Health workers have identified about 4,650 people who were probably exposed to a persistent outbreak of tuberculosis on downtown Los Angeles' skid row and are trying to track them down for testing and treatment.

The Centers for Disease Control has dispatched scientists to Los Angeles to help local health officials determine why the disease is spreading and how to stop it.

Officials say 11 have died since 2007. Sixty of the 78 cases were among homeless people who live on and around skid row.

Scientists have recently linked the outbreak to a tuberculosis strain that is unique to Los Angeles, with a few isolated cases outside the area.

"This is the largest outbreak in a decade," said Jonathan Fielding, director of the Los Angeles County Department of Public Health. "We are really putting all of our resources into this."

Local and federal officials are particularly concerned because the cases are linked to one relatively small geographic area and one vulnerable population. But officials are concerned that the outbreak could spread beyond skid row if action isn't taken.

The public health department issued an alert several weeks ago to doctors at emergency rooms, clinics and urgent-care centers informing them about the investigation within the homeless community.

Most of the patients are men and about 20% are also HIV-positive, according to the alert, which was obtained by The Times. Six of the eight patients who also had HIV have died.

The health department also issued new guidelines for shelters earlier this year on how to effectively screen and identify patients at risk of tuberculosis. The guidelines urge shelters to appoint a TB liaison and to create a "cough alert" log for tracking patients with persistent coughs.

The county also recommended that shelters determine if incoming clients have been screened and refer those who haven't been to health providers.

The county suggests that all employees and volunteers also be screened for TB because they are also at risk.

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