Etiology

C. psittaci affects both psittacine birds (e.g., parrots, parakeets, macaws) and nonpsittacine birds (ducks, turkeys); the known host range includes 130 avian species. The life cycle of C. psittaci is the same as for Chlamydophila pneumoniae (Chapter 217). Strains of C. psittaci have been analyzed by patterns of pathogenicity, inclusion morphology in tissue culture, DNA restriction endonuclease analysis, and monoclonal antibodies, which indicate that there are 7 avian serovars. Two of the avian serovars, psittacine and turkey, are of major importance in the avian population of the USA. Each is associated with important host preferences and disease characteristics.

Epidemiology

From 1988 to 2003 there were 935 reported cases of psittacosis in the USA. Of these, 85% were associated with exposure to birds, including 70% following exposure to caged pet birds, which were usually psittacine birds, including cockatiels, parakeets, parrots, and macaws. Chlamydiosis among caged nonpsittacine birds occurs most often in pigeons, doves, and mynah birds. Persons at highest risk for acquiring psittacosis include bird fanciers and owners of pet birds (43% of cases) and pet shop employees (10% of cases). Reported cases most likely underestimate the number of actual infections owing to a lack of awareness.

Inhalation of aerosols from feces, fecal dust, and nasal secretions of animals infected with C. psittaci is the primary route of infection. Source birds are either asymptomatic or have anorexia, ruffled feathers, lethargy, and watery green droppings. Psittacosis is uncommon in children, in part because children may be less likely to have close contact with infected birds. One high-risk activity is cleaning the cage. Several major outbreaks of psittacosis have occurred in turkey-processing plants; workers exposed to turkey viscera are at the highest risk for infection.

Clinical Manifestations

Infection with C. psittaci in humans ranges from clinically inapparent to severe disease, including pneumonia and multiorgan involvement. The mean incubation period is 15 days after exposure, with a range of 5-21 days. Onset of disease is usually abrupt, with fever, cough, headache, and malaise. The fever is high and is often associated with rigors and sweats. The headache can be so severe that meningitis is considered. The cough is usually nonproductive. Gastrointestinal symptoms are occasionally reported. Crackles may be heard on auscultation. Chest radiographs are usually abnormal and are characterized by the presence of variable infiltrates, sometimes accompanied by pleural effusions. The white blood cell count is usually normal but is sometimes mildly elevated. Elevated levels of aspartate aminotransferase, alkaline phosphatase, and bilirubin are common.

Diagnosis

Psittacosis can be difficult to diagnose because of the varying clinical presentations. A history of exposure to birds or association with an active case can be important clues, but as many as 20% of patients with psittacosis have no known contact. Person-to-person spread has been suggested but not proved. Other infections that cause pneumonia with high fever, unusually severe headache, and myalgia include routine bacterial and viral respiratory infections as well as Coxiella burnetii infection (Q fever), Mycoplasma pneumoniae infection, C. pneumoniae infection, tularemia, tuberculosis, fungal infections, and Legionnaires disease.

The mainstay of diagnosis remains serology using the complement fixation (CF) test, which is genus specific. According to the recommendations from the Centers for Disease Control and Prevention in 2000, a confirmed case of psittacosis requires a compatible clinical illness, usually with a reliable history of avian exposure. Laboratory confirmation may be by 1 of 3 methods: culture of C. psittaci from respiratory secretions; a ≥4-fold increase in CF or microimmunofluorescence (MIF) titer in sera collected at least 2 wk apart; or a single MIF immunoglobulin M titer of ≥1 : 16. A probable case should be epidemiologically linked to a confirmed case or have a single CF or MIF antibody titer of ≥1 : 32 in ≥1 serum sample obtained after onset of symptoms. As with the use of MIF for diagnosis of C. pneumoniae infections, cross reactions with other Chlamydia species and bacteria can occur. False-negative MIF results can occur in acutely ill patients. Early treatment of psittacosis with tetracycline can abrogate the antibody response.

The organism also can be isolated by culture from sputum or pleural fluid. Although C. psittaci will grow in the same culture systems used for isolation of Chlamydia trachomatis and C. pneumoniae, very few laboratories culture for C. psittaci, mainly because of the potential biohazard. Nucleic-acid amplification tests for detection of C. psittaci have been reported in the literature, but these are in-house assays that do not have FDA approval.

Treatment

Recommended treatment regimens for psittacosis are doxycycline (100 mg PO twice daily) or tetracycline (500 mg PO 4 times a day) for at least 10-14 days after the fever abates. The initial treatment of severely ill patients is doxycycline hyclate (4.4 mg/kg/day divided every 12 hr IV, maximum 100 mg/dose). Erythromycin (500 mg PO 4 times a day) and azithromycin (10 mg/kg PO day 1, not to exceed 500 mg, followed by 5 mg/kg PO on days 2-5, not to exceed 250 mg) are alternative drugs if tetracyclines are contraindicated (e.g., children <8 yr of age and pregnant women), but may be less effective. Remission is usually evident within 48-72 hr. Initial infection does not appear to be followed by long-term immunity. Reinfection and clinical disease can develop within 2 mo of treatment.

Prognosis

The mortality rate of psittacosis is 15-20% with no treatment but is <1% with appropriate treatment. Severe illness leading to respiratory failure and fetal death has been reported among pregnant women.

Prevention

Several control measures are recommended to prevent transmission of C. psittaci from birds. Bird fanciers should be cognizant of the potential risk. C. psittaci is susceptible to heat and to most disinfectants and detergents but is resistant to acid and alkali. Accurate records of all bird-related transactions aid in identifying sources of infected birds and potentially exposed persons. Newly acquired birds, including birds that have been to shows, exhibitions, fairs, or other events, should be isolated for 30-45 days or tested or treated prophylactically before adding them to a group of birds. Care should be taken to prevent transfer of fecal material, feathers, food, or other materials between birdcages. Birds with signs of avian chlamydiosis (e.g., ocular or nasal discharge, watery green droppings, or low body weight) should be isolated and should not be sold or purchased. Their handlers should wear protective clothing and a disposable surgical cap and use a respirator with an N95 or higher efficiency rating (not a surgical mask) when handling them or cleaning their cages. Infected birds should be isolated until fully treated, which is generally 45 days.