There are no other cases of suspected local transmission at this time, but health officials continue to conduct disease surveillance activities as part of the state's ongoing response.

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“We knew it was only a matter of time before we saw a Zika case spread by a mosquito in Texas,” said John Hellerstedt, the state health commissioner. “We still don’t believe the virus will become widespread in Texas, but there could be more cases, so people need to protect themselves from mosquito bites, especially in parts of the state that stay relatively warm in the fall and winter.”

The woman's case is not that surprising given Brownsville's location in the Rio Grande Valley, directly across the border from Mexico, which has ongoing local transmission of Zika in multiple communities.

The valley is considered to be at higher risk because of previous outbreaks of dengue, a similar virus spread by the same type of mosquito. Texas authorities have been closely monitoring people for signs of infection as well as checking for populations of the aggressive Aedes aegypti mosquito, which is most commonly found in south Texas and is the primary carrier of Zika.

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Cameron County and state health officials will begin conducting door-to-door Zika screenings Monday evening in a 20-block area in southwestern Brownsville around the area where the woman lives. They will be asking residents to reduce potential mosquito breeding areas on their properties. Authorities also plan to collect voluntary urine samples to determine whether other infections are present.

For the moment, the Centers for Disease Control and Prevention is not issuing a specific advisory warning pregnant women against travel to the area. When Florida confirmed the first local spread of the disease in the continental United States this summer, the CDC issued a travel alert for pregnant women to avoid the Miami area.

"We have not identified more than an isolated circumstance" in Texas, CDC Director Tom Frieden said in an interview.

But health officials and the public, especially pregnant women, need to remain vigilant. The Brownsville case "emphasizes that there are still parts of the United States where Zika can still spread," Frieden said. "The season is not yet over, and we're going to need to continue to strictly monitor what's happening in order to adjust to the risks as they occur."

The CDC has been working with Texas and local health departments to conduct drills on the kinds of testing and investigation that need to be done in response to local transmission. "These are not easy, and they are not quick investigations because of the complexity of how Zika spreads," Frieden said.

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About 80 percent of people who become infected have mild or no symptoms, so diagnosis is difficult. But a Zika infection can be devastating for pregnant women and babies because the virus can cause a range of devastating birth defects and brain abnormalities.

In October, the Texas health department urged clinicians in the Rio Grande Valley to be on the lookout for Zika infections in patients showing fever, rash, joint pain and eye redness and to order the appropriate tests, regardless of the individual's travel history or other risk factors.

State health authorities recommend testing for anyone with at least three of those symptoms and for all pregnant women, even those without symptoms, who have traveled to an area with active Zika transmission.

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Texas had confirmed 257 cases of Zika through last week. Until now, all cases had been associated with travel, including two infants born to women who had traveled during their pregnancy and two people who had sexual contact with infected travelers.

Since Zika was found to be actively spreading through locally infected mosquitoes in Florida in the summer, researchers have learned much more about the way the virus works and the grave consequences of Zika infection in babies.

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In September, researchers reported in the New England Journal of Medicine that in rare cases when patients have severe Zika infections, the levels of virus may be so high that they could pose increased risk for transmission through bodily fluids. That finding stemmed from a mysterious case in Utah where a son caring for his sick father became infected with the virus. The father's death in June was the first related to Zika in the continental United States, and his son's infection was unusual because he had not traveled to a Zika-infected region or had sex with a partner who had done so.

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Last week, the CDC reported disturbing news about the broader risk for Zika-infected babies. U.S. and Brazilian researchers looked at a small group of Zika-infected babies in Brazil, who were born with normal-size heads but developed microcephaly, a condition often accompanied by severe brain damage, five months to a year after birth.

The Brazilian infants developed severe neurological damage, such as decreased brain volume and decreased brain tissue with a specific pattern of calcium deposits indicating damage. In a few cases, the mothers had ultrasounds during pregnancy that showed the infants' brain abnormalities. But in other babies, these problems only showed up during CT scans or MRIs conducted after birth.