(New throughout, previous HOUSTON)
By Jim Forsyth
SAN ANTONIO, April 27 (Reuters) - Texas state health authorities on Monday confirmed a third case of swine flu that is threatening to become a pandemic, and closed 14 schools in an attempt to contain it.
All three confirmed cases involve students at Byron Steele High School in Guadalupe County near San Antonio, the Texas Department of State Health Services said.
Two high school students have recovered and the third is recovering under voluntary isolation at his home in Schertz, officials said.
The new virus has killed up to 149 people in Mexico and there were 40 confirmed U.S. cases, though there are no confirmed U.S. deaths from the virus that spreads quickly between humans [nN27539441].
The World Health Organization on Monday raised its pandemic alert level to phase 4, indicating a significant increased risk of a pandemic, a global outbreak of a serious disease. It would be the world's first flu pandemic since 1968.
Elsewhere in Texas, local officials said they were tracking four possible cases in Nueces County near Corpus Christi and three probable cases in Dallas.
The Dallas cases "haven't been confirmed to be actual cases of swine flu, but the symptoms are consistent," said assistant Dallas County Health Director John Carlo.
"Thankfully, we haven't seen severe cases here, but of course we have to be ready for that possibility," he said.
In Schertz, Texas, a middle class suburb outside San Antonio where the three confirmed swine flu cases live, residents did not show any alarm.
"It doesn't make me nervous, it's only three cases so far," said Yvette Sosa, whose daughter attends Byron Steele, one of 14 area schools that authorities closed indefinitely as a precaution. "I'm not scared yet." (Editing by Alan Elsner)