One new swine flu case was identified, bringing the total number of swine flu cases in the United States to eight, health officials said on Friday.

The new case was identified in San Diego, about 80 miles (129 kilometers) south of Los Angeles, according to the US Centers for Disease Control and Prevention (CDCP).

The most recent victim, a child, has recovered, as did the other seven victims, said Dr. Richard Besser, CDCP's acting director.

Six of the eight U.S. cases were in California, and the other two in Guadeloupe County, Texas. The cases were identified during routine screening of virus samples.

Researchers have so far found no links among the US victims or any common behaviors, Besser said, suggesting that "there has been transmission through several cycles."

He said there might be several intermediaries who passed it among themselves before the virus reached the identified victims.

The new virus is easily passed, but does not appear to be especially virulent, according to the CDCP.

In Mexico where there is a suspected swine flu outbreak, the viruses in 12 cases match six in the United States, prompting U.S. federal authorities to call for intensified screening of samples from flu victims in the area and in flu victims who have recently traveled to Mexico.

U.S. researchers plan to go to Mexico for investigation, according to the CDCP.

The outbreak has caused concern because the virus appears to be spread from human to human, which is the crucial requirement for a new virus to precipitate a large-scale outbreak, Besser said.

None of the American victims has had any contact with pigs and only one of them has traveled to Mexico recently, he said.

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 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.