Psittacosis is a systemic, zoonotic infection that frequently causes pneumonia. It is caused by Chlamydophila psittaci (formerly Chlamydia psittaci), an obligate, intracellular bacterium. From 1988-2003, 935 human cases of psittacosis were reported to the US Centers for Disease Control and Prevention (CDC). Most cases of psittacosis are due to exposure to infected birds. Pet owners and pet shop workers, veterinarians, poultry farmers, and workers in abattoirs are at risk for this infection. Natural infection occurs by inhalation of contaminated dust or aerosols, and as an agent of bioterrorism, this would be the most likely method of dispersal.

Chlamydophila psittaci infects many different species of birds including those in the parrot family (macaws, cockatoos, parakeets, and budgerigars), poultry, pigeons, and pheasants. Infected birds may exhibit symptoms of shivering, dyspnea, diarrhea, discharge from their beaks, and ruffling of feathers; however, infection can also be asymptomatic. Infection in humans is acquired by inhalation of the infective discharge, urine, or dried feces. Exposure to the infected bird can vary from seemingly insignificant and trivial to close contact. Strains from turkeys and psittacine birds are most virulent in humans. Occasional cases in humans are due to nonavian sources like lambing sheep, goats, and cows. Human-to-human and nosocomial transmissions are described but rare.

The incubation period varies from 5-15 days. The symptoms are nonspecific and may include fever, nonproductive cough, headaches, photophobia, myalgias, and chills. Rales or signs of consolidation may be present on examination. Other forms of presentation include a mononucleosis-like syndrome with fever, pharyngitis, and hepatosplenomegaly and a typhoidal form with fever, bradycardia, splenomegaly, and a pink, blanching, maculopapular rash called Horder's spots.

Pulmonary involvement is the most common form, but the infection can progress to other organ systems. Cardiac manifestations like pericarditis, myocarditis, and culture-negative endocarditis have been described. Neurological involvement with meningitis, encephalitis, cranial nerve palsies (includes sensorineural hearing loss), seizures, and transverse myelitis can occur. A hemolytic anemia with both a positive Coombs test and cold agglutinins may be seen.

A syndrome of gestational septicemia and abortion is described in pregnant individuals after contact with lambing sheep. Therefore, pregnant people, in general, and those living on farms should avoid contact with animals with premature births and abortions.