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New CDC Laboratory Study Suggests U.S. H5 Bird Flu Viruses Currently Pose Low Risk to People

A new CDC study describes findings from a series of CDC laboratory experiments designed to improve understanding of the human health risk posed by two H5 bird flu viruses detected in birds in the United States: H5N2 and H5N8. Findings of this study indicate that the H5N2 and H5N8 bird flu viruses detected in the United States were less lethal in mammals and replicated (made copies of themselves during infection) at a lower level than the H5 bird flu viruses from Asia that have caused infections, serious illness and deaths in people. Overall findings suggest that these new U.S. bird flu viruses are unlikely to easily infect or spread between people in their current form and are likely to be associated with mild to moderate illness compared to the more severe illness associated with Asian H5 viruses. These U.S. bird flu viruses would need to undergo additional changes in order to pose a pandemic health risk to people.

This study, published today in the Journal of Virology, involved a combination of laboratory tests, some of which included animals and others that involved human lung cells grown in the laboratory via cell culture. CDC often uses such tests to infer how newly detected flu viruses can impact human health. These studies are part of a routine public health risk assessment process that CDC undertakes whenever a new virus with pandemic potential is identified.

Experiments conducted in this study include the following: tests in mice to determine the severity of disease associated with these viruses, tests in ferrets to determine characteristics of how these viruses spread between mammals and within the body, and tests using cell culture to measure the ability of these viruses to grow in human airway/lung cells (specifically human airway epithelial Calu-3 cells) in a laboratory setting.

Results in mice showed that mice infected with these viruses did not experience severe disease unless given very high doses of the virus. Virus was detectable in the lungs of mice, though, which is a characteristic that can be associated with more serious illness. However, compared to Asian H5 viruses, these U.S. H5 bird flu viruses demonstrated less severe, more moderate disease characteristics. Health researchers consider mice to be a reliable model for how disease associated with H5 bird flu viruses develops and progresses in mammals.

Transmission experiments involving ferrets showed that these U.S. H5 bird flu viruses did not spread between flu naïve ferrets (i.e., ferrets that had never been exposed to flu viruses previously) placed in the same cage as infected ferrets. This indicates that the virus is unlikely to spread efficiently among people, if they were to become infected by close contact with H5N2- or H5N8-infected poultry. Also, illness in the infected ferrets was generally mild, and the viruses did not spread systemically to multiple organs, which is a characteristic associated with more severe disease. These results are consistent with previous studies of H5N8 bird flu viruses in South Korea, which also showed low to moderate virulence in mammals. Ferrets are considered an excellent model for studying flu transmission and they also exhibit signs of disease that are similar to people infected with the flu.

Researchers also evaluated the ability of these bird flu viruses to replicate in human lung cells in laboratory experiments involving cell culture. The ability of a virus to infect human lung and airway cells is a trait that can be associated with more severe illness. These tests showed that H5N2 and H5N8 viruses replicated in human lung and airway cells at significantly lower levels compared to the Asian H5N1 viruses that have caused human deaths in Asia and elsewhere. While replication did occur, it was at a level comparable to human seasonal H1N1 flu virus.

Outbreaks in birds of H5N2, H5N8 and a new H5N1 bird flu virus were detected in the United States first in late 2014. Both of the bird flu viruses involved in this study were detected in Washington State: the H5N8 bird flu virus was obtained from an infected gyrfalcon and the H5N2 virus was obtained from a northern pintail duck. Most of the U.S. poultry outbreaks reported this year have been associated with the H5N2 virus, resulting in the loss of nearly 50 million chickens and turkeys on over 200 farms since the virus was first identified in December 2014. No human infections with these viruses have been detected at this time.

These findings reaffirm CDC’s current assessment that these viruses pose a low risk to the general public. CDC will continue to closely monitor and assess the risk of these viruses to human health as part of its routine pandemic preparedness responsibilities and activities.

This study is available for online viewing via the Journal of Virology. The latest information and guidance related to H5 bird flu viruses detected in the United States is available from the CDC H5 Viruses in the United States website.