Psittacosis is a systemic, zoonotic infection that can develop into pneumonia in humans. It is caused by Chlamydia psittaci (formerly Chlamydophila 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). Since 2010, about 10 cases per year have been reported in the United States, although periodic outbreaks have occurred (eg, in 2018, a multistate outbreak among poultry plant workers with 13 laboratory-confirmed cases). In 2023 and the start of 2024, Austria, Denmark, Germany, Sweden, and the Netherlands reported an increase in cases, including some deaths.

Chlamydia 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 such as 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 such as pericarditis, myocarditis, and culture-negative endocarditis have been described. Neurologic 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.