Swine flu outcompetes seasonal flu, unlikely to get more lethal
A new study by Maryland researchers eases fears that the pandemic H1N1 influenza virus will recombine with seasonal flu to mutate into a more lethal form. The study, reported in the online journal PLoS Currents devoted to research on influenza, shows that the pandemic virus, commonly known as swine flu, grows much faster than seasonal flu viruses and is thus less likely to recombine or exchange genetic material. In fact, the team headed by virologist Daniel Perez of the University of Maryland found no evidence at all of such recombination.
Perez and his colleagues grew the virus in ferrets, which are considered the best animal model for influenza because their respiratory system is very similar to that of humans. They co-infected the animals with the pandemic H1N1 virus and either a seasonal H1N1 virus or a seasonal H3N2 virus. The animals were sickened by both the viruses, but only the swine flu virus went on to infect other ferrets brought into contact with the original animals.
"The H1N1 pandemic virus has a clear biological advantage over the two main seasonal flu strains and all the makings of a virus fully adapted to humans," Perez said. "I'm not surprised to find that the pandemic virus is more infectious, simply because it is new, so hosts haven't had a chance to build immunity yet. Meanwhile, the older strains encounter resistance from the hosts' immunity to them."
The studies also confirmed findings by other researchers that the swine flu virus is able to grow deeper into the victim's lungs, allowing it to cause viral pneumonia. That may be why some victims with underlying medical conditions are more likely to develop severe illness from infections. They also found that the virus can in some cases grow in the animals' intestines as well. Reports in humans have shown that swine flu causes a higher incidence of diarrhea and gastrointestinal distress than seasonal flu.
The highly infectious nature of the swine flu virus "underscores the need for vaccination against both seasonal influenza and the 2009 H1N1 influenza this fall and winter," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
-- Thomas H. Maugh II
Photo: Blackfoot ferrets. Credit: LuRay Parker / Wyoming Game and Fish