Introduction Influenza viruses are members of the orthomyxo family of viruses. Infection or vaccination with a particular subtype generates little if any cross-protective immunity to the other subtypes.
Influenza viruses are a common cause of respiratory disease in humans around the world.
In addition, they cause clinically important disease in a wide variety of other animals as well, including poultry, horses, pigs and even marine mammals. But influenza viruses also infect ducks and other waterfowl, and in these species, infection targets the gastrointestinal tract rather than the respiratory tract, and the infections are completely subclinical. The subclinical nature of these infections, the fact that all 15 HA and 9 NA subtypes of influenza exist among waterfowl, the migratory behavior of waterfowl and the ability of influenza viruses to persist in cold lake water all contribute to the capacity of waterfowl to form an immense, reservoir for influenza viruses in nature. Therefore, it is vitally important to realize the zoonotic potential of certain animal influenza viruses and the roles that animals may play in the generation of novel human influenza viruses. In fact, an understanding of the zoonotic nature of influenza viruses and surveillance for atypical viruses in animal populations are critical to overall planning for the control of future influenza pandemics.