Ornithosis: Psittacosis, Causes, Symptoms, and Treatment

Ornithosis, also known as psittacosis and "parrot fever," is a zoonotic bacterial infection of humans caused by Chlamydia psittaci and transmitted primarily by inhaling aerosols from the droppings and secretions of infected birds. The disease most commonly presents as atypical pneumonia with fever, dry cough, headache, and severe weakness, but the spectrum of severity is broad, from mild forms to severe respiratory and cardiovascular complications. [1]

Transmission occurs from a reservoir in birds: infection is possible through handling pet parrots and canaries, through contact with pigeons, ducks, and turkeys, and on poultry farms, as well as during cleaning of areas with accumulated dry droppings. Direct human-to-human transmission has been described extremely rarely and is not considered epidemically significant. [2]

In Europe, an increase in ornithosis cases with hospitalizations and isolated deaths was recorded in several countries in late 2023 and early 2024, attracting the attention of surveillance services and highlighting the importance of early disease recognition. According to the World Health Organization, the outbreaks coincided with increased contact with domestic and wild birds. [3]

With timely diagnosis and proper treatment, the prognosis is favorable and the mortality rate is low. The key to preventing severe outcomes is the early administration of tetracycline antibiotics when there is reasonable suspicion and strict preventive measures when working with birds and their premises. [4]

Code according to ICD-10 and ICD-11

In the International Classification of Diseases, Tenth Revision, ornithosis is coded under the heading A70 "Chlamydia psittaci infections." This code covers "ornithosis," "psittacosis," and "parrot fever." Code A70 is used for all clinical forms of the disease in humans. [5]

In the International Classification of Diseases, Eleventh Revision, ornithosis is classified under "Other diseases caused by chlamydia" and is coded as 1C22 "Infections due to Chlamydia psittaci." This code is used for current statistics, case reporting, and patient routing in systems that have transitioned to ICD-11. [6]

Table 1. ICD codes for ornithosis

System	Chapter	Code	Name
ICD-10	Certain infectious and parasitic diseases	A70	Chlamydia psittaci infections, including ornithosis and psittacosis
ICD-11	Other diseases caused by chlamydia	1C22	Chlamydia psittaci infections
[7]

Epidemiology

Ornithosis is reported worldwide as sporadic cases and localized outbreaks, most often in families, groups, and workplaces with close contact with birds. In recent years, surveillance in Europe has recorded an increase in detection rates and requests for laboratory testing, which is associated with a real increase in incidence and better recognition. [8]

In February–March 2024, the World Health Organization reported an increase in cases in Austria, Denmark, Germany, Sweden, and the Netherlands, including hospitalizations and several deaths. The European Centre for Disease Prevention and Control confirmed this trend in its weekly bulletin and emphasized the role of contact with poultry and wild birds. [9]

Seasonality is weak and depends on local factors: bird population density, housing conditions, and seasonal cleaning of dovecotes and poultry houses. Outbreaks in humans are often associated with pet shops, bird shows, dovecotes, and turkey and duck farms. [10]

Mortality with modern treatment is low; however, elderly patients and those who present late are at higher risk of severe complications, as confirmed by observations of recent outbreaks. This necessitates prompt consideration of ornithosis in the differential diagnosis of atypical pneumonia in people with contact with birds. [11]

Table 2. Epidemiological landmarks

Indicator	Characteristic	Comment
Tank	Domestic and wild birds	Parrots, pigeons, turkeys, ducks, etc. [12]
Route of transmission	Inhalation of aerosols from droppings and excreta	The risk increases with dry cleaning and poor ventilation. [13]
Foci	Families, pet shops, dovecotes, poultry farms	Clusters in contact groups. [14]
Latest signals	Increase in cases in several European countries in 2023-2024	Clinicians need to be vigilant. [15]

Reasons

The etiologic agent is Chlamydia psittaci, an obligate intracellular bacterial pathogen with a biphasic development cycle, with an infectious elementary form and an intracellular reticular form. This characteristic determines its resistance in the external environment and the ability to persist latently in host cells. [16]

Human infection occurs primarily through airborne transmission, inhalation of dust and aerosols generated during litter removal, cage cleaning, and handling of feathers and bedding. Rarely, contact can occur through contamination of hands and mucous membranes. Identifying the bird species and housing conditions is important for epidemiological surveillance. [17]

Chlamydia psittaci elementary bodies remain viable in the environment for long periods, especially in dry litter and dust. Modern quaternary ammonium compounds, alcohols, and freshly prepared hypochlorite solutions are considered effective disinfectants, while heat treatment under appropriate parameters completely inactivates the pathogen. [18]

Birds can carry the infection asymptomatically or with nonspecific signs, shedding the bacteria into the environment. Parrots, pigeons, and waterfowl play the greatest epidemiological role, while infection is less common in chickens but can spread within certain farms. [19]

Risk factors

Increased risk is observed among poultry owners, pet store workers, veterinarians, pigeon breeders, poultry farm workers, and people cleaning areas with accumulated dry manure. Cases often cluster within families and small groups. [20]

The risk of infection increases with poor ventilation, dry cleaning without moistening dust, lack of personal protective equipment, and failure to maintain hand hygiene after handling cages and accessories. In production settings, engineering measures, cleaning protocols, and personnel training play a crucial role. [21]

Severe disease is more likely in the elderly, pregnant women, and patients with chronic diseases, especially those with respiratory or cardiovascular disease. In these groups, a lower threshold for early treatment and initiation of etiotropic therapy is indicated if a diagnosis is suspected. [22]

Travelling, visiting poultry markets, participating in exhibitions and trades, as well as keeping a large number of birds in one room without regular wet cleaning are considered additional risk factors in terms of outbreak situations. [23]

Table 3. Main risk factors and risk reduction measures

Factor	Why is it dangerous?	What to do
Dry removal of droppings	Aerosols containing bacteria are formed	Wet the surface, use respirators and gloves. [24]
Poor ventilation	The concentration of aerosols is increasing	Provide supply and exhaust ventilation. [25]
Mass keeping of birds	High microbial load	Cleaning and quarantine regulations. [26]
Professional contact	Long exposure	Briefing, protective equipment, medical examinations. [27]

Pathogenesis

After inhalation, elementary bodies attach to the epithelial cells of the respiratory tract, penetrate, and transform into reticular bodies, which actively divide within the membrane inclusion. They then transform back into elementary bodies and exit the cell, infecting adjacent tissues and spreading hematogenously. This cycle explains the systemic nature of the disease. [28]

Pulmonary involvement is associated with reticuloendothelial system involvement, manifested by fever, intoxication, and laboratory signs of inflammation. The host immune response contributes to the clinical picture, and the relative resistance of the intracellular pathogen determines the need for sufficient duration of antibacterial therapy. [29]

The persistence of elementary bodies in the external environment and their tropism for respiratory epithelial cells contribute to increased infectivity in closed, dusty spaces. This justifies sanitary regulations for wet cleaning and local exhaust ventilation in areas where birds are kept. [30]

In severe cases, damage to the endocardium, myocardium, liver, and central nervous system has been described, which is associated with both pathogen dissemination and a systemic inflammatory response. These forms require early recognition and intravenous therapy. [31]

Symptoms

The incubation period is usually 5-14 days, in some cases up to 28 days. The onset is often acute: fever, chills, headache, myalgia, dry cough, chest pain, and severe weakness. "Relative bradycardia" is often observed against the background of fever and disproportionately severe illness. [32]

The clinical picture is dominated by atypical pneumonia: a dry or unproductive cough, shortness of breath, scant physical findings with pronounced radiographic changes. Nausea, vomiting, diarrhea, and abdominal pain are possible, reflecting the systemic nature of the infection. [33]

In severe forms, complications affect the respiratory, cardiovascular, and central nervous systems, including myocarditis, endocarditis, and meningoencephalitis. Skin rashes and photophobia are rare. [34]

Without treatment, symptoms may persist for weeks, but with early administration of tetracyclines, improvement usually occurs within 48-72 hours. Slow recovery of function and residual weakness may persist for several more weeks. [35]

Table 4. Typical manifestations of ornithosis

Group of symptoms	The most common signs
General	Fever, chills, severe weakness, headache. [36]
Respiratory system	Dry cough, chest pain, shortness of breath, atypical pneumonia. [37]
Others	Nausea, vomiting, diarrhea, photophobia, rash. [38]

Classification, forms and stages

Clinically, mild, moderate, and severe forms are distinguished, determined by the severity of intoxication, respiratory failure, and damage to other organs and systems. Complicated forms involving the heart, liver, and central nervous system are described separately. [39]

Depending on the localization of the disease, a distinction is made between primarily respiratory forms and systemic variants with multi-organ damage. Mild forms are possible in children and young adults, but more pronounced manifestations are typical for occupational groups with intense exposure. [40]

Traditionally, in practical healthcare, a distinction is made between "typical ornithosis" with pneumonia and "absent" forms without overt pneumonia, which require active laboratory testing during the epidemiological history. The severity of the condition, rather than the specific form, is crucial for treatment decisions. [41]

Old age, comorbidities, late initiation of etiotropic therapy, and the development of complications have unfavorable prognostic effects. In these situations, early administration of intravenous tetracyclines and inpatient observation are indicated. [42]

Complications and consequences

The most common complications are severe pneumonia, acute respiratory failure, and pleurisy. In systemic cases, myocarditis and endocarditis, hepatitis, and meningoencephalitis have been described, requiring interdisciplinary monitoring. [43]

Fatal outcomes are rare with modern treatment, but can occur with late presentation, in the elderly, and in those with underlying medical conditions. Reports from 2023–2024 noted isolated deaths during outbreaks, emphasizing the importance of early diagnosis. [44]

A protracted course with prolonged weakness and cough is possible, especially if therapy is initiated late or is insufficiently long. Relapses have been reported with too short a course, which dictates the correct duration of treatment. [45]

After recovery, specific immunity is formed, but repeated cases are possible with new exposure, so preventive measures remain relevant for owners and workers in contact with birds. [46]

When to see a doctor

Urgent consultation is required for fever, increasing dry cough, shortness of breath, and headache in a person who has been in contact with birds, cleaned cages, visited dovecotes, or worked on a poultry farm in the past 4 weeks. This combination of symptoms and epidemiological history warrants early initiation of etiotropic therapy. [47]

Immediate evaluation is indicated for signs of severe progression: severe shortness of breath at rest, confusion, chest pain, decreased oxygen saturation, and signs of cardiac damage. In these situations, hospitalization and intravenous antibiotic therapy are required. [48]

People in professional and family settings should inform their doctor about any contact with birds, the nature of their cleaning, and the protective measures they are using. This will help them schedule targeted tests and decide on prompt treatment. [49]

If symptoms occur, veterinarians and poultry workers are required to stop contact with birds until the diagnosis is clarified and to follow production regulations on labor protection and epidemiological safety. [50]

Diagnostics

Basic laboratory tests include detection of antibodies to Chlamydia psittaci by microcomplement fixation or microfluorescence assays with paired sera, as well as polymerase chain reaction on clinical respiratory tract samples. A 4-fold increase in titer in paired sera confirms the diagnosis. [51]

Polymerase chain reaction is used for early etiologic verification, particularly in hospitalized patients and during outbreaks; culture requires special conditions and biosafety. Molecular diagnostic panels for respiratory pathogens that include Chlamydia psittaci are available in some laboratories. [52]

Imaging studies include chest radiography and computed tomography (CT) scans, which demonstrate focal interstitial changes, sometimes disproportionate to the clinical picture. Nonspecific laboratory tests reflect the inflammatory response. [53]

In the epidemiological anamnesis, contact with birds is interpreted as a leading clue for prescribing etiotropic therapy until definitive results are obtained. At the same time, the physician evaluates alternative causes of atypical pneumonia to properly structure the examination. [54]

Table 5. Diagnostic tests and their interpretation

Test	What does it show?	How to interpret
Antibodies by complement fixation or microfluorescence	Serological evidence of current or recent infection	A 4-fold increase in titer in paired sera confirms the diagnosis. [55]
Polymerase chain reaction	Detection of Chlamydia psittaci DNA	Early verification, especially in severe cases. [56]
Sowing	Isolation of the pathogen	Rarely used due to biosafety requirements.[57]

Differential diagnosis

In adults with atypical pneumonia and contact with birds, the differential diagnosis primarily includes legionellosis, mycoplasmal and chlamydial pneumonia, and viral fevers of unknown origin. Epidemiological anamnesis and laboratory verification allow us to differentiate these conditions. [58]

In farm and abattoir exposures, coxiellosis is considered Q fever, as it can mimic atypical pneumonia. It is important to collect a detailed occupational history and clarify the circle of animal contacts. [59]

Influenza and other acute respiratory viral infections are also included in the differential diagnosis during the season of increased incidence, but with ornithosis, persistent fever and dry cough with a disproportion between the severity of symptoms and the auscultatory picture are more often observed. [60]

Pulmonary embolism, bacterial pneumonia of typical etiology and drug-induced pneumonitis are taken into account based on clinical and imaging data, however, the absence of contact with birds does not completely exclude ornithosis, especially with indirect contact through the environment. [61]

Table 6. Distinctive cues for differentiation

Suspected cause	Clue	What to check
Legionellosis	Hyponatremia, contact with water and air conditioners	Molecular tests for Legionella. [62]
Mycoplasma pneumonia	Young age, pharyngitis, scanty wheezing	Specific tests and serology. [63]
Q fever	Contact with farm animals	Serology and polymerase chain reaction for Coxiella. [64]
Ornithosis	Contact with birds, dry cough, relative bradycardia	Testing for Chlamydia psittaci. [65]

Treatment

Tetracyclines are the first-line therapy. If ornithosis is suspected in a patient with a moderate to severe clinical picture, doxycycline should be started immediately, without waiting for laboratory confirmation. A typical regimen for adults is doxycycline 100 mg twice daily orally or intravenously, then switching to oral therapy upon clinical improvement. Duration of treatment is at least 10-14 days and at least 3 days after temperature normalization. [66]

In mild cases in adults, oral doxycycline can be prescribed immediately, provided there is good patient adherence and follow-up. Clinical improvement is expected within 48-72 hours; if there is no improvement, the diagnosis and treatment regimen should be reviewed. It is important to advise the patient to complete the full course to reduce the risk of relapse. [67]

Macrolides, most commonly azithromycin, are an alternative for those intolerant to tetracyclines. However, treatment failures with macrolides have been reported, so tetracyclines are preferred in severe cases and at risk of complications. The choice of alternative for pregnant women is discussed individually, with macrolides often favored, taking into account the balance of benefits and risks. [68]

In children under 8 years of age, many guidelines recommend macrolides as the initial choice due to the risks of tetracyclines, but in life-threatening conditions, the appropriateness of doxycycline is assessed individually. Dosage and duration of therapy are adjusted based on body weight and monitored for efficacy and safety. [69]

In severe cases, with severe respiratory failure or inability to take oral medication, therapy begins with intravenous doxycycline, followed by a transition to the oral form. Concurrent treatment includes oxygen support, correction of fluid and electrolyte imbalances, analgesia, and treatment of complications. [70]

Chloramphenicol is considered a reserve drug in certain situations where other medications are intolerable, but is rarely used due to its safety profile and the availability of alternatives. The decision is made in a hospital setting with monitoring for potential adverse effects. [71]

Treatment duration is critical: too short courses are associated with relapse. Discontinuing therapy before 10-14 days is not recommended, even with early clinical improvement. In professional and family settings, it is important to monitor the completion of the course in all affected individuals. [72]

Antibiotic prophylaxis for asymptomatic contacts is not routinely indicated. The primary focus is on sanitation measures, awareness of early symptoms, and rapid diagnosis when they appear. Decisions regarding individual high-risk exposures are made in consultation with epidemiologists. [73]

Patients should avoid self-administering potentially hepatotoxic medications and herbal supplements during illness. In cases of concomitant cardiac and liver disease, treatment should be managed in consultation with specialized specialists. A plan for returning to physical activity is developed based on clinical and laboratory progress. [74]

Table 7. Etiotropic therapy of ornithosis in adults

Situation	Drug and regimen	Duration
First line	Doxycycline 100 mg 2 times a day orally or intravenously	10-14 days and at least 3 days after temperature normalization. [75]
Alternative for intolerance	Azithromycin according to approved regimens	Based on clinical assessment, tetracyclines are preferred in severe cases. [76]
Reserve	Chloramphenicol according to indications	Individually, with safety monitoring. [77]

Prevention

There is no specific vaccine for ornithosis in humans, so prevention is based on source control and interrupting transmission routes. At home and at work, wet cleaning with pre-wetting of bedding and droppings, good ventilation, the use of respirators and gloves, and hand hygiene after handling cages and equipment are recommended. [78]

Poultry owners should regularly clean cages with damp dust, promptly remove droppings, avoid touching birds, and store feed in closed containers. If a bird shows signs of illness, consult a veterinarian and follow quarantine instructions. [79]

Organizations are required to have ventilation and cleaning procedures, staff training, and the availability of personal protective equipment. Disinfection is carried out using effective disinfectants, and the technological parameters of thermal treatment, if required, are observed. [80]

Outbreak reports require collaboration between public health and veterinary services: investigation of the outbreak, notification of contacts, temporary restriction of access, and adjustment of sanitary procedures. This approach reduces the risk of new human cases. [81]

Table 8. Practical prevention measures for homeowners and businesses

Measure	How to perform	Why is this necessary?
Wet cleaning	Pre-moisten bedding and droppings	Reduction of formation of infectious dust. [82]
Ventilation	Supply and exhaust systems and ventilation	Dilution of aerosols. [83]
Protective equipment	Respirator, gloves, protective clothing	Personal protection during cleaning. [84]
Disinfection	Effective means and correct regimens	Inactivation of the pathogen on surfaces. [85]

Forecast

With early administration of tetracycline antibiotics, the prognosis is favorable: the temperature decreases within 2-3 days, and full recovery takes 2 to 4 weeks. In some cases, weakness and cough persist longer, which does not contradict a favorable outcome. [86]

Fatal outcomes are rare, but may occur in severe cases, in elderly patients, and with late presentation. Isolated deaths recorded during recent European outbreaks confirm the need for vigilance and early treatment. [87]

Re-infection is possible with new exposure, so people at risk should maintain protective measures when working with birds and their premises even after infection. [88]

Return to work and physical activity is planned individually, based on clinical improvement and the absence of signs of respiratory failure. In complicated cases, follow-up visits and, if indicated, instrumental assessment are necessary. [89]

Table 9. Prognostic benchmarks

Scenario	What to expect	What to control
Early treatment	Rapid improvement in 2-3 days	Compliance with the full duration of the course. [90]
Late appeal	Risk of severe complications	Low hospitalization threshold. [91]
Old age and comorbidity	Slow recovery	Joint management with specialized specialists. [92]

Frequently asked questions

How contagious is ornithosis to others?
Human-to-human transmission is extremely rare and epidemiologically insignificant. The main risk is contact with infected birds and their environment. [93]

What should a pet bird owner do if they have a fever and cough?
Consult a doctor, report any contact with birds, avoid dry cleaning, and limit close contact with the bird until the cause is determined. Clean the cage only with a damp cloth and with respiratory protection. [94]

How long should you take antibiotics?
Usually 10-14 days, and at least 3 days after your temperature returns to normal. Stopping treatment early is not recommended due to the risk of relapse. [95]

Is there a vaccine for humans?
There is no specific vaccine. Prevention is based on sanitation, ventilation, wet cleaning, the use of protective equipment, and veterinary supervision. [96]

Which tests are the most informative?
Serology in paired sera and polymerase chain reaction on clinical samples are used for confirmation. Culture methods are rarely used. [97]