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A Flock of Bird Flu Viruses

Amesh A. Adalja, MD, FACP, FACEP, February 20, 2015

In recent months there has been a spate of bird flu viruses causing human and avian disease all over the world. Some strains are infecting only birds; others are infecting both birds and people. This is producing confusion among those who seek to keep abreast of the status of these various viral strains. Indeed, such situational awareness is an essential component of preparedness, as it provides the basis for surveillance for human and animal disease, for understanding the genetics of the viruses, and for guiding vaccine development. Here I give a brief update on the most noteworthy of the varied avian flu viruses now circulating. 


Of all the avian influenza viruses circulating, H5N1, with its 60% case fatality rate, has generated the most concern. Various strains of it have been causing animal and human infections since 1997 and have infected more than 600 people, killing over half. Outbreaks in birds continue to pop up in widespread locations in Asia, Europe, and Africa. Most recently, this virus has been responsible for several poultry outbreaks in Egypt and Nigeria while also causing a limited number of human infections in Egypt. Interestingly, the virus isolated in the poultry outbreaks has been shown to harbor mutations that are associated with efficient human-to-human transmission, but such changes have not been seen in human isolates. 

H5N1 consists of myriad clades, some associated with human infection and some not. Recently, for the first time ever, H5N1 has been detected in North America in avian species in both Canada and the US. The clade detected has not been one associated with human disease. Thus far, there has been just 1 human H5N1 case in North America, in an infected traveler from Asia who ultimately succumbed to the illness. 


Second in terms of the number of human cases is H7N9, which is responsible for an ongoing outbreak in China beginning in 2013 with secondary importations to Taiwan, Malaysia, and Canada. Thus far, more than 500 human cases have accrued, with a case fatality rate of about 1 in 3. Since most cases are linked to direct or indirect poultry exposure, control measures have focused on limiting contact between humans and potentially infected poultry. In recent weeks a surge in new cases has been reported in China. Though links to poultry have been found in most cases, there remains concern regarding limited human-to-human transmission and mild cases serving to spread the virus. 

Other Avian Flu Viruses to Track

Several other flu viruses merit attention as well:

H5N8: This virus, which has yet to cause a human infection, has recently been found in poultry flocks in California and Oregon as well as Taiwan. 

H5N2: Taiwan is reporting 29 poultry farms have been afflicted with this virus, which has also been found in wild birds there. Culling of birds has been performed to minimize spread. This strain of virus has not been demonstrated to cause clinical disease in humans, but seroconversion has been shown in close contacts of infected poultry. 

H5N3: Taiwan has also reported that 3 dead wild birds were noted to be infected with this strain of avian flu, which thus far has not infected humans.

H5N6/H6N1/H10N8: These 3 avian influenza strains have each been recently linked to isolated human cases, marking the first incursions of these strains into humans. 

Continual Vigilance Needed

Influenza viruses are clearly one of the most successful of human pathogens and have proven, in pandemic after pandemic, that they can infect and kill at an alarming rate. As pandemic flu viruses can emanate from avian influenza species, it is crucial to study and survey avian flu viruses as part of influenza preparedness. It is very difficult to predict which flu strain--which H and N combination--will be responsible for the next pandemic. Despite the uncertainty, heightened surveillance of avian flu viruses provides the best chance to be prepared.


Selected References

Avian flu scan for February 16, 2015. CIDRAP website. Accessed February 17, 2015.

Situation updates-avian flu. WHO website. Accessed February 17, 2015.

Flu scan for February 18, 2015. CIDRAP website. Accessed February 19, 2015.