* New virus has many genetic mutations
* Infection causes unusual symptoms in some people
* New mixtures likely inevitable
(Recasts with quotes from news conference)
By Maggie Fox, Health and Science Editor
WASHINGTON, May 7 (Reuters) - The first batch of formal reports looking at the current outbreak of a new kind of swine flu virus shows it is a Frankenstein-like mixture of viruses, researchers reported on Thursday.
It includes a so-called "triple reassortent" virus with swine, human and bird elements that has been circulating since at least 1998.
And such mixtures are likely to occur and infect people in the future, the researchers reported in the New England Journal of Medicine.
The new strain of H1N1 swine flu has spread globally although North American countries are the hardest-hit. The World Health Organization is watching for signs it could tip over into a pandemic.
Dr. Lyn Finelli of the U.S. Centers for Disease Control and Prevention and colleagues reported on 11 people infected with the triple reassortent virus -- apparently one ancestor of the new strain -- since 2005.
"As recent events suggest, the generation of novel influenza viruses through the reassortment of swine influenza viruses with other human and animal influenza viruses may be inevitable," they wrote in one report.
The new virus includes genes from this strain, but two genes are from swine viruses that had only been seen in Europe.
MYSTERIOUS ORIGIN
A gene called "M" carried mutations that made the virus resist the effects of older antiviral drugs but not the newer drugs Tamiflu and Relenza, they said.
The other genes were of clear North American origin.
The neuraminidase, or the "N" of a flu's name, most closely resembles a swine flu first identified in Belgium, they said. "Those genes had never been seen in the United States before," the CDC's Dr. Michael Shaw told reporters. "We have no idea whether they came to this hemisphere by human or animal."
He said it appeared the virus had been circulating unseen somewhere, either in humans or animals.
"It was already well adapted for transmission in humans before it popped up," Shaw added.
"In this context, the possibility of novel influenza viruses causing epidemic and pandemic disease ... remains a major ongoing public health threat," the report reads.
Among the 11 patients infected with the older swine virus, symptoms were slightly different from those of seasonal flu, but 100 percent of the patients had typical symptoms such as cough and nine of out 10 had fever. A third had diarrhea, which is not usually seen with seasonal flu. Some were very sick but all recovered.
A similar pattern is seen with the new virus, said Dr. Fatimah Dawood of CDC's Epidemic Intelligence Service, which indicates it may spread through the fecal-oral route as well as traditional influenza routes such as sneezing or coughing.
"This is a new virus and we still learning how transmission occurs," Dawood said.
The two U.S. patients who died were the Mexican toddler who had been born with an autoimmune disease called myasthenia gravis and a pregnant woman in her 30s, Dawood and colleagues said in another report.
Both also had several other conditions that put people at greater risk of dying from influenza.
The first person to be identified with the flu was a 9-year-old girl from Imperial County, California, on the border with Mexico, who developed a cough and fever on March 28.
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