Transmission

Because several diseases affecting humans can be caused by other species of Chlamydia, the disease resulting from the infection of humans with C. psittaci frequently is referred to as psittacosis rather than chlamydia. Most C. psittaci infections in humans result from exposure to pet psittacine birds. Infection with C. psittaci usually occurs when a person inhales the organism, which has been aerosolized from respiratory secretions or dried feces of infected birds. Other means of exposure include bird bites, mouth-to-beak contact, and the handling of infected birds’ plumage and tissues. Even brief exposures can lead to symptomatic infection; therefore, some patients with psittacosis may not recall or report having any contact with birds.

Mammals occasionally transmit C. psittaci to humans. Certain strains of C. psittaci infect sheep, goats, and cattle, causing chronic infection of the reproductive tract, placental insufficiency, and abortion in these animals. These strains of C. psittaci are transmitted to persons when they are exposed to the birth fluids and placentas of infected animals. Another strain of C. psittaci, feline keratoconjunctivitis agent, typically causes rhinitis and conjunctivitis in cats. Transmission of this strain from cats to humans appears to occur rarely.

Human-to-human transmission has been suggested but not proven. Standard infection-control precautions are sufficient for patients with psittacosis, and specific isolation procedures (e.g., a private room, negative pressure air flow, and masks) are not indicated.

Clinical Signs and Symptoms

For persons infected with C. psittaci, the onset of illness follows an incubation period of 5–14 days. The severity of this disease ranges from inapparent illness to systemic illness with severe pneumonia. Before antimicrobial agents were available, 15%–20% of persons with C. psittaci infection were reported to have died. However, <1% of properly treated patients now die as a result of the infection.

Persons with symptomatic infection typically have abrupt onset of fever, chills, headache, malaise, and myalgia. They usually develop a nonproductive cough that can be accompanied by breathing difficulty and chest tightness. A pulse-temperature dissociation (fever without elevated pulse), enlarged spleen, and rash are sometimes observed and suggest a diagnosis of psittacosis for patients with community-acquired pneumonia. Auscultatory findings may underestimate the extent of pulmonary involvement. Radiographic findings include lobar or interstitial infiltrates. The differential diagnosis of psittacosis-related pneumonia includes infection with Coxiella burnetii, Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and respiratory viruses such as influenza. C. psittaci can affect organ systems other than the respiratory tract and result in endocarditis, myocarditis, hepatitis, arthritis, keratoconjunctivitis, and encephalitis. Severe illness with respiratory failure, thrombocytopenia, hepatitis, and fetal death has been reported among pregnant women.

Diagnosis

A patient is considered to have a confirmed case of psittacosis if clinical illness is compatible with psittacosis and the case is laboratory confirmed by one of three methods: a) C. psittaci is cultured from respiratory secretions; b) antibody against C. psittaci is increased by fourfold or greater (to a reciprocal titer of ?32 between paired acute- and convalescent-phase serum specimens collected at least 2 weeks apart) as demonstrated by complement fixation (CF) or microimmunofluorescence (MIF); or c) immunoglobulin M antibody against C. psittaci is detected by MIF (to a reciprocal titer of ?16). A patient is considered to have a probable case of psittacosis if clinical illness is compatible with psittacosis and a) the case is epidemiologically linked to a confirmed case of psittacosis or b) a single antibody titer ?32, demonstrated by CF or MIF, is present in at least one serum specimen obtained after onset of symptoms. CDC and the Council of State and Territorial Epidemiologists established these case definitions for epidemiologic purposes ( 2 ). These definitions should not be used as the sole criteria for establishing clinical diagnoses.

Until recently, the diagnosis almost always was established by using serologic methods in which paired sera were tested for Chlamydia antibodies by CF test. However, because Chlamydia CF antibody is not species-specific, high CF titers also may result from Chlamydia pneumoniae and Chlamydia trachomatis infection. Acute-phase serum specimens should be obtained as soon as possible after the onset of symptoms, and convalescent-phase serum specimens should be obtained ?2 weeks after the onset of symptoms. Because treatment with tetracycline can delay or diminish the antibody response, a third serum sample might help confirm the diagnosis. All sera should be tested simultaneously at the same laboratory. If the patient’s epidemiologic and clinical history indicate a possible diagnosis of psittacosis, MIF assays can be used to distinguish C. psittaci infection from infection with other chlamydial species. Information about laboratory testing often is available at state laboratories. The infectious agent also can be isolated from the patient’s sputum, pleural fluid, or clotted blood during acute illness and before treatment with antimicrobial agents; however, culture of C. psittaci is performed by few laboratories because of technical difficulty and safety concerns.

Treatment

Tetracyclines are the drugs of choice for treating patients with psittacosis. Most persons respond to oral therapy (100 mg of doxycycline administered twice a day or 500 mg of tetracycline hydrochloride administered four times a day). For initial treatment of severely ill patients, doxycycline hyclate may be administered intravenously at a dosage of 4.4 mg/kg (2 mg/lb) body weight per day divided into two infusions per day (up to 100 mg per dose). In past years, tetracycline hydrochloride has been administered to patients intravenously (10–15 mg/kg body weight per day divided into four doses per day), but a preparation for injection is no longer available in the United States. 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 <9 years and pregnant women).