Scientists Still Baffled About Origins of Swine Flu
May 9 -- FRIDAY, May 8 (HealthDay News) -- Researchers say they remain puzzled as to the roots of the swine flu outbreak currently circling the globe.
A study led by researchers at the U.S. Centers for Disease Control and Prevention and released online Thursday by the New England Journal of Medicine notes that 11 cases of infection with a swine flu virus similar to that involved in the current outbreak have been recorded in the United States since 2005. These viral strains were so-called "triple-reassortant" viruses, mean that -- like the current H1N1 strain -- they contained genes from bird, pig and human viruses.
A companion paper, also from CDC researchers, determined that the virus -- first infecting people in 2005 -- appears to be a new one of unknown ancestry.
"The genetics are indicating that the origin of this virus apparently appeared before anyone was aware of it occurring in animals or humans," said Michael Shaw, a microbiologist with the CDC's National Center for Immunization and Respiratory Disease (NCIRD), at a news conference held late Thursday. "Six of the genes are similar to what had already been seen in the Americas circulating in pigs that we knew about. The acquisition of two new genes from the Eurasian lineage has never been seen in the U.S."
Pigs are sometimes imported to Europe and Asia from the Americas for breeding purposes, but not the other way around, Shaw noted.
"If it came into this hemisphere by a person or animal, we have no idea," he said. "We're not in a position to say right now."
Officials believe there may have been a gap in surveillance. Researchers are poring through archives and digging through refrigerated samples to see if they may unknowingly possess information to fill this gap.
All but two of the 11 cases outlined in the first paper involved people who had direct or indirect contact with pigs, but "in another patient, human-to-human transmission was suspected," wrote a team led by Dr. Lyn Finelli of the CDC's Influenza Division. The patients were typically young -- an average age of 10 -- and four of the 11 cases were severe enough to require hospitalization, with two needing invasive mechanical ventilation. Four patients were given Tamiflu, and all eventually recovered.
