Psittacosis overview

# Overview
Psittacosis is a zoonotic infectious disease caused by a bacterium called Chlamydophila psittaci (formerly Chlamydia psittaci). The disease is contracted not only from parrots, such as macaws, cockatiels, and budgerigars, but also from pigeons, sparrows, ducks, hens, seagulls, and many other species of birds. The incidence of infection in canaries and finches is believed to be lower than in psittacine birds. As a systemic zoonotic infection with protean clinical features, the major risk factor is exposure to birds. Hence, bird owners, veterinarians, those involved with breeding and selling birds, and commercial poultry processors are most at risk. Patients typically present with 1 week of fevers, headache, myalgias, and a nonproductive cough. Although pneumonia is the most common manifestation, all organ systems can be involved. Serology remains the mainstay of diagnosis; however, polymerase chain reaction techniques offer a rapid and specific alternative. Doxycycline is the treatment of choice.

# Historical perspective
The word psittacosis is derived from the Greek work "psittakos," which means parrot. Psittacosis infects psittacines (parrots, parakeets, cockatoos). "Ornithosis" is the term used if it infects other types of birds. It is assumed that the origin of psittacosis is in South America, where the rain forests are populated with many species of psittacine birds. Aboriginal tribes were fond of these birds and used their feathers as parts of their ceremonial clothing. Other psittacine birds were kept as pets in aboriginal villages.

# Classification
There is no established classification system for psittacosis.

# Pathophysiology
The major risk factor for acquiring psittacosis is exposure to birds. Transmission can occur either by inhalation of aerosolized organisms in form of dried feces or respiratory secretions or by direct contact with birds. The exact molecular details of bacterial uptake are not well understood. It is speculated that chlamydial cell contact is a two-step process: reversible binding followed by irreversible attachment. The key to understanding the pathogenesis of C. psittaci is that frequent and repeated episodes of reinfection are needed for the development of severe disease. Several studies also highlighted the critical importance of host microfilaments, microtubules, and microtubule motor proteins (kinesin and dynein) for uptake and intracellular development of C. psittaci and other Chlamydia spp. As with other intracellular zoonoses such as Q fever and brucellosis, the clinical conditions associated with psittacosis have been seen in many organ systems such as the pulmonary, hepatic, and central nervous systems.

# Causes
Psittacosis is caused by the organism Chlamydia psittaci. Chlamydia is understood to be a Gram-negative bacterium belonging to the genus Chlamydia or Chlamydophila in the family Chlamydiaceae. Parachlamydiaceae, Waddliaceae, and Simkaniaceae  also belong to the order Chlamydiales, class and phylum Chlamydiae. Chlamydiales are obligate intracellular infectious agents in eukaryotic cells characterized by a unique developmental replication cycle.

# Differential diagnosis
Psittacosis must be differentiated from other diseases that cause atypical pneumonia and febrile illness without localizing signs and extrapulmonary manifestations such as gastroenteritis, hepatitis, meningitis, or encephalitis. The three main diseases to differentiate psittacosis from are Chlamydia pneumoniae pneumonia, Mycoplasma pneumoniae pneumonia, and Legionella infection, as they tend to have similar clinical manifestations that can only be differentiated by taking appropriate histories and laboratory investigations. There are other conditions to watch out for which may also present similarly to psittacosis.

# Epidemiology and demographics
Since 1996, fewer than 50 confirmed cases of psittacosis have been reported in the United States each year. Many more cases may occur that are not correctly diagnosed or reported. In the United States, the incidence of psittacosis is 0.01 per 100,000 persons. The prevalence and incidence of psittacosis does not vary by gender. There is no racial predilection for psittacosis, as it has been identified in all parts of the world including Africa, China, Europe, and the United States.

# Risk factors
Bird owners, pet shop employees, persons who work in poultry processing plants, and veterinarians are at increased risk for contracting psittacosis. Typical birds involved are parrots, parakeets, and budgerigars. Other animals documented with C. psittaci infection include horses, cattle, and koalas, but they have not been documented to transmit the infection to humans.

# Screening
According to the USPSTF, there is insufficient evidence to recommend routine screening for psittacosis.

# Natural history, complications and prognosis
If left untreated, psittacosis usually presents as flu-like symptoms or an atypical pneumonia. In the first week of psittacosis, the symptoms mimic typhoid fever, including prostrating high fevers, arthralgias, diarrhea, conjunctivitis, epistaxis, and leukopenia. Headache can be so severe that it suggests meningitis and some nuchal rigidity. Towards the end of the first week, stupor or even coma can result in severe cases. The second week is more akin to acute bacteremic pneumococcal pneumonia with continuous high fevers, cough, and dyspnea. Some complications include respiratory failure, acute tubular necrosis, hemolytic anemia, endocarditis, hepatitis, encephalitis and, in some cases, death.

# Diagnosis

## History and Symptoms
The hallmark of psittacosis is a flu-like reaction with a history of exposure to birds. However, a history of exposure to birds may not always be present. Psittacosis is characterized by a wide range in both disease severity and spectrum of clinical features, but it typically presents with fever, prominent headache, myalgia, and a nonproductive cough. The mainstay of diagnostic testing is serology, although molecular techniques are being increasingly utilized.

## Physical Examination
Physical examination findings in a patient with psittacosis include rose spot rashes on the skin called Horder's spots and splenomegaly, which is frequent toward the end of first week. Diagnosis can be suspected in the case of respiratory infection associated with splenomegaly and/or epistaxis.

## Laboratory Findings
Exposure history is critical to diagnosis. Complete blood count shows leukopenia, thrombocytopenia, and moderately elevated liver enzymes. Culture of C. psittaci is demanding and requires a level 3 laboratory isolation facility because of the risk of laboratory transmission, so it is rarely performed. Serology is the most widely available method for laboratory diagnosis of C. psittaci infection. Complement fixation, microimmunofluorescence, and EIA are the most commonly used techniques.

## Electrocardiogram
There are no electrocardiogram findings associated with psittacosis. However, bradycardia may be noticed on electrocardiogram.

## Chest X-Ray
Chest X-rays show lobar consolidation, patchy infiltrates, a diffuse whiteout of the lung field, or pleural effusions.

## CT scan
On high resolution CT, infiltrates may be nodular and surrounded by ground glass opacity.

## MRI
There are no MRI findings associated with psittacosis.

## Echocardiography or ultrasound
There are no echocardiography or ultrasound findings associated with psittacosis.

## Other imaging findings
There are no other imaging findings associated with psittacosis.

## Other diagnostic studies
Biopsy, culture, and serology have been found useful in the diagnosis of psittacosis.

# Treatment

## Medical therapy
Psittacosis is treated with tetracyclines. 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. For pregnant patients or infants, the preferred treatment is azithromycin because tetracyclines are contraindicated.

## Surgery
Surgical intervention is not recommended for the management of psittacosis.

## Primary prevention
Avoid exposure to birds that may carry this bacteria, such as imported parakeets. Medical problems that lead to a weak immune system increase the risk for this disease and should be treated appropriately.

## Secondary prevention
The primary and secondary prevention strategies for psittacosis are the same.