Avian influenza is an infection in birds caused by avian strains of influenza A viruses; representatives of each of the known subtypes of influenza A have been isolated from aquatic birds worldwide, in both wild and domesticated species. The natural reservoir of the virus is wild water birds in which the infection is generally avirulent, such “low pathogenic avian influenza virus” (LPAIV) strains causing little, or no, evident symptoms. LPAIV strains may be readily transmitted to poultry species (chickens, turkeys) which are more susceptible to infection, however only mild clinical symptoms may result, for example ruffled feathers and a drop in egg production. The mode of transmission from aquatic to domesticated birds may be through direct physical contact of the animals, or through contact with surfaces, water or feed that have been contaminated with the virus.

The majority of LPAIV strains replicate in the lungs and the cells lining the intestinal tract of aquatic birds with high titers of virus excreted with feces into the water leading to the continuation of an oral-fecal cycle of transmission. Virus may also be excreted in the animal’s saliva and nasal secretions.