psittacosis, infectious disease of worldwide distribution caused by the bacterial parasite Chlamydia psittaci and transmitted to humans from various birds. The infection has been found in about 70 different species of birds. However, the principal sources of human infection are pet birds, such as parrots and parakeets (Psittacidae, from which the disease is named), as well as ducks and geese and poultry, particularly turkeys.
The association between the human disease and sick parrots was first recognized in Europe in 1879, although a thorough study of the disease was not made until 1929–30, when severe outbreaks, attributed to contact with imported parrots, occurred in 12 countries of Europe and America. During the investigations conducted in Germany, England, and the United States, the causative agent was revealed. Strict regulations followed concerning importation of psittacine birds, which undoubtedly reduced the incidence of the disease but did not prevent the intermittent appearance of cases. The infection was later found in domestic stocks of parakeets and pigeons and subsequently in other species. Infected turkeys, ducks, or geese have caused many cases among poultry handlers or workers in processing plants.
Psittacosis usually causes only mild symptoms of illness in birds and humans. Humans usually contract the disease by inhaling dust particles contaminated with the excrement of infected birds. The bacterial parasite thus gains access to the body and multiplies in the blood and tissues. In humans, psittacosis may cause high fever and pneumonia. Other symptoms include chills, weakness, head and body aches, and an elevated respiratory rate.
Before modern antibiotic drugs became available, the case fatality rate was approximately 20 percent, but penicillin and the tetracycline drugs reduced this figure almost to zero. Although uncommon, complications such as endocarditis or nerve inflammation can occur, typically requiring hospital care.