Investigators from the Chinese Center for Disease Control and Prevention hurried to the scene of the outbreak. On the basis of DNA evidence, they quickly concluded that it had been caused by human granulocytic anaplasmosis, a bacteria transmitted by tick bites.

Now, though, subsequent studies have shown that original conclusion was incorrect, and that a previously unknown and dangerous virus has been responsible for seasonal outbreaks of the disease in six of China's most populated provinces.

"We expected to find a bacterial infection behaving in an unexpected way -- human anaplasmosis has a less than one percent fatality rate in the U.S., and it rarely causes abdominal pain or vomiting or diarrhea," said Dr. Xue-Jie Yu of the University of Texas Medical Branch at Galveston, lead author of a paper on the discovery now appearing in the "online advance" section of the New England Journal of Medicine. "Instead, we found an unknown virus."

Researchers have dubbed the newly discovered pathogen Severe Fever with Thrombocytopenia Syndrome virus, and placed it in the Bunyaviridae family, along with the hantaviruses and Rift Valley Fever virus. Later investigation has placed its mortality rate at 12 percent, still alarmingly high.

Yu, a specialist in tick-borne bacteria like the species responsible for HGA, first suspected that a virus might be responsible for the outbreaks after close examination of patients' clinical data showed big differences from symptoms produced by HGA, and blood sera drawn from patients revealed no HGA or HGA antibodies.

Yu became certain that a virus was at fault after sera taken from patients retained its ability to kill cells, despite being passed through a filter that blocked all bacteria. Still, initial genetic tests failed to generate a match with a known pathogen.

"Clearly, we had a virus, but what virus?" Yu said. "I told the people I was working with that they needed to be even more careful, because we were working with an unknown."

That caution seemed appropriate when electron microscope studies of deactivated virus particles revealed what appeared to be a hantavirus -- associated in Asia with hemorrhagic fever and in the Americas with a deadly pulmonary syndrome. But when Yu and his colleagues managed to extract the virus' entire genetic code, they found that it didn't match any other known virus.

When researchers from the Chinese Center for Disease Control and Prevention led by study author Dr. Yu Wang analyzed sera taken from 241 symptomatic patients from Henan, Hubei, Shandong, Anhui, Jiangsu and Liaoning provinces, they found 171 contained either the previously unknown virus itself or antibodies against it. In addition, the scientists found the virus in 10 out of 186 ticks collected from farm animals in the area where the patients lived.

"This seems to be a tick-borne disease, and the disease comes out when the ticks come out, from late March to late July," Yu said. "Fortunately, even though the full life cycle is not clear, we know that for the virus humans are a dead end -- we don't have human-to-human transmission as we did with SARS."

Note: If no author is given, the source is cited instead.

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