The natural reservoir for Type A influenza viruses is wild water birds such as ducks and geese. New influenza A subtypes are continually emerging in the waterfowl population due to the constant mutation of the virus. While avian influenza is caused by Type A viruses, seasonal influenza outbreaks in people, which occur almost every winter, are caused by either Type A and Type B influenza viruses. Influenza Type C viruses cause a mild respiratory illness in humans, but are not usually responsible for outbreaks of the flu. Type A viruses are found in both people and animals, whereas Type B viruses are normally only found in humans.
Avian influenza viruses are classified in two ways, one is by the host’s immune response and the other severity of infection. Influenza subtypes are named for two antigens present on the surface of the virus. These are:
There are 16 possible H antigens and 9 possible N antigens. Virus subtypes are named H9N2, H5N1, etc., depending on their combination of antigens.The other classification of avian influenzas is by the severity of the disease they cause in domestic poultry, and the designations are:
Low pathogenicity avian influenza (LPAI)
High pathogenicity avian influenza (HPAI)
Why the concern about H7N9?
This influenza A (H7N9) virus is one subtype among the large group of H7 viruses. Although some H7 viruses (H7N2, H7N3 and H7N7) have occasionally been found to infect humans, this is the first time H7N9 infections have been reported in humans. As such, H7N9 has the potential to spark a pandemic due to the lack of immunity to this virus in the human population. In May 2013, Chinese researchers reported H7N9 appears to have evolved as wild birds mingled with each other and then with domestic flocks. (Lancet). The H gene might have originated from avian influenza viruses of duck origin, and the N gene might have transferred from migratory birds infected with avian influenza viruses as they traveled the East Asian flyway. The research indicated the six internal genes of this virus likely originated from two different groups of H9N2 avian influenza viruses. These viruses have been isolated from chickens. Further analyses suggest ducks and chickens were the likely the intermediate hosts for the virus leading to the new H7N9, however experts caution other intermediate hosts may exist.
For the most recent confirmed human cases of H7N9 and deaths attributed to the virus please see the World Health Organization (WHO) Disease Outbreak News.
Human-to-human transmission of H7N9
As of May 3, 2013 Chinese officials reported four family “cluster” cases where two or three members of the family were infected. Officials have offered a number of possibilities for the infections and have not ruled out human-human transmission. However, the data are not clear as family members appear to have been infected at different times, short and long lag times which means they may have been infected by the same source rather than each other.
Thus far the majority of infections have occurred in areas with comparatively high number of poultry carrying the virus, per Chinese officials in the area.
May 23, 2013 Chinese, US, and Canadian researchers reported limited aerosol transmission of the virus in ferrets. Ferrets are used for this research as they mimic humans with respect to contracting the virus. This is good news with respect to human-to-human transmission of the virus at this time. The researchers also found H7N9 virus shedding occurred before most clinical signs developed, a pattern seen with pandemic and seasonal flu.”
The also cautioned should H7N9 becomes endemic in China’s poultry, the likely source of the of the virus, the risk of the virus evolving to become more transmissible among humans or spreading to pigs would increase.
How can infection with H7N9 virus be prevented?
Though at this time it is not known for certain the source or mode of transmission, as with any influenza virus, following basic hygiene practices and food safety measures are recommended to reduce the risk of infection.
Wash your hands before, during, and after you prepare food; before you eat; after you use the toilet; after handling animals or animal waste; when your hands are dirty; and when providing care when someone in your home is sick. If possible use a disposable towel to open public doors, push shopping carts, etc.
Wash your hands with soap and running water when visibly dirty; if not visibly dirty, wash your hands with soap and water or use an alcohol-based hand cleanser.
Cover your mouth and nose with a medical mask, tissue, or a sleeve or flexed elbow when coughing or sneezing; throw the used tissue into a closed bin immediately after use; perform hand hygiene after contact with respiratory secretions.
Traveling to China:
At this time, WHO does not advise the application of any travel measures with respect to visitors to China nor to persons leaving China. See CDC travel recommendations for visiting China.
Is it safe to consume poultry and game birds?
Influenza viruses are not transmitted through consuming well-cooked food. Because influenza viruses are inactivated by normal temperatures used for cooking ( reaches 165 degrees F in all parts and no pink parts), it is safe to eat properly prepared and cooked meat from poultry and game birds.
Diseased animals and animals that have died of diseases should not be eaten.
In areas experiencing outbreaks, meat products can be safely consumed provided that these items are properly cooked and properly handled during food preparation. The consumption of raw meat and uncooked blood-based dishes is a high-risk practice and should be discouraged.
One-Page Factsheet, Webinar, Webinar PPT, and Resources
H7N9 One-Page Factsheet
H7N9 Avian Influenza: What You Need to Know , Not Fear
EDEN H7N9 Webinar
World Health Organization
Centers for Disease Control
CIDRAP – Center for Infectious Disease Research & Policy
Avian Influenza Homepage