The 18 were among 557 cases of human granulocytic anaplasmosis reported in Henan province since May 2007, with most in Shangcheng County, authorities said.
HGA is a disease infecting white blood cells that produces flu-like symptoms of fever, severe headache, muscle aches, chills and shaking. Most patients are age 40 to 70.
Researchers believe the disease, which is rarely fatal if treated in its early stages, comes from the same ticks that produce Lyme disease.
China's Center for Disease Control and Prevention ordered the Henan CDC to start monitoring the disease May 31, at the beginning of the summer infection season, provincial CDC Vice Director Xu Bianli said.
Local doctors had little knowledge of the disease, Xu told the state-run Xinhua news agency.
Officials are still monitoring the disease, he said.
The Beijing News daily criticized local officials Wednesday for withholding information about the deaths out of fear of public "commotion" if the word got out.
"As the pathogen and the transmission are not yet known, we cannot release any special information about the disease in order to avoid a commotion among the public," Shangcheng County CDC Director Yu Fang told the newspaper.
The newspaper said that, on the contrary, the lack of official information about the disease actually created anxiety.