Psittacosis Pneumonia

Etiology

Chlamydia psittaci is an obligate intracellular bacterium. Besides poultry (turkeys and ducks), pet birds including parrots, cockatiels, macaws, and parakeets are the main culprits. C. psittaci can also be found in non-avian domestic animals, such as cattle, sheep, swine, horses, goats, cats, and wildlife, but cases of clinical disease and transmission to humans have not been proven.

Epidemiology

Chlamydia psittaci is an important cause of community-acquired pneumonia (CAP). It is thought to comprise approximately 1% of all CAP cases. Fewer than 50 cases per year of psittacosis are reported in the United States.[3] Given that birds are the primary reservoir, exposure is greatest in owners of pet birds and workers in poultry-processing centers, pet-shops, aviaries, and veterinary facilities. Due to lack of routine testing, incidence and prevalence are hard to establish. As mentioned above, the organism is transmitted by air droplets from feces, urine, respiratory secretions, and feather dust. Dried organisms remain viable for months. Psittacosis is statistically more common in young to middle-aged males, although this may be related to higher exposure potential.[1]

Pathophysiology

Infection of C. psittaci begins with inhalation of aerosolized bacteria when exposed to secretions from infected birds. The infection begins in the upper respiratory tract and may persist as a prolonged asymptomatic condition of the mucosal surfaces. Infection of the mucosal membrane causes these cells to expose outer membrane proteins, which may promote a host immune response, leading to an autoimmune reaction and manifesting as atypical pneumonia.[4]

Histopathology

Elementary bodies (EB) of C. psittaci infect the lung by direct interaction with the surface microvilli. EBs are subsequently engulfed by endocytosis, hence creating inclusions called reticulate bodies (RB) that avoid lysosomal fusion. These bodies then transform back into EB and are released during cell lysis, restarting the cycle. The pathogen can also cause systemic symptoms due to hematogenous spread, perhaps within macrophages. Persistence of infection has been observed when pathogens remain viable but non-cultivable, and the host immune system is unable to eliminate them. Hence, alveolar-lining cells with intracytoplasmic inclusions are pathognomonic.[2]

History and Physical

Exposure to birds or occupational exposure is the most important clue in the patient's history. C. psittaci commonly presents in young to middle-aged males who complain of fever (most common symptom), chills, headache, myalgia, and dry cough. Symptoms vary from mild to severe systemic disease. In systemic involvement, the patient may present with hepatosplenomegaly and gastrointestinal symptoms. Endocarditis and myocarditis have also been described, and a minimal percentage of patients present with altered mental status. The incubation period is usually 5 to 19 days but can be as long as 28 days. In the pre-antibiotic era, fatal cases were common. Today, imported bird quarantine, treatment, and improved hygiene measures have rendered psittacosis cases rare.[1]

Pulmonary symptoms include a dry cough, and lung auscultation during physical exam may reveal rales or rhonchi. Uncommonly, a pleural rub may be appreciated. A pulse-temperature dissociation (fever and relative bradycardia) has been described for psittacosis pneumonia. Patients may also show signs of photophobia, epistaxis, tinnitus, deafness, gastrointestinal symptoms, and arthralgia if systemically compromised.

Evaluation

The best method for confirming C. psittacosis infection is with serologic testing, namely micro-immunofluorescent antibody test (MIF). Cultures are highly discouraged due to being difficult, infectious to lab personnel, and only performed in specialized labs. MIF testing uses chlamydia-specific surface antigen. A test result with antibodies above 1:16 is considered evidence for exposure, while a 4-fold titer rise with clinical correlation is considered diagnostic. Complement fixation (CF) was the previously diagnostic test of choice, but it cannot differentiate between chlamydial species.[3]

If obtained, laboratory testing may evidence a normal white blood cell count with a "left shift." Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) may be elevated. If a systemic illness is present, the patient may evidence abnormal liver enzymes, hyponatremia, and elevated BUN/creatinine.[5]

Chest radiographs most often show single lower lobar changes. Computed tomography has higher sensitivity and may reveal nodular pulmonary infiltrates with surrounding ground-glass opacities.

Treatment / Management

The primary treatment for C. psittaci consists of tetracyclines. Tetracycline 500 mg by mouth (PO) 4 times per day or doxycycline 100 mg PO or intravenously (IV) 2 times per day are described as a treatment of choice for 7 to 10 days. Minocycline 100 mg PO or IV 2 times per day may also be effective. Macrolides are second-line agents. Azithromycin 250 to 500 mg PO once daily is favored over erythromycin. Macrolides are the treatment of choice for children.[1]

Differential Diagnosis

The initial signs and symptoms of C. psittaci, including fever, headache, and dry cough, can lead to a very broad differential diagnosis. Other culprits of atypical pneumonia, such as Chlamydia pneumoniae, mycoplasma infections, Legionnaires' disease, Q fever, and Tularemia, may be considered when presenting with C. psittacosis. Patients presenting only with fever have a broader differential that includes influenza, septicemia, endocarditis, other causes of pneumonia (bacterial and fungal), and other sources of infection. History of exposure to birds or bird droppings, as might be found under bridges, should raise psittacosis on the differential diagnosis.

Pertinent Studies and Ongoing Trials

Recent studies have shown that C. psittaci copes better than other chlamydial strains with the host's pro-inflammatory effectors during early immune response.[6] In a recent case study,[7] a nested PCR of a nasopharyngeal swab of a patient and feces of his pet parakeet confirmed C. psittaci infection when serology was negative, pointing to a possible need for broader implementation of PCR for early diagnosis. C. psittaci has also been found in non-avian hosts, although its clinical importance to humans has not been proven.[6]

Prognosis

Prognosis for C. psittacosis depends largely on the patient's baseline health status, the severity of disease, and early diagnosis and treatment. Full recovery may take from 6 to 8 weeks, including residual changes on chest films. Per the CDC, the current mortality rate in the United States is less than 1% with early diagnosis and appropriate treatment.[3]

Complications

Apart from pneumonia and its pulmonary complications, systemic infection with C. psittaci can lead to endocarditis, hepatitis, and encephalitis.[1]