23 March 2011

New Chinese virus has "alarmingly high" mortality rate

by Kate Melville

Writing in the New England Journal of Medicine, scientists explain how a previously unknown and dangerous virus carried by tics has been responsible for seasonal outbreaks of the disease in six of China's most populated provinces.

The disease first came to attention five years ago, when large numbers of farmers in central China began falling victim to a mysterious disease marked by high fever, gastrointestinal disorder and a mortality rate initially estimated at 30 percent.

Chinese investigators initially concluded that it had been caused by human granulocytic anaplasmosis (HGA), a bacteria transmitted by tick bites. Now, however, subsequent research has shown this to be incorrect, and that a previously unknown virus has been responsible.

"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 the paper detailing the discovery. "Instead, we found an unknown virus."

The newly discovered pathogen has been dubbed Severe Fever with Thrombocytopenia Syndrome virus. It has been placed in the Bunyaviridae family, along with the hantaviruses and Rift Valley Fever virus.

Yu, a specialist in tick-borne diseases, 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. 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.

Tellingly, the scientists found the new 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."

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Source: University of Texas Medical Branch at Galveston