The child, whose exact age was not disclosed, was symptomatic but has already recovered, the city's health department says.
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In total, Long Beach has documented 20 confirmed or probable cases of monkeypox.
"To date, no one has required hospitalization and all are either recovered or are isolating and recovering at home," the Long Beach health department said.
The case in the child was described as probable but additional testing will be performed by the CDC for confirmation.
There have been at least four cases of monkeypox documented in children across the United States, though it appears that number does not include the new Long Beach infection.
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The announcement of the case comes one day after Gov. Gavin Newsom declared a state of emergency in California to provide more state resources in fighting the virus.
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And on Tuesday, Los Angeles County made a similar emergency declaration.
The state health department says there have been 786 cases of monkeypox documented in California, with 11 of them requiring hospitalization. Prior to Tuesday the state only had indications of a single case that was in a person under age 18.
Los Angeles County has seen 423 cases, according to county health department data.
Nationally, the CDC has documented more than 5,800 cases.