Two mysterious human cases of swine flu have been found in Imperial and San Diego counties in Southern California, it was reported on Wednesday.

Local, state and federal health officials have launched an investigation to find the source, according to the Los Angeles Times.

A 9-year-old girl in Imperial County and a 10-year-old boy in San Diego County were identified as having had the virus. Neither needed hospitalization and both have recovered, the paper said, quoting health officials.

The cases puzzled health officials because neither patient had been in contact with pigs or with each other, and the strain of the flu is one never seen before in the United States, said the paper.

Officials said there is no indication the virus is spreading. "We do not have an epidemic," said Dr. Wilma Wooten, San Diego County health officer.

Family members and other people who know the two patients were being interviewed and tested, according to the paper.

Although it commonly causes respiratory problems in pigs, swine flu is rare in humans. Only 12 other cases of human infection have been detected since 2005, according to the U.S. Centers for Disease Control and Prevention (CDCP). Of the 12 cases, 11 of the patients had had contact with pigs.

Symptoms include fever, lethargy, cough and lack of appetite.

Documented cases of swine flu being transmitted from one human to another are rare, according to the CDCP.

(Xinhua News Agency April 23, 2009)