Health officials reported the outbreak to the county Board of Supervisors, which requested a briefing due to the proximity of LA to San Diego, which has had more than 420 cases and 16 deaths since early this year.
The Los Angeles County outbreak was declared because two of 10 confirmed cases could not be traced back to either San Diego or to Santa Cruz County, some 300 miles (480 kilometers) to the north, where there have been 69 hepatitis A cases since April, the Department of Public Health said.
The two apparently locally acquired infections were the most recent.
"We've met the definition for an outbreak," department Director Barbara Ferrer told the board. "As of this morning we've confirmed that we have two community acquired cases."
Of the other cases, four had been in San Diego and one had been in Santa Cruz during their exposure period. Three secondary cases occurred in a health care facility in Los Angeles County.
Most of those infected in the outbreaks have been homeless or using illicit drugs. Several people who provide services to the homeless have also been infected.
Los Angeles County Interim Health Officer Dr. Jeffrey Gunzenhauser said vaccination is the best protection against hepatitis A and outreach teams and clinics are offering free vaccine to the homeless, active drug users and people who work with those groups.
The homeless are considered at high risk for infection because of poor sanitation. The latest annual tally found nearly 59,000 homeless people in the nation's most populous county.
Hepatatis A spreads when someone comes in contact with an infected person's feces, sometimes when hands are not properly washed after going to the bathroom or changing diapers and the virus is then spread through food or objects. It can also be spread through sex or by sharing drug paraphernalia.
Health officials say the incubation period ranges from 15 to 50 days, and symptoms include fever, malaise, dark urine, anorexia, nausea and abdominal discomfort, followed by jaundice.
In addition to vaccination and thorough hand washing, medical experts said people should avoid sex with anyone who has hepatitis A, and not share food, drinks, cigarettes, towels, toothbrushes or eating utensils.