COLUMBIA, S.C. (AP) – Thirteen cases of swine flu were confirmed in South Carolina on Thursday and dozens of residents were voluntarily isolating themselves in case they were infected, state health officials said.

Meanwhile, a few public schools announced they would close.

Health officials said they expected the number of confirmed cases would continue to increase. They said the people who had been sickened by the virus appeared to be recovering.

All the confirmed cases were linked to a trip taken by private school students to Mexico about two weeks ago.

The number of swine flu cases in the United States has topped 100, with confirmed infections in more than 10 states, according to the Centers for Disease Control and Prevention.

The federal agency reported 10 confirmed cases in the state while South Carolina officials put the number at 13. Such discrepancies have been seen in other states since the breakout of the virus, and federal officials said they are to be expected because of the fluid nature of how cases are being reported.

"The important point is that we absolutely, definitely have this swine-origin virus in our state," said state health department chief Jerry Gibson as he announced the state's first confirmed cases.

Newberry Academy remained closed for the week because some students who returned from Cancun reported flu-like symptoms. Public schools in Newberry County planned to close Friday to "mitigate the potential spread of the flu virus," district spokesman Hugh Gray said.

A school in Mauldin announced Thursday it would close for the rest of the week as a precaution after students returned from Walt Disney World with symptoms. Gibson said that there was no indication they have swine flu, and that he didn't recommend that officials close the Mauldin school.

To control the spread of the virus, health officials have asked that more than 50 people who may have swine flu isolate themselves, but nobody had been ordered to be quarantined, Mr. Gibson said. He said everyone was cooperating.

The confirmations of swine flu came as no surprise to parents and school officials in the area where probable cases had been reported for days.

Newberry Academy Headmaster Bob Dawkins said he expects the school to reopen Monday.

"None of my people are panicking," said Mr. Dawkins, adding that the school has had larger amounts of flu-stricken students in past flu seasons. "I think everyone has already figured out it's just a regular flu."

already figured out it's just a regular flu."

Federal health officials reported Monday that the number of swine flu cases in the United States has topped 100, with confirmed infections in several states.

The Centers for Disease Control and Prevention's count for the first time Thursday adds South Carolina. The CDC update said there have been 109 cases including 50 in New York, 26 in Texas and 14 in California.