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.
Cases of this type of influenza H1 virus appeared to pick up more recently, with eight of the 11 cases being reported by the CDC after June 2007, the team noted. They also said that these were only those cases spotted via the CDC's routine "passive" flu surveillance systems -- additional but unidentified cases may have occurred.
"It's not surprising that they have reports of cases going back further," said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City. "There may have been more than 11 sporadic cases but it probably reached a critical mass where it could spread or it reassorted. Reservoirs of virus found in animals do come to humans."
But unlike those early cases, the H1N1 virus is now hopping easily from human to human.
"What's unusual about this particular case is that it's able to establish sustained [human-to-human] transmission," Shaw said.
"We don't yet know why this one is passing so much better than the other ones," added John Quarles, head and professor of microbial and molecular pathogenesis at Texas A&M Health Science Center College of Medicine. "This is actually still pretty early in the business." Quarles repeated a message appearing in editorial in the journal: This is not the last new influenza virus we will see.
According to Shaw, the new swine flu strain and the circulating seasonal H1N1 influenza strains can all be traced back to the virus that sparked the 1918 pandemic, which killed millions worldwide. "All have been evolving along separate tracks," he said.
Another interesting development: About 38 percent of cases of the current swine flu involve vomiting and diarrhea, which is not typical of the seasonal flu.
Again, scientists don't know why this might be the case, said Dr. Fatima Dawood, a medical epidemiologist with CDC NCIRD, but clinicians and the public need to be aware that the virus may transmit through the gastrointestinal route in addition to the traditional respiratory route.
The number of confirmed or suspected cases of swine flu in the United States was approaching 1,900, federal health officials said Thursday, with most new cases now caused by person-to-person transmission and not some link to Mexico, as was the case when the outbreak began nearly two weeks ago.
On Thursday, the World Health Organization was reporting almost 2,400 confirmed cases of swine flu in 24 countries, with Canada, Spain and the United Kingdom having the most cases outside of Mexico and the United States.
Dr. Carolyn Bridges, a medical epidemiologist also with CDC NCIRD, said that experts would be watching developments in the southern hemisphere closely. "This may give us some clues as to what we might expect in upcoming winter months."
SOURCES: John M. Quarles, Ph.D., head and professor, microbial and molecular pathogenesis, Texas A&M Health Science Center College of Medicine; May 7, 2009, teleconference with Michael Shaw, Ph.D., microbiologist, National Center for Immunization and Respiratory Disease (NCIRD), U.S. Centers for Disease Control and Prevention, Carolyn Bridges, M.D., medical epidemiologist, NCIRD; and Fatima Dawood, M.D., medical epidemiologist, NCIRD; Len Horovitz, M.D., pulmonary expert, Lenox Hill Hospital, New York City; May 7, 2009, New England Journal of Medicine, online