Pregnant SF woman tests Zika-positive after Central America trip

FILE - In this Tuesday, Jan. 26, 2016 file photo, a health workers stands in the Sambadrome spraying insecticide to combat the Aedes aegypti mosquito that transmits the Zika virus in Rio de Janeiro, Brazil. In the 1940s and 1950s, Brazilian authorities made such a ferocious assault on Aedes aegypti that the mosquito, that it was eradicated from Latin America's largest country by 1958. But eradication experts say there is little chance that Brazil can come anywhere near stamping out the pest like it did a half century ago. less FILE - In this Tuesday, Jan. 26, 2016 file photo, a health workers stands in the Sambadrome spraying insecticide to combat the Aedes aegypti mosquito that transmits the Zika virus in Rio de Janeiro, Brazil. In ... more Photo: Leo Correa, AP Photo: Leo Correa, AP Image 1 of / 28 Caption Close Pregnant SF woman tests Zika-positive after Central America trip 1 / 28 Back to Gallery

A pregnant San Francisco woman who had recently been in Central America tested positive for the Zika virus, public health officials said Friday.

The woman has experienced no symptoms of Zika, but got tested because of known risks to babies born to women who were infected during pregnancy. Her test came back positive Thursday.

She is San Francisco’s second case of Zika infection since the virus began spreading widely in Central and South America late last year, and it is the second case of Zika in a pregnant woman in the Bay Area.

The San Francisco Public Health Department is not revealing what country the woman was in when she was exposed to Zika or any other information about her.

Zika is known to cause birth defects, in particular microcephaly, a condition in which the head is smaller than normal. Microcephaly can be fatal and may cause severe problems with brain development and neurological function.

But the relationship between Zika and birth defects, including the degree of risk, is not yet fully understood. Not every pregnancy exposed to Zika will result in a child with birth defects.

The San Francisco woman will be followed closely by her doctor through her pregnancy, officials said.

There is no cure for Zika or for the birth defects it can cause. The U.S. Centers for Disease Control and Prevention has been constantly updating guidelines for doctors on how to monitor pregnant women exposed to Zika.

“This is an emerging infection, so knowledge is evolving,” said Dr. Cora Hoover, director of communicable disease control and prevention with the San Francisco Department of Public Health. “The CDC and the professional specialty organizations are helping to guide providers through this.”

Zika is spread primarily through mosquito bites and occasionally through sex. The virus is not yet spreading in the United States, so any cases reported here are in people who acquired the infection elsewhere or who had sex with someone who acquired it elsewhere.

Symptoms of infection include fever, rash, joint pain and red, watery eyes. But up to 80 percent of people infected have no symptoms. In those who do get sick, the illness is not usually serious.

The CDC has recommended that women who are pregnant or considering becoming pregnant soon avoid more than two dozen countries in Central and South America where Zika is widespread. For more information about Zika and where it is endemic, visit the CDC website.

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @ErinAllday