Psittacosis

INTRODUCTION — Psittacosis, also known as ornithosis, is usually a disease with prominent systemic manifestations and some respiratory symptoms. This infection, caused by Chlamydia psittaci, is transmitted to humans predominantly from birds.

The epidemiology, clinical features, diagnosis, and treatment of psittacosis will be discussed here. Pneumonia caused by Chlamydia spp is discussed separately. (See "Pneumonia caused by Chlamydia pneumoniae in adults" and "Pneumonia caused by Chlamydia pneumoniae in children".)

MICROBIOLOGY — C. psittaci is an obligate intracellular bacterium. There are 10 known genotypes based on sequencing of the major outer protein gene, ompA [1]. Each genotype has host preferences and virulence characteristics, although these overlap. Knowledge of the pathogenesis at a cellular and molecular level is growing [2].

EPIDEMIOLOGY — Psittacosis has been recognized throughout the world, including the United States, the United Kingdom, Europe, the Middle East, and Australia. In a meta-analysis of observational studies, C. psittaci, the causative agent of psittacosis, was estimated to cause approximately 1 percent of cases of community-acquired pneumonia [3]. However, the precise incidence and prevalence of psittacosis is difficult to establish, likely due to lack of routine testing [4] and varying sensitivity and specificity of commonly used diagnostic tests. As the use of molecular diagnostic assays increases, knowledge of the epidemiology of psittacosis is expected to grow [5-7]. Certain genotypes may predominate in specific regions [8].

Birds are the primary reservoir, but transmission from other animals has also been reported. The disease usually occurs sporadically, but outbreaks caused by contact with an infected bird are not uncommon [9]. As an example, in 2018, an outbreak of 82 cases occurred in two poultry slaughter plants in the United States [10]. Outbreaks related to pet shops, aviaries, veterinary facilities, poultry flocks, and turkey and duck processing have also been described [11-16].

Birds — Most patients with psittacosis have a history of contact with birds. C. psittaci has been documented in at least 460 species from 30 bird orders, including turkeys, pheasants, chickens [17], and even ostriches and penguins [18]. Migratory birds, including geese, may carry this pathogen [19]. Most humans are infected from birds of the order Psittaciformes, which includes parakeets (budgerigars), cockatiels, cockatoos, and parrots. In France, mule ducks are associated with human disease through heavy shedding, although the ducks are asymptomatic [20]. Each bird order tends to be infected by a predominant genotype of C. psittaci.

Of 1136 patients reported to the United States Centers for Disease Control and Prevention (CDC) between 1975 and 1984, 72 percent had a history of contact with birds as pets or in a domestic setting, 6 percent had contact with wild birds, 12 percent were poultry workers, and only 10 percent had no recognized avian contact [9]. Pigeons may be an especially significant source in springtime [21]. In a Chinese study, 65 percent of patients had exposure to poultry or parrots [22]. In one Australian study, infection was much more prevalent in captive birds (8 percent) than wild birds (0.7 percent) [23]. Veterinarians, veterinary technicians, zoo workers, pet shop owners [24], duck farmers, poultry workers [25-27], custom officers [28], and farmers can be infected. Fatalities have occurred in occupationally exposed workers [29]. In China, 12 percent of adult pet birds were seropositive in a 2014 survey [30]. Investigation of an outbreak in Sydney, Australia, revealed that, in some regions, psittacosis was endemic through bird exposure [31].

The incubation period in birds is extremely variable, from three days for acute disease to years for reactivation of latent infection, often at a time of stress. Infection in birds is usually asymptomatic or may cause lethargy, anorexia, ruffled feathers, ocular or nasal discharge, or diarrhea. Some birds die rapidly; others become wasted or dehydrated. The organism is shed in feces, urine, and respiratory secretions. Once feces dry, the organism may become airborne as droplet nuclei. Dried organisms remain viable for months at room temperature.

Transmission from infected birds may not always require direct contact. In an investigation of 16 infected patients during an outbreak in a rural Australian town, cases spent more time gardening and were more likely to have mowed the lawn in the three weeks preceding illness than controls, but the feeding and keeping of domestic or wild birds was not associated with infection [32].

Other species — Transmission of infection from other animals to humans is rare. Certain strains of C. psittaci may infect sheep, goats, cats, dogs, dairy cattle, and horses, causing placental insufficiency, abortion, and a chronic respiratory infection. Exposures to the birth fluids and placentas of infected animals can lead to infection in humans. For example, an Australian outbreak affecting five staff in a veterinary school was traced to horse exposure through equine fetal membranes [33]. Equine psittacosis should be considered in the diagnosis of equine pregnancy losses and neonatal foals, with veterinary exposure risk at these events [34]. Severe human disease in pregnancy contracted from lambing sheep is also well recognized [35]. To avoid this risk, pregnant women should avoid contact with birthing sheep, goats, and other farm animals and their gestational products. Feline keratoconjunctivitis agent (a strain of C. psittaci), which produces rhinitis and conjunctivitis in cats, is rarely transmitted to humans [36]. Children have acquired psittacosis from infected dogs; clinical manifestations in dogs include recurrent conjunctivitis, small litter size, and respiratory distress [37].

Human disease — Humans are usually infected by inhalation of organisms in dried feces when caged birds exercise their wings or in bird feather dust. Cage cleaning may pose an infection risk [15]. Bird bites, mouth-to-beak contact, and even transient exposure, such as visiting a bird park, have also been implicated in the transmission of this infection. Virtually all pet birds can carry C. psittaci, but psittacine birds are the most likely to be infected. Less frequently, pigeons, doves, and mynahs carry infection. In an Italian series of 76 cases, most human infections were contracted from poultry [38]. Outbreaks associated with chicken slaughter plants have occurred, with those working with eviscerating birds most at risk. Some workers experienced severe illness requiring intensive care unit admission [23].

Human-to-human transmission may occur, but has been considered rare, with limited reports in the past. In Sweden, one severely ill patient with psittacosis transmitted infection to 10 others, including a hospital roommate and 7 hospital staff [39,40]. In an outbreak in a duck meat processing plant in China, transmission initially occurred from avian to human; later secondary and tertiary human-to-human transmission occurred. This included several transmissions from asymptomatic carriers, and transmission by health care workers. With metagenomic diagnosis revealing new understandings of the epidemiology of this infection, C. psittaci should be considered a biosecurity risk [41].

Psittacosis is most common in young and middle-aged adults, although it has been described in all age groups. In several series, but not all, a marked predilection for males is noted, presumably related to increased potential for exposure [42,43].

Infection is not always associated with a major illness. When avian chlamydiosis was detected in a large shipment of pet birds, households exposed were investigated. Eleven percent of exposed persons developed an acute respiratory illness. Clinical or serological evidence of infection was found in 30 percent of households. In many, infection led to mild illness or was asymptomatic [44]. However, deaths are occasionally reported [45].

In 2014, a total of 8 psittacosis cases were reported in the United States [46], compared with an average of 10 cases (range 2 to 21) reported annually during the period 2005 to 2011 [47].

CLINICAL FEATURES — C. psittaci infection of humans most commonly presents in young or middle-aged adults as fever of abrupt onset, pronounced headache, and dry cough. Infection can also be asymptomatic. Patients usually have a recent history of bird exposure. The incubation period is usually between 5 and 14 days but can be as long as 39 days [48]. In one study of 135 patients, systemic manifestations were prominent with fever in all patients, rigors occurring in 61 percent, and sweats and myalgias in most patients [42]. Though cough was present in 82 percent of patients, it frequently developed late. Other respiratory symptoms, including dyspnea, chest pain, and hemoptysis, were present in 24 percent of patients. Eighteen percent of patients had no respiratory symptoms.

The clinical presentation may mimic coronavirus disease 2019 (COVID-19). In a case series reported in China, the white cell count, neutrophil count, and calcium were more likely to be decreased in patients with COVID-19. The lung lesions in patients with psittacosis were more likely to be unilateral, involve fewer lobes, and have pleural effusions (although these are not common) [49].

Headache is common, usually severe, and may be accompanied by photophobia. One-third of the 135 patients in the above series underwent lumbar puncture for possible meningitis [42].

Other less common but important symptoms include pharyngitis, diarrhea, and altered mental status. Diarrhea may be seen in up to 25 percent of patients and is typically mild. It can be quite severe, however, and even be the most prominent symptom in some patients [42]. Twelve percent of patients present with altered mental status, which can range from mild confusion to, rarely, overt encephalitis [50].

Physical examination — Most patients have some abnormalities on chest examination (usually rales), occasionally have evidence of consolidation, and uncommonly have a pleural rub. Relative bradycardia with pneumonia is said to be suggestive [51]. Pleural effusions occur infrequently and are rare enough to prompt reconsideration of the diagnosis. Splenomegaly and hepatomegaly occur in approximately 10 percent of patients [42].

Organ system complications — Complications in a number of organ systems may arise in patients with psittacosis. These are uncommon manifestations of the illness, but some can be severe.

Pulmonary — Occasionally, respiratory failure occurs requiring ventilatory support and can be fatal. In a report from the Netherlands, 8 of 12 patients with severe respiratory failure died [52]. Patients with fulminant pneumonia often also manifest neurologic, renal, and gastrointestinal complications [52-54]. Severe pneumonia was associated with higher levels of procalcitonin, urea nitrogen, lactate dehydrogenase, creatinine kinase, myoglobin, interleukin 6, and lower lymphocyte and CD8-positive T cell counts [55]. Progression less acutely to a life threatening, probable organizing pneumonia has also been described, when there may be a role for corticosteroid therapy [56].

Hematologic complications in fulminant disease include disseminated intravascular coagulation [57,58] and hemophagocytic syndrome [59,60].

Renal disease — Mild renal involvement with proteinuria and occasional oliguria is well recognized [61]. Rarely, acute tubular necrosis may occur in fulminant psittacosis with respiratory failure. Acute tubulointerstitial nephritis and acute proliferative glomerulonephritis have also been reported [62].

Hematologic complications — Cold agglutinins are sometimes noted in psittacosis, but hemolytic anemia with jaundice is rare [63]. Acute thrombocytopenic purpura, severe pancytopenia secondary to hemophagocytic syndrome, and thrombotic thrombocytopenic purpura in a patient with respiratory failure have also been described.

Liver disease — Icteric hepatitis occurs uncommonly [64]. One patient with multiple nodules in the liver and spleen who had a pericardial rub but no evidence of pulmonary disease was found to have hepatic granulomas and serological evidence of psittacosis [65].

Neurologic disease — A wide range of neurologic syndromes are infrequently seen, although abnormal neurologic findings are less common than with mycoplasma infection. Encephalitis was seen in 2 of 156 patients in a series from the United Kingdom [66]. Cerebellar disturbance may be prominent. Psychiatric symptoms and intracranial hypertension have been described [9]. True meningitis with a cerebrospinal fluid pleocytosis may occur rarely. Positive metagenomics sequencing of the cerebrospinal fluid has been reported [67,68]. One patient presenting with status epilepticus suffered a protracted and severe neurologic illness. Direct invasion of the central nervous system was demonstrated by Chlamydia deoxyribonucleic acid (DNA) in cerebrospinal fluid [69].

Uveitis has been seen in association with meningitis. Palsies of the 4^th, 6^th, 7^th, and 12^th cranial nerve have been described, often accompanying overt encephalitis [70]. Diplopia may persist for months. Transient sensorineural deafness associated with pneumonia has been reported [71]. Transverse myelitis with fever can also be the clinical presentation of psittacosis [72]. The neurologic deficit may or may not resolve. In addition, Guillain-Barré syndrome can follow acute infection [50].

Musculoskeletal — Reactive arthritis, though rare, has been reported. The onset may be a week or more after the first symptoms of psittacosis [73]. Migratory small and large joint arthritis, symmetrical polyarthritis, and reactive arthritis (formerly Reiter syndrome) have been described [74,75]. Human leukocyte antigen-type associations have been noted [76]. Rhabdomyolysis has also occurred with severe pneumonia [42,77,78].

Skin manifestations — Dermatologic manifestations are infrequent. Erythema nodosum is the most frequent, but erythema multiforme, erythema marginatum, "Horder spots," and panniculitis also have been reported [79].

Chronic follicular conjunctivitis — Conjunctivitis with enlarged preauricular nodes and punctuate epithelioid keratitis, without any associated systemic symptoms, has been reported in bird fanciers and laboratory workers [80,81]. This may be unilateral, and diagnosis made on polymerase chain reaction testing rather than serology [82].

Endocarditis, myocarditis, and pericarditis — Endocarditis caused by C. psittaci is rare [83]. It should be considered in the differential diagnosis of culture-negative endocarditis usually lasting weeks or months, especially if there is a history of bird exposure. Serologic evidence of infection should then be sought. Surgery is often required, and a mortality of 50 percent has been reported [84,85]. If the patient comes to surgery, electron microscopy and immunofluorescent staining of valvular tissue is suggested. Although there is anecdotal evidence to suggest tetracycline treatment without surgery may at times be adequate, the optimal duration of treatment is unknown [86]. Myocarditis and pericarditis are also rare. Myocarditis may lead to left ventricular dilatation, but limited reports suggest resolution of myocardial damage with therapy [87,88].

Infection in pregnancy — Infection in pregnancy may be life-threatening, particularly in the late second or third trimester [89]. Respiratory failure, liver dysfunction, and disseminated intravascular coagulation may threaten the life of the mother. Eleven of 14 pregnant women died in a psittacosis outbreak in 1938. Fetal outcome is poor, with fetal death in 11 of 14 reported cases [90].

The infection may cross the placenta and cause intrauterine fetal compromise. The organism has a predilection for the placenta. Placental histopathologic findings consist of an intense acute intervillositis, perivillous fibrin deposition with villous necrosis, and large irregular basophilic intracytoplasmic inclusions within the syncytiotrophoblast [91]. These changes can impair fetoplacental blood flow and increase the risk of bacterial embolization to the fetus [89]. Although rare, infections with ovine C. psittaci are a particular hazard for pregnant women, in whom there is severe placentitis and frequently fetal loss. Such infections are mainly associated with contact with lambing sheep [90,91].

Association with ocular adnexal lymphoma — There are conflicting reports regarding a possible causal association between psittacosis and ocular adnexal lymphoma (OAL) as well as the response of OAL to treatment with antibiotics. Reports suggest a response rate of this indolent lymphoma to antibiotic therapy for C. psittaci of 33 to 65 percent [92]. (See "Clinical manifestations, pathologic features, and diagnosis of extranodal marginal zone lymphoma of mucosa associated lymphoid tissue (MALT)", section on 'Chlamydia psittaci'.)

Laboratory studies — The white cell count is usually normal, but the differential may show toxic granulation or a "left shift." Occasionally, there is a marked leukocytosis. The erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and procalcitonin are very often elevated. In an Australian series, the mean peak ESR was 50 mm/hour, and CRP was 129 mg/L [42]. The liver enzymes are abnormal in approximately half of hospitalized patients, usually with mild elevation of the aspartate transaminase and low serum albumin levels. Hyponatremia is quite common, as is mild elevation of serum creatinine and blood urea nitrogen. Serum creatinine kinase levels are infrequently elevated [93].

Chest radiology — The chest radiograph is abnormal in approximately 80 percent of cases, most often with lobar changes; such changes were observed more frequently in the lower lobes in some series [94]. Approximately one-quarter of patients have multilobar changes, and slightly fewer have a normal chest radiograph [43]. Air space consolidation or ground-glass opacities have also been reported. Pleural effusions are rare but can occur. Migratory infiltrates have also been described [95]. On high-resolution computed tomography, the pulmonary infiltrates may be nodular surrounded by ground-glass opacities [96].

Cerebrospinal fluid — In patients undergoing lumbar puncture, elevation of cerebrospinal fluid protein levels is frequently observed. Elevated cerebrospinal fluid white blood cell counts occur rarely.

PATHOLOGY — There is limited knowledge of the pathology of psittacosis, since lung biopsies are rarely performed in this setting.

Infectious elementary bodies enter respiratory epithelial cells and convert to replicating reticulate bodies, which are metabolically active structures that are 10 to 100 times larger than elementary bodies [97,98]. These subsequently change back to elementary bodies that are released to infect other cells in the same individual or to infect others.

At the macroscopic level, the lungs have patchy consolidation that may be lobar or lobular. The consolidated area is usually hemorrhagic. Adjacent pleura may show fibrinous pleurisy, but effusions are uncommon.

Microscopically, as with other causes of bacterial pneumonia, exudation is more marked than interstitial changes. The exudate consists of fibrin, neutrophils, and hemorrhage. There is a mixed lymphocytic and neutrophilic infiltrate in the interstitium. The changes are concentrated in terminal bronchioles spreading to adjacent alveoli and then the whole lobule. Necrosis of bronchiolar and bronchus epithelium may occur, as may capillary thrombosis leading to alveolar necrosis. Later, the alveoli develop a lining of swollen epithelial cells. Healing through repair may lead to an organizing pneumonia.

The Chlamydia are just visible with the light microscope as cytoplasmic inclusion bodies 0.25 to 0.5 mm diameter. These are basophilic and best seen with Giemsa stain.

DIAGNOSIS — The typical clinical features (fever, headache, myalgias, dry cough) in a patient with a history of bird contact should suggest the diagnosis and should prompt the initiation of appropriate treatment. In a patient presenting with pneumonia, severe headache, splenomegaly, and failure to respond to beta-lactam antibiotics may be other clues to the diagnosis.

Differential diagnosis — The differential diagnosis of psittacosis is broad but can be narrowed based on the particular clinical presentation of the patient.

●For a patient with atypical pneumonia, the other etiologies to consider include Chlamydia pneumoniae, Mycoplasma pneumoniae, and Legionella infection. (See "Pneumonia caused by Chlamydia pneumoniae in adults" and "Pneumonia caused by Chlamydia pneumoniae in children" and "Mycoplasma pneumoniae infection in adults" and "Clinical manifestations and diagnosis of Legionella infection" and "Mycoplasma pneumoniae infection in children", section on 'Clinical manifestations'.)

●If the patient primarily has a febrile illness without localizing signs, influenza, endocarditis, septicemia, vasculitis, Coxiella burnetii infection, leptospirosis, and brucellosis should also be considered. (See "Leptospirosis: Epidemiology, microbiology, clinical manifestations, and diagnosis" and "Brucellosis: Epidemiology, microbiology, clinical manifestations, and diagnosis".)

●When extrapulmonary manifestations predominate, the patient should be evaluated for the causes of the most prominent manifestation such as gastroenteritis, hepatitis, meningitis, or encephalitis.

Specific diagnostic tests — The diagnosis of psittacosis is typically established by serologic testing. Culture is discouraged since C. psittaci is highly infectious when cultured and is only performed in specialized laboratories. All of the available diagnostic tests have major limitations, as discussed below.

Serology — Since culture is both difficult and dangerous, serology is the principal method of confirming the diagnosis. There are two types of serologic tests available: complement fixation (CF) and microimmunofluorescent (MIF) antibody test. When available, we favor MIF testing, but CF testing may be used as an alternative.

●MIF test – The MIF test is the most sensitive and specific serologic test for C. psittaci but is only available in specialty laboratories [99]. The antigen used is Chlamydia species-specific surface antigen. An MIF test showing a fourfold rise in antibodies or an immunoglobulin (Ig)M antibody titer ≥16 is interpreted as diagnostic. As the serologic response seen in both CF and MIF is often unimpressive, a single antibody titer of ≥32 in either test combined with a typical history is sufficient for a diagnosis of "probable psittacosis."

A number of studies reevaluated by MIF cases that were diagnosed as psittacosis based upon clinical history and a positive CF test. C. psittaci was confirmed in 32 to 77 percent of patients [99-101]. A history of bird contact increased the probability of the diagnosis of C. psittaci, and clustering of cases in a family increased the probability of C. pneumoniae.

●CF test – The CF test is the test that has been used traditionally to make the diagnosis. It is the most widely available test but unfortunately cannot differentiate among the chlamydial species. The antigen used is a lipopolysaccharide present in the outer membrane of all Chlamydia spp. Paired acute and convalescent sera should be obtained at least two weeks apart, and a fourfold rise in antibody levels is significant. If there are clinical grounds for suspecting psittacosis and no antibody response is detected, the CF test should be repeated in another two weeks. Treatment with tetracycline may delay or diminish the antibody response.

Monoclonal antibody techniques — Monoclonal antibody techniques, which use antibodies directed against the chlamydial antigens (genus-specific lipopolysaccharides), show promise. Direct immunofluorescent antibody staining of respiratory secretions (sputum, pharyngeal swabs) has been used for rapid diagnosis [102]. Studies that demonstrate the sensitivity and specificity of these techniques are needed.

DNA-based assays — Polymerase chain reaction (PCR) methods for the detection of C. psittaci have been developed [103,104]. They have been used in outbreak investigations and give a rapid and specific diagnosis [10,105,106]. Initial results from nested PCR for C. psittaci suggest good specificity but less sensitivity than for C. pneumoniae [105]. Touchdown enzyme time-release PCR assays have been used to amplify different DNA sequences in the variable regions of the 16S and 16S-23S spacer ribosomal ribonucleic acid (rRNA) genes specific for Chlamydia trachomatis, C. pneumoniae, and C. psittaci and have potential as improved tests for sensitive diagnosis and rapid species differentiation in chlamydial infections [106,107]. A real-time PCR assay to detect DNA in human clinical samples targeting ompA genes also offers potential as a sensitive, specific, and rapid method of diagnosis from respiratory samples [108]. DNA may be detected in whole blood, throat specimens, urine [109], and feces [10]. In a United States outbreak, C. psittaci was detected by real-time PCR in only 7 percent of upper respiratory specimens, but more frequently in lower respiratory tract specimens (59 percent) and feces (four of five patients). PCR testing of sputum, when available, in all community-acquired pneumonia patients in a Dutch study led to an increased recognition of psittacosis as a pathogen compared with previous studies of pneumonia in that setting (4.8 versus 0 to 2.1 percent) [110]. A subsequent study that used multiplex PCR assays to evaluate patients with lower respiratory tract infections also found that the prevalence of psittacosis was higher than previously estimated [6]. Diagnosis through next-generation metagenomic sequencing has been described and will likely allow rapid diagnosis and increase recognition of this disease in the future [45,111-114]. Some platforms also include information regarding antibiotic susceptibility [115]. Bronchoalveolar lavage specimens may give the optimal diagnostic yield, over sputum and blood. A Chinese report suggests superior sensitivity over real-time PCR [116].

Commercial multiplex PCR assays do not test for C. psittaci [117,118].

Culture — Culture is discouraged, except in specialized laboratories. C. psittaci can be isolated from sputum, pleural fluid, and autopsy specimens of lungs, liver, and spleen [48]. However, it is highly infectious when cultured, and the danger of aerosol transmission to laboratory personnel is significant.

C. pneumoniae, a frequent consideration in the differential diagnosis, is particularly difficult to culture. (See "Pneumonia caused by Chlamydia pneumoniae in adults", section on 'Diagnosis'.)

TREATMENT — The tetracyclines are preferred for the treatment of psittacosis, but other agents have been proposed as alternatives. Macrolides are frequently used in the treatment of community-acquired pneumonia in the absence of a microbiologic diagnosis and are highly likely to be effective for psittacosis.

Tetracyclines — Tetracyclines are the drugs of choice. Doxycycline (100 mg orally twice daily) usually produces a rapid clinical response in patients with mild to moderate disease. Doxycycline can be given intravenously in critically ill patients. In one series, 92 percent of patients defervesced within 48 hours of commencing treatment with one of these agents [42]. However, occasional treatment failures and relapses are recognized. Minocycline has also been used with success. Although the optimal duration of therapy for psittacosis has not been determined, we generally treat for 7 to 10 days when using a tetracycline.

Macrolides — A macrolide, such as erythromycin or azithromycin, is usually recommended as the second-line therapy when the tetracyclines are contraindicated. In one study of five patients treated with erythromycin, the response appeared equivalent to tetracyclines [119]. If azithromycin is used, treatment duration may be shortened to five days if there is a prompt clinical response. (See "Pneumonia caused by Chlamydia pneumoniae in adults".)

Macrolides remain the agents of choice in children with mild to moderate infection. Treatment in pregnancy is controversial and is influenced by anecdotal reports. Two women with gestational psittacosis failed to respond to erythromycin but improved with doxycycline and delivery [89,120]. Some authors have therefore suggested that doxycycline should be the agent of choice in pregnancy. Azithromycin shows good activity in vitro and in animal models, but adequate clinical data are lacking. We believe that azithromycin is a reasonable choice in pregnant women and children <8 years of age, in whom doxycycline is typically avoided. We favor azithromycin over erythromycin since azithromycin is better tolerated.

Other drugs — Chloramphenicol and rifampin have been used successfully, but relapses have occurred with both agents. Ofloxacin proved effective in the treatment of 13 patients [121], but the role of quinolones in this infection requires further evaluation.

PREVENTION

Vaccine — Early work identifying vaccine candidate antigens in mice has been undertaken, but no vaccine has been developed to date [122,123].

PUBLIC HEALTH ISSUES — Many health authorities require clinicians to report patients with psittacosis [9]. Reporting facilitates the recognition of outbreaks and the evaluation and treatment of the birds that are the source. If the source bird has the diagnosis of psittacosis confirmed and has been purchased from a shop or breeder in the preceding two months, an epidemiologic investigation should be undertaken. Others in contact with infected birds should be warned of the possibility of disease. Infected birds should be isolated and treated.

Patients recovering from psittacosis will often ask if they should destroy their pet bird if they developed psittacosis following exposure to the bird, but this is not usually necessary. Psittacosis of the bird can usually be cured or suppressed with antibiotic treatment alone. If the bird is kept in poor or crowded conditions, stress may be contributing to the shedding of organisms and should be corrected.

Aviaries with infected birds should be cleaned according to published guidelines [124]. Techniques to prevent aerosolization of infected particles during this procedure are necessary. In some poultry plants, where ducks and other poultry living in an aqueous habitat are slaughtered and processed, there may be a high risk of C. psittaci infection and ornithosis morbidity [125]. Preventive measures (eg, gloves, masks, information, and medical examinations of the workers) should be implemented in such plants when C. psittaci is common [126]. In a community outbreak situation, gardening practices may need to be examined and modified [32].

The Swedish outbreak with in-hospital transmission described above [39] raises concerns that stricter hospital infection control measures than those currently used may be indicated. (See 'Human disease' above.)

C. psittaci is a "category B" bioterrorism agent. Disease could be spread by aerosol [127]. (See "Identifying and managing casualties of biological terrorism", section on 'Organisms of concern'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Community-acquired pneumonia in adults".)

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●Basics topic (see "Patient education: Psittacosis (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Avian borne respiratory illness − Psittacosis is a disease with prominent systemic manifestations and some respiratory symptoms. This infection, caused by Chlamydia psittaci, is transmitted to humans predominantly from birds. (See 'Introduction' above.)

●Worldwide geographic distribution − Psittacosis has been recognized throughout the world, including the United States, the United Kingdom, Europe, the Middle East, and Australia. Birds are the primary reservoir, but transmission from other animals has also been reported. The disease usually occurs sporadically, but outbreaks caused by contact with an infected bird are not uncommon. (See 'Epidemiology' above.)

●Bird exposure and incubation period − Humans are usually infected by inhalation of organisms in dried feces when caged birds exercise their wings or in bird feather dust. Cage cleaning may pose an infection risk. Bird bites, mouth-to-beak contact, and even transient exposure, such as visiting a bird park, have also been implicated in the transmission of this infection. The incubation period is usually 5 to 14 days. (See 'Human disease' above.)

●Clinical manifestations − C. psittaci infection of humans most commonly presents in young or middle-aged adults as fever of abrupt onset, pronounced headache, and dry cough. Patients usually have a recent history of bird exposure. Most patients have some abnormalities on chest examination (usually rales), may have evidence of consolidation, and uncommonly have a pleural rub. (See 'Clinical features' above and 'Physical examination' above.)

●Complications − Complications in a number of organ systems may arise in patients with psittacosis. These are uncommon manifestations of the illness, but some can be severe. Examples include respiratory failure, hepatitis, endocarditis, and encephalitis. Infection in pregnancy may be life-threatening. (See 'Organ system complications' above.)

●Chest imaging − The chest radiograph is usually abnormal, most often with lobar changes. Approximately one-quarter of patients have multilobar changes, and slightly fewer have a normal chest radiograph. On high-resolution computed tomography, the pulmonary infiltrates may be nodular surrounded by ground-glass opacities. (See 'Chest radiology' above.)

●When to suspect psittacosis − The typical clinical features (fever, headache, myalgias, dry cough) in a patient with a history of bird contact should suggest the diagnosis and should prompt the initiation of appropriate treatment. In a patient presenting with pneumonia, severe headache, splenomegaly, and failure to respond to beta-lactam antibiotics may be other clues to the diagnosis. (See 'Diagnosis' above.)

●Confirming the diagnosis − The diagnosis of psittacosis is typically established by serologic testing. When available, we favor microimmunofluorescent antibody testing, but the complement fixation assay may be used as an alternative. Culture is discouraged since C. psittaci is highly infectious when cultured and is only performed in specialized laboratories. (See 'Specific diagnostic tests' above.)

●Preferred antibiotic treatment − We recommend the tetracyclines as first-line therapy for psittacosis. Doxycycline (100 mg orally twice daily) usually produces a rapid clinical response in patients with mild to moderate disease. Tetracycline hydrochloride or doxycycline (4.4 mg/kg per day divided into two daily doses) is given intravenously in critically ill patients. We generally treat for 7 to 10 days. (See 'Tetracyclines' above.)

●Alternative treatment − A macrolide such as erythromycin or azithromycin is usually recommended as second-line therapy when the tetracyclines are contraindicated. (See 'Macrolides' above.)

ACKNOWLEDGMENT — UpToDate gratefully acknowledges John G Bartlett, MD (deceased), who contributed on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Infectious Diseases.