All 37 patients are from five localities of Zhongshan city's Huangpu township.
The Guangdong provincial disease control and prevention center said more cases are likely to be reported.
He Jianfeng, an expert at the center, said the patients had been hospitalised since July 16 with mild symptoms of muscle pain and skin rashes.
None of them had made trips to Southeast Asia, where most infections occur, indicating that the outbreak was not imported, state-run news agency Xinhua quoted him as saying.