Avian pox is a slow-developing disease caused by a large virus from the avipoxvirus group. Some strains of this virus can infect multiple species of birds, while others are species-specific. The virus is transmitted by mosquitoes that have fed on infected birds or pox lesions. The virus can also infect through cuts, abrasions, or mucous membranes. The disease occurs worldwide, and has occurred mostly in songbirds, upland game birds, and marine birds, although prevalence in wild bird populations is unknown. Pox outbreaks are commonly reported at aviaries and rehabilitation centers, where close contact facilitates transmission of the virus. Bird feeders are also the source of avian pox outbreaks, and routine cleaning of bird feeders and birdbaths is recommended as a preventative measure (cleaning a bird feeder). There is no evidence that avipoxviruses can infect humans.
Signs of avian pox are wart-like nodules on one or more of the featherless areas of a bird, often causing the bird to appear weak and emaciated if the lesions interfere with feeding. If breathing and feeding is not impaired, the lesions regress and the bird commonly recovers. Mortality can result from secondary bacterial infections that are common with this disease. An internal form of avian pox, called wet pox, appears as moist lesions on mucous membranes of the mouth, upper digestive and respiratory tracts, and most likely has greater morbidity and mortality in wild birds. Wet pox has been reported in house finches, ruffed grouse, and mourning doves.