised alarms as it spreads in the Americas, especially among pregnant women. Zika causes microcephaly, a severe birth defect characterized by babies with small heads and brains. The Maryland health department has confirmed cases of Zika among pregnant women but is not reporting the number.
Most people with the virus have no symptoms. For children and adults who do exhibit symptoms, they are typically mild, though the virus also has been linked in a small number of people to Guillain-Barre syndrome, which can cause temporary paralysis.
"All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami," Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said in a statement. "We continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present — and especially pregnant women — take steps to avoid mosquito bites. We will continue to support Florida's efforts to investigate and respond to Zika, and will reassess the situation and our recommendations on a daily basis."
Aedes aegypti mosquitoes, notable for their white-and-black markings and thought to be the primary means of Zika transmission, range across the southern United States and as far north as southern Connecticut on the Eastern Seaboard.
The CDC launched a Zika monitoring effort months ago and has provided guidance to clinicians who treat pregnant women that includes testing of those who have traveled to heavily affected countries, whether they have symptoms or not.
Those who might have been exposed to the virus are receiving extra ultrasounds, as there is no treatment, though researchers, including some in Maryland, are working on developing a vaccine.
The CDC reports there are 1,658 known cases of Zika in the United States, and 433 pregnant women infected with the virus. Fifteen cases were sexually transmitted.
President Barack Obama has requested $1.9 billion to respond to the virus, but the money has not been approved by Congress. The CDC has distributed funding from other sources to states for emergency preparedness.
In addition to public surveillance and mosquito control campaigns, the agency recommends that people use mosquito repellent, wear long sleeves and stay indoors when possible. They also recommend draining standing water that provides breeding grounds for the mosquitoes, which also carry other viruses that can sicken people such as West Nile, dengue and chikungunya. All three diseases have been reported in Maryland in recent years.
Local and state health departments in Maryland have been trying to keep the public informed about Zika recommendations and have handed out kits to pregnant women that include repellent, larvicide and condoms. State agriculture officials have been conducting their annual spraying in some areas and plan to spray in any place where the virus is reported.
"Maryland's confirmed cases of Zika have all been travel-related, and there are no locally transmitted cases from mosquitoes," said Dr. Howard Haft, a deputy state health secretary for public health. "The Department of Health and Mental Hygiene is continuing to educate the public about Zika through a statewide public awareness campaign, including what preventive measures they can take to protect themselves from Zika transmission, and how they can help reduce the adult mosquito population."
Dr. Leana Wen, Baltimore City health commissioner, said locally transmitted cases have been anticipated in the mainland United States for months. That's why public health officials have been making the effort to get information out about prevention.
Baltimore residents can call 311 to report standing water, and city health workers will come to drain it or drop in larvicide. But Wen said that the city has limited resources and residents should stay vigilant, especially after rainstorms, emptying pet bowls, flower pots and other receptacles of standing water outside and picking up trash to reduce the mosquito breeding grounds.
In the meantime, officials will continue monitoring for Zika-carrying mosquitoes and cases of infection.
"We are prepared for this in Baltimore City," Wen said. "Mosquitoes tend to stay where they are, so we're not worried about mosquitoes traveling from Florida, but the kind of mosquitoes that carry Zika are found in Baltimore."
CDC officials said they will continue coordinating with state and local governments to prepare for the likelihood of more transmission. Additional "homegrown" Zika cases are likely in the coming weeks, said Dr. Lyle Petersen, incident manager for the CDC's Zika virus response.
"Our top priority is to protect pregnant women from the potentially devastating harm caused by Zika," Petersen said in a statement.
State and federal officials do not plan to limit travel to affected areas, though Wen said it was important that pregnant women and those planning to become pregnant and their partners avoid areas with active transmission. She said men who do travel to those areas should wear condoms or abstain from all forms of sex if their partners are pregnant.
Others, including Dr. D.A. Henderson, distinguished scholar at the UPMC Center for Health Security in Baltimore, a nonprofit research group, said those who are not pregnant and not planning to conceive can safely visit places affected by Zika, including Puerto Rico, which has many more locally transmitted cases of Zika than the mainland United States.
"Unless you are pregnant or planning to conceive, there's no reason to avoid traveling to destinations where Zika may be present," said Henderson, also dean emeritus of what is now the Bloomberg School of Public Health at the Johns Hopkins University. "If you use common-sense precautions to avoid mosquito bites, like applying insect repellent and wearing protective clothing, any threat of Zika infection can be easily managed. The overreaction and hype about Zika is not warranted or helpful."