Infected birds shed the bacteria through feces and nasal discharges, which can remain infectious for several months. Many strains remain quiescent in birds until activated under stress. Birds are excellent, highly mobile vectors for the distribution of chlamydial infection because they feed on, and have access to, the detritus of infected animals of all sorts.

In humans, after an incubation period of 5–14 days, the symptoms of the disease range from inapparent illness to systemic illness with severe pneumonia. It presents chiefly as an atypical pneumonia. In the first week of psittacosis the symptoms mimic typhoid fever: prostrating high fevers, arthralgias, diarrhea, conjunctivitis, epistaxis and leukopenia. Rose spots can appear and these are called Horder's spots.^[4] Splenomegaly is frequent toward the end of first week. Diagnosis can be suspected in case of respiratory infection associated with splenomegaly and/or epistaxis. Headache can be so severe that suggests meningitis and some nuchal rigidity is not unusual. Towards the end of first week stupor or even coma can result in severe cases.

The second week is more akin to acute bacteraemic pneumococcal pneumonia with continuous high fevers, cough and dyspnoea. X rays show patchy infiltrates or a diffuse whiteout of lung fields.

Bloodwork shows leukopenia, thrombocytopenia and moderately elevated liver enzymes.

Differential diagnosis must be made with typhus, typhoid and atypical pneumonia by Mycoplasma, Legionella or Q fever. Exposure history is paramount to diagnosis.

Complications in the form of endocarditis, hepatitis, myocarditis, arthritis, keratoconjunctivitis, and neurologic complications (encephalitis) may occasionally occur. Severe pneumonia requiring intensive-care support may also occur. Fatal cases have been reported (less than 1% of cases).

Diagnosis

Diagnosis involves microbiological cultures from respiratory secretions of patients or serologically with a fourfold or greater increase in antibody titers against C. psittaci in blood samples combined with the probable course of the disease. Typical inclusions called "Leventhal-Cole-Lillie bodies"^[5] can be seen within macrophages in BAL (Bronchial Alveolar Lavage) fluid. Culture of Chlamydia psittaci is hazardous and should only be carried out in biosafety laboratories.

Epidemiology

Psittacosis was first reported in Europe in 1879.^[6]

In 1929, a highly publicized outbreak of psittacosis hit the United States. Although not the first report of psittacosis in the United States, it was the largest up to that time. It led to greater controls on the import of pet parrots.^[6] The aftermath of the outbreak and how it was handled led to the establishment of the National Institutes of Health.^[7]

From 2002 through 2009, 66 human cases of psittacosis were reported to the Centers for Disease Control and most resulted from exposure to infected pet birds, usually cockatiels, parakeets, parrots, and macaws. Many more cases may occur that are not correctly diagnosed or reported.

Bird owners, pet shop employees, zookeepers and veterinarians are at risk of the infection. Some outbreaks of psittacosis in poultry processing plants have been reported.

Treatment

The infection is treated with antibiotics. Tetracyclines and chloramphenicol are the drugs of choice for treating patients with psittacosis.^[8] Most persons respond to oral therapy doxycycline, tetracycline hydrochloride, or chloramphenicol palmitate. For initial treatment of severely ill patients, doxycycline hyclate may be administered intravenously. Remission of symptoms usually is evident within 48–72 hours. However, relapse can occur, and treatment must continue for at least 10–14 days after fever abates. Although its in vivo efficacy has not been determined, erythromycin probably is the best alternative agent for persons for whom tetracycline is contraindicated (e.g., children aged less than 9 years and pregnant women).^{[citation needed]}